Part 41

=The sporangia-bearing leaves--Sporophylls.= In the Mosses the asexual generation is only represented by the sporogonium, and if the theory is correct which considers the sporogonium to be an embryo consisting of a rudimentary stem and terminal leaf, then the spores are produced on the leaves in these plants. The sporangia in the Filicinæ are situated in groups (sori) on the back or on the edge of the leaves. The number of sporangia in the sorus diminishes very greatly in the Marattiaceæ and Gleicheniaceæ (three to four in the latter, Fig. 213). In the Equisetinæ the sporangia are situated in a small number on the underside of shield-like leaves, and in Lycopodinæ, singly, in the axils of the fertile leaves, which are alike and bear either micro- or macrosporangia. In the Phanerogams there is a great difference between the stamens and carpels.

=Stamens.= In the lowest Phanerogams (Cycadeæ) there are many indications of relationship to the Ferns. The stamens are flat and broad, and have on the back many pollen-sacs (microsporangia) arranged in small groups (true sori), which even have a small “placenta,” similar to the one possessed by the Ferns, and open towards the inside by a longitudinal cleft (Fig. 241, compare Fig. 213). A section of the Coniferæ agree more closely with the Equisetaceæ, in having a few (three to eight) pollen-sacs arranged on the underside of more or less shield-like leaves (Figs. 242, 243, compare with Fig. 224 a, c, d). In the Abietaceæ the number of sporangia is diminished to two, which are placed also on the lower side (Fig. 267) of a stamen. The number of pollen-sacs (microsporangia) in the Angiosperms is nearly always four to each stamen; they are longitudinal projections which are placed in pairs on each side of the central line of the stamen, two on the edge, and the other two generally on the side which is turned inwards; the pollen-sacs generally dehisce longitudinally (quadrilocular anthers, Fig. 244). A few, for instance Orchidaceæ and Asclepiadaceæ, have only two pollen-sacs (bilocular anthers); and in others, such as Solanum and the Ericaceæ, they open by pores; in Lauraceæ and Berberidaceæ, by valves. The part of the stamen which bears the pollen-sacs is termed the anther. Most frequently this is supported by a stalk known as the filament.

=Carpels.= The simplest forms of carpels are found in Cycas. In this genus both the foliage and fertile leaves are pinnate, and hence present great similarity; the ovules (macrosporangia) are situated on the margin of the central portion, just as the sporangia are placed on the edge of the fertile leaf of Ophioglossum (Fig. 245, compare with Fig. 209). The carpels of the other Cycadeæ present greater divergence from the foliage-leaves, being peltate, for instance, in Zamia and Ceratozamia (Fig. 246). The ovules in the Coniferæ are situated on the upper side and near the base of the ovuliferous scales, almost in the same position as the sporangia in the Lycopodinæ (Figs. 269, 272, 273 H, compare Figs. 230, 239). In Taxus the uninclosed ovule is placed on the apex of a shoot (Fig. 264). In all these plants the ovules are not enclosed by the carpels, that is, they are not enclosed in chambers formed by the turning in of the walls of the carpel, and hence the name Gymnospermæ is given to them. In the higher Flowering-plants, the Angiospermæ, the ovules are distinctly situated on the edge, the upper surface, or base of the carpel; but the carpel closes round the ovules which are therefore enclosed in a chamber--the ovary. In a few cases, for example in the Polygonaceæ, an ovule is situated apparently on the apex of the stem itself, as in the Yew; in other cases, as in the Primulaceæ, many ovules are apparently developed on the apex of the stem, which seems to have been specially adapted as a placenta, but it is also possible and correct in these cases to suppose that the ovules are in reality developed on the carpels. A single fully-developed carpel or a collection of carpels joined together is termed the pistil. The extremity of the carpel, which is specially developed to catch the pollen-grains and form a suitable nidus on which they may germinate, is called the stigma. The united edges of a carpel which bear the ovules are termed the ventral suture. The back of the carpel forms the dorsal suture. The Marsiliaceæ take a position among the Hydropterideæ analogous to that occupied by the Angiosperms; the sporangia are in a corresponding manner enveloped in a closed leaf.

The collection of stamens in a flower is termed the andrœcium, and all the carpels, whether individually free or united into one pistil, the gynœceum.

