Is pine a land plant
Evolution and biodiversity of plants
3.2 The evolution of the generational change
3.2.1 The location of the gametangia as the first crucial step
3.2.2 The need to form spores
3.2.3 The generation change of liverworts and its evolutionary biological potency
3.2.4 The generational change of the moss and its evolutionary potential
3.2.5 The different generation changes of the fern plants (Pteridophyta)
184.108.40.206 The ferns (Filicopsida)
220.127.116.11 The generation change of horsetail
18.104.22.168 The generation change of the moss ferns (Selaginellales)
3.2.6 The generation change of gymnosperms
22.214.171.124 Cycas as an example of gymnosperms with spermatozoid fertilization
126.96.36.199 Gymnosperms with (simple) pollen tube fertilization
3.2.7 Further improvements and the development of angiosperms
The first land plants were with some certainty not modern land plants, but more or less thallosely organized and still regularly flooded and only occasionally dry. They were therefore able to maintain their mode of fertilization by freely swimming spermatozoids and initially only had to solve the problem of limiting the fertilization process to the time when the water was covered. The complex search and attracting strategies already developed for the algae could thus be retained. The generation change of the first land plants was already marked by heterogamy. The egg cell was no longer released, but fertilized within the characteristic gametangium, which always contains only one egg cell and can be referred to as oogonium (oogoniogamy). This characteristic oogonium is called the archegonium. The male gametangium is called the antheridium and releases flagellated spermatozoids. In contrast to the most highly developed green algae and fungi, the land plants always form a shell of sterile cells around the gametes or cells forming the gametes, i.e. the wall of the gametangium consists of cells and not just a cell wall. Such a cellular wall of the gametangium does not occur anywhere else and is a new acquisition (autapomorphy) of land plants.
If an organism is regularly flooded, the gametangia are expediently arranged on the surface of a thallus. If there is a lack of water, however, they are better on the underside, since a film of water is kept between the thallus and the substrate in which the spermatozoids can swim.
As long as all germ cells were released into the open water, the zygote was automatically a suitable dispersal unit for colonizing new locations. This is no longer the case with an oogamous, land-living organism. Even the male germ cells that are still released cover only very short distances, even under favorable conditions. Under unfavorable conditions, even the viability of the entire organism is in danger.
It was therefore necessary for such organisms to develop permanent stages that made it possible to survive more severe dry spells. Such permanent stages are expediently formed in larger numbers and are small in order to keep the loss low if the permanent stages are not used. While many algae use the free-swimming zygote for this and form so-called cystozygotes, this is inexpedient here, since in a heterogamous fertilization mode with only one egg cell per gamtangium, comparatively few egg cells are present and thus far too few zygotes are formed. The zygote must therefore carry out a series of mitoses before the unicellular permanent stages can be formed which, in contrast to the egg cell, also have to be released. Very small permanent stages are also well suited for the spread of the species to new locations by wind. From this starting point, the development of unicellular spores can be understood. For the mixing of the gene pool it is most expedient if the released spores are haploid and form the starting point of a new gametophytic generation. In fact, the spores of all land plants are haploid and arise directly from a spore mother cell through meiosis and without additional mitoses. This maximizes the number of meioses for the formation of a given number of spores and thus optimizes the chance for the evolutionarily important recombination.
The spores are formed in characteristic containers, the sporangia, which in land plants are always part of the diploid sporophyte. Like the gametangia, the sporangia form a cellular wall of sporangia which, in contrast to the wall of the gametangia, usually consists of several cell layers in adaptation to rural life. For the spread of the haploid spores, different ways are used to improve the chances of spread.
The simplest liverworts are thallous and have antheridia and archegonia on top of the thallus. Good water coverage is required for the spermatozoids to swim to the archegonium. However, the water cover is usually only sufficient to swim in the surface film on a thallus and not to get from one thallus to another. However, when water drops hit, spermatozoids can be thrown away or sprayed away and thus reach another thallus. The sporophyte that develops from the zygote forms a long stalk that carries a single sporangium that opens with flaps. The sporophyte is very short-lived and can no longer be found a short time after the spores are scattered.
