A handbook of systematic botany

by Eugenius Warming

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.