The =Sporangia=. The asexual generation of the Mosses is the sporogonium, in which the spores arise in tetrads from the mother-cells. The sporangia in the Filicinæ take their origin either from a single cell (Leptosporangiatæ) or, what probably may be regarded as an older stand-point, from a group of cells (Eusporangiatæ). In both cases there may be distinguished in a mature sporangium three tissues, which have different significance (Fig. 204): (1) an external layer, the sporangium-wall, most frequently composed of one layer of cells made up of cells of dissimilar structure, so that on desiccation the wall is ruptured and the sporangium opens in a definite manner; (2) an internal group of cells, consisting of the spore-mother-cells, developed from an archesporium, and which by division into four gives rise to the spores; (3) a layer of cells lying between the two already mentioned, which is dissolved before maturity. The intermediate cellular layer, which directly surrounds the spore-forming cells, is in form and contents more worthy of note than the others, and is termed the tapetum. The construction of the sporangium in the Equisetinæ and Lycopodinæ is in the main the same.

In the PHANEROGAMS the =Microsporangia= are termed =Pollen-sacs=. They take their origin from a large group of cells, which, in the Angiosperms, lie immediately beneath the epidermal cells of the anther. In the developed, but not yet mature, sporangium (pollen-sac) there are to be found: (as in the Vascular Cryptogams) (1) an internal group of mother-cells which give rise to the pollen-grains (microspores), in this case also formed in tetrads; (2) a group of cells surrounding these, of which the internal ones form a tapetal layer, similar to that in the Vascular Cryptogams; the tapetum and some of the cells surrounding it in this group, become dissolved before maturity; the more external ones, on the other hand, are provided with peculiar thickenings, and form the “fibrous” layer by the aid of which the dehiscence of the anther takes place; (3) an external layer, the epidermis, enclosing all the other layers (Fig. 247).

In some Coniferæ (Cupressus, Thuja, and several species of Juniperus) the microsporangia (pollen-sacs), which are situated on the under side of the stamen, are covered by a thin structure which seems to be a continuation of the lamina and which is supposed to be homologous with the indusium of the Ferns.

=The Ovule= in the Phanerogams arises most frequently on a projecting portion of the carpel, termed the placenta. The ovules (compare the sporangium of the Eusporangiatæ and especially the pollen-sac) take their origin from a group of cells which lies beneath the epidermis (Fig. 248 A, B). First of all a small papilla is formed, which is later on provided with a vascular bundle and becomes the funicle; this probably has the same value as the projections (“placenta”) on which the sori in the Ferns are attached. Only one =macrosporangium= (nucellus; Fig. 248 nc) is developed at the apex of the funicle. This arises by a process of cell-division exactly corresponding to that by which the pollen-sacs are formed (Fig. 248 C-E), with this difference only, that while a great many cells may be distinguished in each pollen-sac, which forms pollen-grains by tetrad-division, only a few are found in the ovule, and all these moreover are suppressed, with one single exception which developes into the =macrospore= (=embryo-sac=) without undergoing a division into tetrads. The wall of the embryo-sac, in the Gymnosperms, may be thick and divided into two layers and partly cuticularized, as in the spores of the Cryptogams which are to be set free. In the Angiosperms, on the other hand, the wall is extremely thin.

The pollen-sac thus stands in the same relation to the nucellus as the microsporangium does to the macrosporangium: in the pollen-sacs and microsporangia a number of spores arise by the tetrad-division of several mother-cells; in the nucellus and macrosporangium, a reduction of the cells already formed takes place to such an extent that the number of macrospores becomes one (Salvinia, Marsilia, Phanerogams) or four (Selaginella), or rarely a large number as in Isoëtes.

In the Ferns, as stated on page 210, etc., indusia covering the sori very often occur. Horsetails and Club-Mosses have no indusium; but in all Phanerogams cupular or sac-like structures (integuments) are found which envelop the nucellus. These develope from the upper end of the funicle (ii and ie, in Fig. 248; y and i, in Fig. 249) and enclose the nucellus on all sides as a sac, leaving only a small channel at the apex of the nucellus--the micropyle--(Fig. 249) through which the pollen-tube proceeds to the embryo-sac. The ovules of the Gymnosperms have only one integument (Figs. 251, 264, 269, 274) and the same is the case with the majority of the Sympetalæ and a few Choripetalæ; but the Monocotyledons and most of the Choripetalæ have two integuments (Fig. 249).