The "spray spread" of the spermatozoids is apparently something very important because it has probably been independently improved several times. The easiest way to do this is to move the gametangia to the tip of upright thallus sections. In the fountain liverwort (Marchantia polymorpha), the gametangia are lifted up on long stalks so that water droplets hit can hurl the spermatozoids further away. The stem of the umbrella-shaped structure and the umbrella itself are thallus lobes on which the gametangia are sunk into the top. In the case of the antheridia this is most useful for spreading the spray, but not in the case of the archegonia, since drops of water that hit it would flow down and wash off the spermatozoids. In the female gametangia stands, the archegonia are therefore shifted to the underside of the umbrella through secondary growth processes. A sporophyte develops from the fertilized egg cell, which consists of only a short stalk and a single sporangium. The sporangia are exposed through the stalk of the gametangia and therefore no longer need their own stalk.
Some thallous liverworts have a haploid secondary cycle in which so-called "brood bodies" are formed on the thallus surface in special "breeding cups", from which thalli can grow again.
Although the liverworts have already achieved a considerable morphological differentiation in the leafy liverworts, their evolutionary biological potency is low. They are restricted to humid habitats.
The generation change of the moss corresponds in all essential parts to that of the liverwort. The gametangia are, however, basically shifted to the tip of leafy sprouts and grouped together in gametangia levels. The gametangia levels are more rarely hermaphroditic and mostly unisexual. In the latter case, archegonia stands and antheridia stands can occur on different branches of a plant (monocial) or they can be distributed over different plants (dioecious). To improve spray spreading, the antheridia stands are often surrounded by a conspicuous shell of sterile leaflets called "perianth". The bowl shape of the perianth significantly favors the splashing away of the spermatozoids by impacting raindrops. The female gametangia classes lack such a perianth. The bases of the moss leaflets adjacent to the moss shoots allow water to rise capillary, and spermatozoids can still reach the archegonia through this phenomenon known as "outer water conduction", even if the archegonial state is not hit directly by a drop of water with spermatozoids. A perianth like the antherid stand would be a hindrance here.
The sporophyte initially develops completely in the archegonium. In the course of its development it bursts the archegonium and lifts it up as a hood (calyptra) over the sporangium. The sporophyte grows on top of the gametophyte and therefore never becomes an independently living organism and must therefore inevitably be nourished by the gametophyte. Its lifespan must therefore be shorter than that of the gametophyte. Although the sporophyte forms only a single sporangium in the deciduous moss, it is morphologically and histologically much more differentiated than the sporophyte of the liverwort. The long stem (seta) already has simple guiding elements for water (hydroids) and assimilates (leptoids) and at the base of the capsule, the so-called apophysis, one can find functional stomata. The capsule has a complicated opening (peristome), the opening mechanism is reversible in contrast to liverworts. The capsule is open when dry and closed when moist. The scattering of the dry spores ("spore dust") can therefore take place over a longer period of time, and the sporophyte of the moss persists much longer than the sporophyte of the liverwort. That is why the sporophytes of the mosses are found much more frequently than the short-lived sporophytes of the liverworts. From the germinating spore, a short thread first develops, which is called a protonema. It is only on this branched or unbranched thread that the typical moss plants emerge.
The problem of homozygosity in original haplomonözischen mosses also makes it understandable why in mosses the sporophyte does not gain the upper hand over the gametophyte, as is the case with the ferns and all following groups. A completely homozygous sporophyte cannot benefit from heterosis effects like a heterozygous organism and the diploid situation therefore does not initially offer any advantage over the haploid.
There is an understandable limit to the size of plants that use spray-spreading spermatozoids. If the plants get so big that a drop of water no longer splashes onto neighboring plants, the system no longer works. Another disadvantage is that it has to rain at least when the spermatozoids spread. If there is a lack of sufficient precipitation, spermatozoids that did not hit exactly one archegonium by chance are not able to swim up to the archegonium on female moss plants. Some mosses (pleurocarp mosses) can help here, however, by moving the archegonia not at the tip of the main shoots, but at the tip of short basal side shoots. Many mosses in our latitudes place the reproductive phase in the wetter winter half of the year, where periods with longer lasting moisture are more frequent. The spores then spread in the following summer, when the drier weather favors a long-distance spread of the spores by wind.
The ferns have taken a completely different evolutionary path, which does not eliminate the water dependency of the fertilization process, but significantly mitigates its consequence. All spurs are designed the same. A monözischen gametophyte develops from the spores, which is called prothallium (pre-germ). At best, self-fertilization is restricted by a different time of maturation of antheridia and archegonia on a prothallium. The gametangia are shifted to the underside of the gametophyte. The spermatozoids can swim in the film of water that forms between the gametophyte and the subsurface when the weather is damp. You have to forego the advantage of spray spreading. In addition, the ferns need a much stronger sporophyte, since the gametophyte does nothing to expose the sporangium.
