Part 21

The asexual generation which follows from the further development of the fertilised oosphere, is, in the _Mosses_, only the sporogonium (according to one theory it is perhaps homologous with a spore-bearing leaf, situated upon a short stem, see p. 187); in _Filicinæ_, _Equisetinæ_, and _Lycopodinæ_, on the other hand, it is a highly developed plant differentiated into stem, leaf, and true root, and bearing the sporangia on its leaves. The ~MODIFICATION OF THE SHOOT~ is very slight in _Filicinæ_. The first leaves of the embryo are very simple in form (Fig. 205), but after a certain age all the leaves which arise are essentially alike. The fertile leaves do not differ from the barren ones, and are found associated with them, and their formation does not limit the growth in length of the stem. It is only in a few of the true Ferns, and in the Hydropterideæ, that the fertile leaves differ considerably from the barren ones. A division of labour in which certain leaves are set apart for nutrition, and others for reproduction, is found more pronouncedly in the _Equisetinæ_ and _Lycopodinæ_, for in these groups, with a few exceptions, the fertile and barren leaves are very dissimilar; the former are collected in special ear-like _cones_, which _terminate the further growth_ of the short stems on which they are borne. In connection with the cone, leaves are sometimes developed which form a transition from the barren to the fertile ones (the “annulus” in Equisetaceæ), and in these cases the first indication of a flower with perianth or floral-leaves is to be traced. Among the Cryptogams the highest division of labour is found in _Selaginella_ and _Isoëtes_, which have the two kinds of sporangia borne on _different_ leaves. The division of labour (modification) is, however, still more pronounced in the _Phanerogams_: the leaves which bear the microsporangia (“pollen-sacs”) have quite different forms from those which bear macrosporangia (the “nucellus” in the ovule), the former are termed _stamens_, the latter _carpels_; in certain instances, too, there is even a contrast between the “male plants” and the “female plants.” Moreover, a division of labour, in a much greater degree, takes place in the leaves which do not directly take part in reproduction, and it is thus possible in many plants to draw a sharp line not only between stamens and carpels, but also between four or five distinct kinds of leaves, which differ in _form_, _structure_, and corresponding _functions_, and which appear in regular sequence on the shoot: namely, between “scale-leaves” and “foliage-leaves,”[21] both of which occur in the Cryptogams, and the “floral-leaves,” including the bracts and leaves of the “perianth,” which latter often differ from each other in form and colour, and are then separated into _sepals_ and _petals_. The _leaves_--stamens and carpels--_which bear the sporangia_ are termed sporophylls, and the shoot, or extremity of a shoot, whose leaves are modified into sporophylls, is _terminated in its further growth by their production, and is known as a flower_. The flower which is most completely furnished has calyx, corolla, stamens, and carpels arranged in this order. If the only sporophylls present are stamens, then it is said to be a _male_ (_staminate_) flower, and if only carpels, then a _female_ (_pistillate_) flower, and in both these cases the flowers are _unisexual_, or diclinous. If stamens and carpels are both present in the same flower, it is termed _hermaphrodite_. Diclinous plants in which the female flowers are situated on one plant, and the male flowers on another, are termed _diœcious_; and those in which the same plant bears the two kinds of flowers are termed _monœcious_. When the male, female, and hermaphrodite flowers are found in the same species, the plant is said to be _polygamous_.

=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.[22] 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).

[Goebel (1884 and earlier) with Strasburger considered the entire ovule of the Phanerogams as homologous with the macrosporangium, the integuments however as new structures in contradistinction to the Ferns: the funicle then corresponds to the stalk of the sporangium. The integuments of the ovule (according to Goebel, 1882) differ from the indusium of the Fern-like plants in being developed from the basal portion of the nucellus and are not, as in the Ferns and _Isoëtes_, a portion (outgrowth) of the leaf which bears the sporangia (_K_).]

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.