In shape the integuments resemble very closely the cupular indusium of the Hymenophyllaceæ, certain Cyatheaceæ (Fig. 212 E), and Salvinia (Fig. 218); that they are really homologous with these is probable, but is not proven. Some authorities regard them as structures found only in the Phanerogams.

The ovule is thus a “monangic” (i.e. reduced to 1 sporangium, the nucellus) sorus, situated on a funicle, and enclosed by one or two cupular indusia, the integuments. Some of the ovules are erect (orthotropous), others curved (campylotropous), the majority reversed (anatropous) (Fig. 249).

The nucellus is the only macrosporangium which never opens; the macrospore remains enclosed in it, and the macrosporangium remains attached to the mother-plant. It is therefore essential that the method of fertilisation which is employed should be very different from that of the Cryptogams. The pollen-grains must be transferred to the ovule, and retained either by a drop of mucilage at the micropyle (Gymnosperms) or by the stigma on the carpels (Angiosperms). Fertilisation by spermatozoids, which are freely motile in water, is abandoned in the Phanerogams.

Many other modifications, unknown in plants of more simple structure, take place, for instance, in the shoots which bear the fertile leaves; especially in the form of the stem or thalamus (hypogynous, perigynous, epigynous); in the development of the perianth which stands in intimate connection with the special means employed to effect fertilisation; with respect to the different grades of union found in the leaves; in the union of the flowers into aggregations of a higher order (inflorescences), and at the same time the production of “floral-leaves” (page 235).

=The sexual generation. The Fertilisation.=

The sexual generation in the Mosses is relatively well developed, because not only the protonema, but all the other vegetative parts of the Moss-plant, in addition to the archegonia and antheridia, belong to it. In the groups which follow, a gradual but increasing reduction of the sexual generation takes place, and at the same time an indication of sex is found in the prothallia, which finds expression in the forms of the spores themselves. In the majority of cases among the isosporous Vascular Cryptogams, the sexual generation--prothallium--is a green, leafy expansion which can sustain itself by the assimilation of carbonic acid, and by the absorption of nutriment from the soil by means of root-hairs. In some plants (Ophioglossaceæ, Lycopodium annotinum) the prothallium is a subterranean, pale, tubercular body, but in these instances it is relatively large. In the heterosporous Vascular Cryptogams and in the Phanerogams, the prothallium is much more reduced, both as regards its size, and also with respect to the number and structure of the antheridia and archegonia.

1. =The Microspores.= The PROTHALLIUM in all Vascular Cryptogams which have unequal spores, consists of a single, vegetative (barren) cell, which plays a very unimportant part in the life of the prothallium (Fig. 233 A). In Salvinia it is somewhat elongated and tubular, because it must break through the sporangium (Fig. 214); but in other cases it is very small and lenticular. In all these plants only one antheridium is formed. In Salvinia it consists of 2 cells whose walls are ruptured in order that the spermatozoids may be liberated (Fig. 214 B, C). In Marsilia, Isoëtes, and Selaginella the prothallium does not leave the spore, and consists for the most part of primordial spermatozoid-mother-cells without cell-wall, which on germination are ejected so that the spermatozoids are set free.

In the Phanerogams, the microspores have from olden times been termed pollen-grains.

In the GYMNOSPERMS the prothallium is reduced to 1, 2 or 3 small cells, placed on one side of the mature pollen-grain (at the top in Fig. 250 I, II, and in Fig. 267 N) and which do not play any part in the germination of the pollen-grain. The antheridium is represented by the remaining portions of the interior of the pollen-grain, that is, it consists of a large cell with a nucleus which does not even go so far as the antheridium of Selaginella and become divided into spermatozoid-mother-cells without cell-wall, for even these cells are not formed. The unicellular antheridium grows, on the germination of the pollen-grain, into a tubular body known as the pollen-tube, formed from the inner wall of the pollen-grain (Fig. 250), which works its way down the micropyle to the oosphere. The fertilisation takes place by diosmosis through the cell-wall, and consists here also of the coalescence of the nucleus of the pollen-tube (the sperm-nucleus, male pronucleus) with that of the oosphere.