Since the young sporophyte develops on the underside of the prothallium, it inevitably receives early contact with the ground and thus the opportunity to take root and become nutritionally independent. This possibility is used to make the fern sporophyte larger and, above all, more durable. While the moss sporophyte only sporulates once and then dies, the fern sporophyte is not a single-use product, but is used several times. It no longer forms just a single sporangium, but a great many. The sporangia are located on the underside of large leaves, which are commonly referred to as fronds and are grouped together, which are called Sori (sing. Sorus). In many groups of ferns, the sori are protected by a veil called indusium, which only shrinks when the sporangia mature and allows the spores to be thrown off freely. The sporangia are thus well protected against rain and the very large fall distance to the ground compared to mosses significantly improves the chances for the spores to spread in the wind.
The wall of the sporangia is multilayered in the original case. Ferns with a multilayered sporangia wall are called eusporangiat. Most of our native ferns have a greatly reduced, single-layer sporangia wall and are known as leptosporangiate.
The size of the sporophyte is directly related to the fact that it is no longer the gametophyte but the sporophyte that represents the persistent, long-lived generation. The fern sporophyte quickly overgrows its gametophyte, so that it dies as soon as the sporophyte has deprived it of all nutrients. The multiple use of the sporophyte also makes a larger use of material economical. In the case of ferns, tree-like growth forms and especially tree-like sizes are therefore achieved for the first time. One of the decisive factors for this great evolutionary step was the invention of the root, which, in contrast to the differentiation into shoot and leaf, does not exist in mosses.
The evolutionary steps required to achieve tree-shaped growth forms and sizes are thus completed. In the further evolution, the aim is to make the phase, which is still water-dependent in the fern plants, during the transition from the gametophytic generation to the sporophytic generation, independent of water, and thus to make drier locations colonizable. Within the large group of fern plants, various progressions can be identified, up to and including relationships that are similar to those of the seed plants.
In mosses, the vegetation body consists almost entirely of the gametophyte, the sporophyte is limited to the moss capsule and the capsule stem. For fertilization, spermatozoids have to swim from the antheridia to the archegonia, which presupposes that these organs are at least temporarily surrounded by water. All life processes of the gametophyte in mosses are adapted to this excess water. Where there are adaptations to drought, they consist primarily in the fact that all vital functions are stopped and the organism waits in a state that is often described as "latent life" until water is again available in abundance.
Ferns have made considerable progress with the development of perennial, rooted sporophytes. They can remain in the competitive sporophytic phase for a very long time and, if necessary, avoid the haplophase, which is dependent on favorable conditions, and in particular the water-dependent fertilization process, for a long time. However, they have not made any progress to limit or even eliminate the water dependence of the fertilization process.
Interesting approaches to solutions can be observed here with horsetail.A first and still very simple step is to reduce the distance the spermatozoids have to swim. Not only will you arrive more safely, but the smaller swimming pool will also require less water. Since self-fertilization on a prothallium is evolutionarily unfavorable because of the homozygosity that occurs, the problem of the short distance must be solved for the much more difficult case of diocese prothallium. That is, mechanisms are needed to ensure that male and female prothallia are as close together as possible.
In the case of horsetail, the spores are held together by attachments of the spores (Hapteren) and spread together for this purpose. So there is always a group of prothallia. The first germinating spore always develops into a female prothallium, but all subsequent spores into male prothallium. The sex of the prothallia is therefore determined by the environmental conditions. This type of sex determination of the gametophyte is called "haplomodificatory". With this trick, the horsetail succeeds in avoiding the homozygous trap. The disadvantage of the method, however, is that spores of the same sporophyte are almost always next to one another, and the effect therefore corresponds exactly to that which occurs when self-pollination occurs on flowering plants.
From this point of view, it is more beneficial not to determine sex haplomodification but genetically, and to ensure that prothallia of different sexes come together as closely as possible. In dense populations of a species it is ensured in this way that pairings of male and female prothallia from spores of different sporophytes occur approximately as often as pairings that go back to one sporophyte. In the case of solitary plants, however, there is no difference to the conditions in horsetail.
If one wants to bring two prothallia of different sexes together, then that is a task that has already been set in a similar way, namely when the aim is to bring gametes together. Apparently there is only one solution to this, because the problem was solved in a consistent manner everywhere. In the gametes, there was a differentiation into small, large, immobile macrogametes, which contain the first nutrients for the macroprothallium (and more rarely also for the future embryo) and mobile, very small and therefore produced in large numbers. The same now applies to the spurs. Spores of different sizes are formed. The large macrospores are produced in small numbers and are accordingly only spread over shorter distances. They form large prothallia, which also contribute a great deal to the nutrition of the young sporophytes that later "parasitize" on them and exclusively form macrogametangia (archegonia) each with one macrogamete (egg cell). The much smaller microspores, on the other hand, are produced in large numbers. They become more widespread and can only do their job if they land directly on the macroprothallium. The small microprothallium then has no other task than to produce microgametes. It is therefore increasingly reduced to the microgametangium (antheridium) in the course of phylogeny. Since the microprothallium germinates on the macroprothallium, the hit rate for the microgametes (spermatozoids) is very favorable and their number can therefore be reduced.
These principles are implemented in an excellent way by the Selaginelles. The micro and macro spores arise in different sporangia, which are correspondingly referred to as micro and macro sporangia. The macrosporangium contains only a single macrosporangium, while the microsporangium contains a large number of tetrads. Because the microspores are much smaller than the macrospores, both sporangia are about the same size, although the microsporangium contains many more spores. Externally, however, the macrosporangium can be recognized by the shape of the macrosporangium that is visible through the wall of the sporangia. In the Sellaginellales, the macroprothallium develops completely within the macrospore. As a result of the cell division, there is no increase in biomass, but only a differentiation of the existing material. The macroprothallium is well protected against drying out by the spore wall. The spore opens on only three lines at its proximal pole. The microspore must fall into these openings, otherwise fertilization cannot take place. This means a low hit rate, but also that only a very small drop of water is required to enable fertilization. Although several archegonia are formed, as with mosses and ferns, only one fertilized egg cell can develop into a sporophyte. The others succumb to the competition of the strongest sporophyte. The microprothallium also develops within the spore wall and only develops a functionless rhizoid cell and an antheridium consisting of eight wall cells. The antheridium contains several spermatogenic cells, from each of which a spermatozoid develops.
In some fossil relatives of the Selaginella, the macrospores were fertilized before they fell out of the open macrosporangium. In some, only one spore of the tetrad developed, and the macrospore did not fall out of the macrosporangium, but rather the macrosporangium fell with the only fertilized macrospore. Such complexes can already be understood as simple seeds, although there are still evolutionarily significant differences to the seeds of the seed plants. These groups were appropriately referred to as the seminal whiskers.
A significant difference in the arrangement of the sporangia is decisive for this development into seminal larvae. In ferns, the sporangia are located on the underside of the sporophylls. This ensures that the sporangia do not get wet when it rains. However, if the macrospore remains in the macrosporangium, it is almost impossible for a microspore to fall into the opened macrospore. This is only possible if the sporangia are exposed on the leaf margins or, as with all selaginella and bear moss, on the upper side of the sporophyll. Plants with an organization such as the leptosporangiate ferns are therefore excluded from an evolution that leads to seed plants from the outset.
Even in dense stands it is not easy to get the microspores exactly into the opening gap of the macrospores. Since the macrospore of the Sellaginella is already given the full nutrient content, all of the material used is lost if no microspore hits. It would be better to use a method that enables only successful macrospores (i.e. those whose macroprothallium will later carry a fertilized egg cell) to be provided with nutrients. The distance to be covered in the open water has been drastically shortened for the Selaginella, but in principle the dependence on free water still exists.
Convincing solutions have been developed for these problems in the gymnosperms. The ancestors of the naked samos, the so-called progymnosperms, presumably followed the same evolutionary path as the club moss family in parallel and independently and developed comparable seed-like formations. The initial situation for further evolution was therefore similar to the conditions known from the selaginella and seminal bear lobes.