In the ANGIOSPERMS the reductions proceed still further. The barren cell, which represents the prothallium, was in the last group separated from the antheridium by a true cell-wall, but in the Angiosperms a membrane at most, but no firm cell-wall, is formed. The pollen-grain contains two cells, a vegetative and a free generative cell. Both these pass into the pollen-tube, but the vegetative cell disappears about the time the pollen-tube reaches the ovule; while the generative cell divides into two: one, the sperm-nucleus coalescing with the nucleus of the oosphere, the other being absorbed (Lilium, after Guinard).

The Gymnosperms prove in yet another point that they are more closely related to the Cryptogams than are the Angiosperms. When the pollen-grain begins to germinate the external wall ruptures as in the Cryptogams (Fig. 250), but in the Angiosperms special germ-pores are formed in the cell-wall for the emergence of the pollen-tube.

2. =The Macrospores.= The prothallium in Salvinia and Marsilia is still rather large, green, and capable of the independent assimilation of carbon. It projects more or less from the macrospore and bears (in Marsilia only one, in Salvinia several) archegonia, which however are embedded to a greater degree in the prothallium, and are more reduced than the archegonia of the true Ferns and Horsetails (Figs. 215, 216). The prothallium is still more reduced in Isoëtes and Selaginella; partly because it is smaller and is in a higher degree enclosed in the spore, it also contains less chlorophyll, or is entirely without chlorophyll, and in consequence incapable of independent existence, whilst the number of archegonia is less; and partly because the archegonia are themselves reduced, the cells of the neck are fewer and embedded to the level of the surface of the prothallium without any, or with only a very slight projection (Figs. 235, 236).--Finally, the prothallium with its archegonia begins to develope in Selaginella while the macrospore is still within its sporangium, and before it is set free from the mother-plant. After the spores are set free and germination has commenced, the spore-wall ruptures and the prothallium is exposed.

The GYMNOSPERMS go still further. The macrospore (embryo-sac) germinates and forms internally a cellular tissue, designated in former times by the name of albumen (endosperm), which is homologous with the prothallium. It always remains enclosed in the embryo-sac, and is a parenchymatous mass containing a large supply of nourishment. In the upper part of the endosperm a number of archegonia are developed which are in the main constructed in the same manner as those in the Cryptogams, but are still more reduced, the neck consisting only of 4, 2, or 1 cell (Figs. 251, 252). The ventral canal-cell is also formed, in the majority, as a small portion cut off from the large central cell just beneath the neck; the larger remaining portion becomes the oosphere. When the pollen-tube has passed down to the oosphere (Fig. 253) and fertilisation has been effected, the oospore commences a cell-formation, the final result of which is the formation of an embryo (the asexual generation) which is provided with a thinner, lower end, termed the suspensor. The embryo is forced more or less into the endosperm in which it may rest for a longer or shorter time, and generally is developed to such an extent that it has a distinct primary-root (radicle) and stem (plumule) with one or more embryo-leaves (cotyledons).

When the oosphere has been fertilised its nucleus sinks down to its lower end, and by repeated division into two, forms four cells lying in one plane (Fig. 253, see base of the left archegonium). Three tiers of cells are now formed by transverse division of these four. It is the intermediate one of these which elongates and forms the suspensor, or four suspensors, if they separate from each other, which push the lowermost four cells deep down into the endosperm. It is from these four lower cells that the embryo (or four embryos when the suspensors separate) is developed, but never more than one embryo attains full development. As several archegonia are contained in one and the same ovule, all of which are capable of forming embryos, there is the possibility that several embryos may be developed in a seed (polyembryony), but usually only one embryo attains perfect development.

At the same time that the embryo is being developed, other changes are taking place in the ovule, especially in the integument which becomes the shell of the seed (testa). The endosperm grows, and the embryo-sac supplants the cells of the nucellus. The seed is now formed, and it consists in its most complete development, as in this instance, of three parts:

(1) The testa of the seed, formed by the enveloping integuments, with the remainder of the tissue of the nucellus lying outside the embryo-sac (the macrosporangium).

(2) The endosperm or prothallium.

(3) The embryo.