As a first important step, the use of nutrients for unfertilized and thus lost macroprothallia was reduced. This is easy if only macroprothallia with fertilized egg cells are provided with nutrients. In this context, two important innovations have occurred. Firstly, the spore wall of the macrospore was reduced so that a supply of further nutrients was possible even after fertilization of the macroprothallium. Second, from now on the microspore is no longer captured by the macroprothallium, but the receiving organ is now primarily the macrosporangium. The spore wall of the macrospore is reduced or even absent in order to enable the transport of nutrients into the spore. The protective function that was previously performed by the spore wall is now taken over by the sporangia wall and above all by a shell around the sporangium, the so-called integument. The development of the macrosporangium can initially be recognized as a less differentiated "tissue core" surrounded by a shell (the integument). It is traditionally called nucellus in all seed plants. The integument releases a pore called a micropyle.
The macrosporangium, which continues to grow on the sporophyte, with enveloping integuments and the resulting macroprothallium and embryo is called the ovule. With ripening, the ovule becomes the seed. This is the first time that the young sporophyte is a unit of spread. It is enveloped by protective layers and nutrient tissues from the previous gametophytic and sporophytic generation.
In almost all gymnosperms, a pollination droplet formed by the macrosporangium (nucellus) is used to capture the microspores. Scar-like structures are only formed in a few species. Within the gymnosperms, very different strategies were used to optimize the critical steps of the generation change. Three of the most important of these are explained below.
All cycads are dioecious, so there are male plants (microsporophytes) and female plants (macrosporophytes). The sporophylls form egg-shaped to cylindrical, compact sporophyll stands, which are called cones. The cones are unbranched and usually have a limited growth, so they can be viewed as flowers. Only the female cones of the genus Cycas continue to grow vegetatively after flowering (through-growing cones) and therefore, strictly speaking, do not meet the definition of the flower. The macrosporophylls show in exceptional cases (Cycas revoluta) in the distal part the feathering typical of the trophophylls. In the lower part (Fig. 30 A) they have several ovules on their edges. The ovules have only one integument. Deep in the nucellus, a single cell becomes the macrospore mother cell and then undergoes a meiotic division, from which a linear macrospore tetrad emerges (Fig. 30 B, C). A macroprothallium emerges from the spore, which is oriented towards the stalk of the ovule, and the other macrospores degenerate. The formation of the macroprothallium begins with free nucleus divisions without subsequent cell divisions, so that a polyenergid spherical cell is created, which can be several millimeters in diameter and which can be seen with the naked eye in halved ovules. The polyenergid cell then gradually begins to become cellular, progressing from the periphery. At about the same time, the nucellus begins to secrete a sugar-containing drop of liquid to which the pollen grains spread by the wind adhere (Fig. 30 D). This pollination droplet is then reabsorbed and the pollen grain gets into a cavity that is formed by the nucleus and integument and is called the pollination chamber (Fig. 30 E). The nucellus closes over the pollination chamber so that the remainder of the pollination droplet lies entirely within the nucellus. In the pollination chamber, the pollen grain germinates and forms a microprothallium, which is anchored in the nucellus with a rhizoid-like pollen tube and is nourished by it. In the macroprothallium 2 (-5) archegonia are formed, the egg cells of which can be up to 1 cm in size and are therefore the largest in the plant kingdom (Fig. 30 F). Even before the seeds ripen, the outer part of the integument becomes fleshy. The necessary protection of the embryo is provided by the inner sclerified layer of the integument, so that the seed coat of the mature seed is structured like the pericarp of a stone fruit (Fig. 30 G). The vigorously developing macroprothallium, called the primary endosperm (secondary endosperm, see page 103), serves as the nutrient tissue for the embryo. The embryo is dicotyledonous, the two cotyledons serve to absorb the nutrients from the endosperm and remain in the seed during the hypogeic germination process (Fig. 30 H, I)
The microsporophyll stands consist of numerous sporophylls with several to many sporangia on the underside (Fig. 30a). These are usually grouped into several on short stalks, so that pedunculated synangia are present (Fig. 30b). The epidermis of the individual sporangia has local wall reinforcements which, when the epidermis dries out, lead to the opening of the sporangium; the epidermis is designed as an exothecium. From a primary archesporal cell, mitotic divisions give rise to nothing but pollen mother cells, which after meiosis each become four microspores (Fig. 30c). The development of the microprothallium begins in the closed microsporangium with a first division from which a prothallium cell and an initial emerge (Fig. 30d). The initial is again divided into a generative and a vegetative cell or pollen tube cell. In this three-cell stage, the now mature pollen grain is released from the microsporangium, which can therefore be called the "flying prothallium" with good reason (Fig. 30e, f). In this state it is captured by the pollination droplets of an ovule and only continues its development when it is sucked into the pollination chamber of the ovule. During germination, the generative cell initially enlarges by bulging (Fig. 30g). The generative cell is then divided into a ring-shaped stem cell and a spermatogenic cell completely encircled by this. The ring-shaped stem cell is therefore hit twice in the longitudinal section (Fig. 30h). The pollen tube cell grows into a tubular, haustorial cell that penetrates the nucellus (Fig. 30 i, k). The spermatogenic cell forms two spherical spermatozoids that carry a screw-like eyelash band and are released into the former pollination chamber, which has now been expanded to become a fertilization chamber. There they swim to the archegonia and fuse with the egg cells to form zygotes (Fig. 30 G).
First of all, every fertilized zygote begins to develop into an embryo (polyzygous polyembryony). However, the strongest embryo soon crushes the others and these degenerate and are resorbed.
After fertilization, the zygote first undergoes a series of simultaneous nuclear divisions without increasing in size, so that a polyenergid cell is present (Fig. 30 II). At the pole facing away from the micropyle, however, a small-cell meristem develops (Fig. 30 III), which is attached to the polyenergid residue known as the basal body. The young embryo develops from it (Fig. 30 IV). Its top layer of cells is particularly differentiated histologically and presumably has the task of absorbing nutrients from the endosperm. The cells of this layer are called cap cells because of their location and arrangement. On the opposite side, the cells bordering the basal body stretch strongly and, with multiple transverse divisions in this area, the suspensor develops in the form of a multicellular stalk (Fig. 30 V, VI). The embryo, which still has no cotyledons, can divide into two embryos several times in the course of further growth (monozygous or split polyembryony). Of these, however, only one of the embryos resulting from the split will soon develop further, so that in the end the embryo that has developed completely is at the end of the longest suspensor arm (Fig. 30 VII, VIII).With the beginning of the development of the two cotyledons, the cap cells no longer bordering on the endosperm lose their special differentiation and the nutrient uptake takes place via the surface of the cotyledons (Fig. 30 VIII). In the course of embryogeny, the basal body and suspensor are more and more crushed. The base of the embryo is formed by a strong mass of tissue (root calotte), in the interior of which the attachment of the radicle develops (Fig. 30 XI). The embryo forms a plumula consisting of several scale-like lower leaves and a deciduous primary leaf. Germination is hypogean, the cotyledons remain in the seed and the primary leaf is the first assimilating organ.
Spermatozoid fertilization occurs in gymnosperms only in cycads and ginkgo. The spermatozoids of the two groups look practically identical. The development of the embryo also shows great consistency and also with ginkgo germination takes place hypogeaically and before the first assimilating foliage leaf a series of scale-shaped lower leaves are formed.
The essential progress of the spermatozoid-fertilized gymnosperms is that fertilization has become completely independent of external water through the secretion of a drop of liquid. Perhaps this was the pre-adaptation for using this drop to collect the pollen grains. With pollination invented in this way, the distance covered by the spermatozoids is reduced to a minimum. As a result of the good fertilization rate achieved in this way, the number of spermatogenic cells per microprothallium can be reduced to a single one. The microprothallium consists only of one (cycads) or two cells (ginkgo), the antheridium itself consists of two cells and the spermatogenic cell and is thus reduced almost beyond recognition.
In ginkgo the haustorial pollen tube is more strongly developed than in the cycads and is strongly branched (but still single-celled!). The macrosporangia are on long stems and not in a clear positional relationship to leaves. Whether Ginkgo has macrosporophylls at all or whether the sporangia are at the ends of the axes (stachysporia) is therefore a matter of controversy. Since the microsporangia are clearly formed on microsporophylls, it seems reasonable to assume that the position of the macrosporangia is also derived from a position on sporophylls (phyllosporia), but this has not been proven.
The free exposure of a drop of pollination produced by the plant, which we have just come to know as an important evolutionary step, also brings with it new problems. The pollination droplet evaporates quickly, for example, if no new liquid is supplied, or it can simply be blown away by a strong wind. Developments can therefore already be observed within the cycads that eliminate or at least minimize these deficiencies. First, the evaporation of the dust droplet is reduced by dissolved sugars. Due to the formation of nectar-like, viscous dusting droplets, these can also become larger than would be possible with pure water without flowing or dripping off. At the same time, the pollination drops also become interesting as a source of food for insects. Secondly, there is a further possibility of protection in not exposing the pollination droplet freely, but rather concealing it inside the cone. In fact, in many genera (e.g. Zamia) the only two ovules of the macrosporophyll are oriented in such a way that the micropyle points inwards towards the spindle. At first glance, this seems to contradict the task of the pollination drop as a pollen catcher. Investigations on cones and cone models in the wind tunnel have shown, however, that the pollen accumulates in the interior of the cone, i.e. exactly where the pollination drop is. This reversal of the ovule will subsequently prove to be an important step towards modern gymnosperms and angiosperms.
As the best studied example, the change in generation of the pine (Pinus) will be presented here. Various modifications and improvements to this principle can then be shown in a simplified manner. As with the cycads, the cones of modern conifers (Pinanae) are basically unisexual. The male and female cones are mostly distributed in one house, so they occur on the same individual. The male cones are unbranched sporophyll stands and can therefore be called flowers. Each sporophyll usually has two microsporangia on the underside. The female cones are more complex and represent branched systems, which must therefore be viewed as inflorescences. They are made up of two different types of cone scales. The cover scales are inserted on the tenon spindle. In the armpit of the cover scale is the seed scale, which bears two ovules on the side facing the spindle. Since, according to the leaf position rules, another leaf may not stand directly in the axilla of a leaf (the cover scale), the seed scale is now mostly understood as a flax sprout (cladodium). In fossil gymnosperm species (Lebachia, Voltziales), however, short axillary shoots with several stalked ovules were found and the interpretation of the female cones as inflorescences was thus confirmed.
The pollen is four-celled when it spreads, but the two prothallium cells have already degenerated and collapsed, so that only two intact, nucleated cells are left. In Pinus, the air flow causes the pollen to reach the two tail-like appendages of the micropyle, which are arranged radially one behind the other in relation to the longitudinal axis of the cone. The pollen remains on the inside of these attachments on a sticky secretion. The pollination drop is exposed only a few hours after midnight. It is pulled over the pollen grains adhering to the appendages and detaches them from them. Through the two air sacs, the pollen grain swims in the pollination droplet up to the nucleus (buoy effect). In order to prevent the dust droplet from flowing away when touching the cone scales, the cone scales are made non-wettable by a thick layer of wax. In order to ensure that the pollen grain germinates at the right time, the stimulus for germination of the pollen must come from the pollination drop. If a pollen grain gets stuck somewhere, the outgrowing pollen tube may grow behind the pollination drop. Since the spermatogenic cell lies entirely within the pollen tube cell, it can be transported to the target through the pollen tube and fertilization is still possible, even if the pollen grain itself has not been sucked in as far as the nucellus.
Fertilization is no longer done by spermatozoids. The pollen tube no longer "roots" in the nucellus as in Ginkgo and the cycads, but grows through the nucellus directly to one of the several fertile archegonia. There the pollen tube opens to release the gametes at the tip. This is the first time that the male gametes are deposited from the pollen tube at their destination. Pollen tube fertilization is invented. At the same time flagellated spermatozoids become superfluous and consequently do not appear anywhere in further evolution from here on. As with Cycas and Ginkgo, the pollen gets into the micropyle with the absorbed pollination droplets, but since the pollen tube grows through the nucellus, a pollen or fertilization chamber is superfluous and is absent in all species with pollen tube fertilization. Since the pollen tube grows directly towards the egg cell, the second sperm cell of the pollen tube has no chance of fertilization and degenerates. Other archegonia within the same ovule can only be fertilized by sperm cells from other pollen tubes.
Some conifers no longer form large, spherical pollination droplets. The pollen is caught by scar-like, receptive tissue that is formed by the integument in the area of the micropyle (Larix). In some rare cases (araucaria), a receptive tissue is formed from the cover scales. This receptive tissue is functionally identical to the stigma of the flowering plants. In order to be homologous to the stigma of the flowering plants, however, it would have to be formed by the seed scale, which in the araucarias is reduced to a small bulge.
In many conifers, the formation of the macroprothallium only occurs after pollination. In this way, the energy required for the formation of the nutrient tissue is saved in the case of non-pollinated ovules.
With the gymnosperms, the generation change has achieved a development that is independent of free water and in principle enables the colonization of all non-aquatic habitats and any size of individuals. Further improvements are therefore primarily aimed at improving the success of the reproduction or minimizing the use of materials for reproduction. To this end, paths are taken further in the evolution of angiosperms, which are already beginning to be recognizable in gymnosperms.
The basic type (normal type or polygonum type according to the group on which it was first examined) is shown in Fig. 32. Macrosporophylls and microsporophylls (carpels and stamens or groups of stamens) are usually grouped together in hermaphroditic flowers (Fig. 32 A). The ovules develop on the edges of the carpels. At first they are still upright or only slightly curved and the nucellus (macrosporangium) is not yet completely enclosed by the two integuments (Fig. 32B). In the course of further development, the curvature takes place in the anatropic form. Inside the macrosporangium, a single cell differentiates into the macrospore mother cell and undergoes meiosis (Fig. 32B-D). In angiosperms, the macrospore mother cell is either located directly under the epidermis of the nucellus and is thus covered by only one layer of nucellus tissue (tenuinucellate ovule), or is sunk several cell layers deep into the nucellus (crassinucellate ovule). The macrospore normally (normal type = polygonum type) undergoes three core divisions (Fig.32E-G). The resulting 8 nuclei form 7 cells (Fig.32H Fig.?). The largest of these is the binuclear macroprothallium, which appears as a glassy, translucent sac in the light microscope image and is called an embryo sac cell because the first steps in the development of the new embryo take place within this area. The other 6 cells form two groups of three that lie entirely within the wall of the embryo sac and thus within the macrospore wall. Both groups can be viewed as an archegonium with an egg cell reduced to two cells. Of these two groups, however, only the group below the micropyle represents a functioning, reduced archegonium with an egg cell. The other group is at the opposite end of the embryo sac and is sterile. Because of this location, these three cells are called antipodes. The two lateral cells of the functional archegonium are called synergids because they have assumed a supporting role in fertilization. The embryo sac cell together with all the cells in it is called an embryo sac. The cells within the embryo sac remain without cell walls. In this state, fertilization takes place through the pollen tube, which penetrates through the micropyle to the embryo sac.
A number of primary archesporal cells develop in the pollen sacs, from which pollen mother cells emerge through further divisions (Fig. 32b). These spheres against each other and at the time of meiosis (Fig. 32d, e) are surrounded by a callose shell, on which they can be easily distinguished from other cells of the anther in preparations. After the formation of the spore wall, the formation of the microprothallium begins within the sporangium with a first division into a vegetative and a generative cell. The generative cell is initially still in contact with the sensor wall, but soon comes to lie within the vegetative cell. The division of the generative cell into two sperm cells can take place, depending on the species, either in the sporangium or, as shown in the figure, only on the scar. Here, too, the ripe pollen grain is a two- or three-cell, flying prothallium.
The pollen grain only germinates on the scar. A single-cell, unbranched thread is formed, the pollen tube, which grows along the scar and further over the pollen tube tissue formed by the carpel edge on the placenta to the ovules. Typically, the pollen tube penetrates the ovule through the micropyle (porogamous fertilization). After the pollen tube has penetrated the embryo sac, the pollen tube opens and one generative cell fuses with the egg cell, the other with the embryo sac cell. In the egg cell the two nuclei fuse to form the zygote, in the embryo sac the two nuclei present there and the second generative nucleus fuse to form a triploid nucleus, the secondary embryo sac nucleus, from which the secondary, triploid endosperm (primary endosperm see p. 96) develops.
The further development of the endosperm begins with a phase of free nuclear divisions (Fig. 32L, nuclear endosperm formation; for other forms of endosperm formation see Chapter 1). Later, in all angiosperms, the endosperm is completely subdivided into mononuclear cells (Fig. 32M). The development of the embryo involves the formation of a multicellular, but single-row suspensor that ends in an enlarged, mononuclear basal cell (Fig. 32N). Suspensor and basal cell can no longer be recognized on the embryo that is ready for germination (Fig. 32O). After the seeds have spread and germinated, a new sporophyte develops (Fig. 32, Fig. O). In the gymnosperms it is necessary that all scales develop evenly within a cone so that the protection of the seeds by the cone is guaranteed. So investments must also be made in seed scales that do not have any fertilized ovules. This disadvantage is overcome with angiosperms. Perhaps the most crucial step in the evolution of angiosperms was the development of a common pollen collecting tissue for several ovules.
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