Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/445

Rh VEGETABLE.] REPRODUCTION 427 individual is constituted, it is clear that the protoplasm of plants is imperfectly differentiated physiologically. Never- theless all plants produce cells to which the work of repro- duction is especially assigned. It is of interest to recall the fact that a suppression of spore-formation, either asexual or sexual, may occur, and vegetative multiplication be reverted to, as in aposporous and some apogamous plants. It has been shown above that the reproductive cells of plants are of two kinds those, namely, which are indivi- dually capable, and those which are individually incapable, of giving rise to a new organism ; the former are the
 * .e:nl asexual, the latter the sexual reproductive cells. It has

'! also been indicated that the latter are to be regarded as derivatives of the former, a point which may now be xia ] f somewhat more fully established. It was pointed out, namely, that the gametes of Ulothrix will, if they fail to conjugate, germinate independently; the sexual differen- tiation of these gametes is clearly imperfect, and they differ but little from asexual zoospores. The same thing has been observed in Botrydium, and this is a specially interesting case inasmuch as it throws some light upon the conditions which determine sexual differentiation of the reproductive cells in these lowly organized plants. It has been ascertained that the nature of the cells produced from the resting-spore, in the manner described above, depends upon the age of the spore producing them : when the spore is young, the cells produced by it are sexual gametes ; if they fail to conjugate they perish ; when the spore is old, the cells produced by it are entirely asexual zoospores ; they never conjugate, but each by itself gives rise to a new indi- vidual. The imperfect sexual differentiation of the gametes has also been observed in Ectocarpus ; if they fail to con- jugate they germinate independently. The occurrence of this in Ectocarpus is rather surprising when it is remem- bered that the gametes of this plant are to some extent sexually differentiated as male and female (see above). From these cases in which the typically sexual repro- ductive cells still possess the properties of asexual spores we pass to others, like Acetabularia, in which they have entirely lost these properties. The planogametes of this plant are definitely sexual ; but they are quite similar, as far as external appearance goes, to each other ; there is no perceptible distinction between male and female cells. This is the case also in the Mesocarpeae and the Desmidiese among the Conjugate ; here the non-ciliated conjugating masses of protoplasm (aplanogametes) are externally similar Dn-en- and take an equal part in the sexual process. In Cutleria ^ the planogametes, and in the Zygnemeae the aplanogametes, to ile i ye indications of further sexual differentiations ; in Cut- 3e,. leria the female gamete is much larger than the male and comes sooner to rest ; in the Zygnemese the one aplanoga- mete passes over into the cell producing the other aplano- gamete and fuses with it ; the former is to be regarded as male, the latter as female. Finally, in the oosporous Algas, in the Muscineae, and in the Pteridophyta the two cells are quite distinct in form, size, and behaviour ; the male cell (antherozoid) alone retains the character of a planogamete, the female (oosphere) is non-motile and is many times larger than the antherozoid. In this series the gradual differentiation of the highly differentiated sexual cells from asexual cells can be clearly traced. If the sexual reproductive cells are to be traced back to lived af:<3xua l s P re s, then the organs which produce the sexual fi a reproductive cells are also to be forced back to those which produce the asexual spores, namely, the sporangia; the a 'a. most highly differentiated sexual organ the antheridium, the oogonium, the archegonium, the carpogonium is derived from the sporangium. The question now arises as to the nature of the differ- ence between sexual and asexual reproductive cells. It would appear that the former are in some way incomplete, that something is lacking to them which the latter possess, and that this lack is supplied in the sexual process. In many cases facts have been observed in connexion with the development of the sexual cells which indicate that they are thus incomplete. In Acetabularia the whole of Difference the protoplasm of the gametangium is not used up in the between formation of the gametes, and- in the Peronosporeae only a !^ ual portion of the protoplasm of the oogonium forms the asexual oosphere ; the remainder is simply the periplasm. In cells. Vaucheria and other Algae a mass of protoplasm escapes from the oogonium when it opens. In other cases a pro- cess of cell-division has been observed to accompany the formation of the oosphere which recalls the production of the " polar bodies " in the developing eggs of animals. In the Archegoniata the central cell of the archegonium does not directly give rise to the egg, but a portion, the ventral canal-cell, is first cut off ; this takes place also in the cor- pusculum of most Gymnosperms. Similarly in the develop- ment of antherozoids, the whole of the protoplasm of the mother-cell is never used up in their formation. In the germinating microspore of most of the Heterosporous Vascular Cryptogams and of the Phanerogams a process of cell- division takes place which Strasburger interprets as the formation of a polar body. The protoplasm of the microspore undergoes division so that two cells are formed, which may be distinguished as the vegetative and the generative, the former being much smaller than the latter in the Vascular Cryptogams and in the Gymno- sperms, whereas in the Angiosperms the converse is the case ; usually the separation of the two cells is permanent, but in most Angiosperms it is transitory, the only permanent indication of the cell-division being the presence of two nuclei in the pollen -grain; in some Gymnosperms two or three more vegetative cells may be cut off from the generative cell. The antheridium is in all cases formed from the generative cell. These vegetative cells Stras- burger regards as o'f the nature of polar bodies. The nucleus of the generative cell undergoes division, to form in the Heterosporous Vascular Cryptogams the nuclei of the mother-cells of the anthero- zoids, and in the Phanerogams the nuclei which take part in the sexual process, as will be more fully described below. The assumption of the incompleteness of the sexual cells may be extended to those sexual reproductive organs which, like the procarpia of the Flor ideas, the pollinodial antheridia of the Peronosporeae, the ascogonium and pol- linodial antheridia of the sexual Ascomycetes, do not give rise to differentiated sexual cells. There is reason to believe that the sexual reproductive cells are spores which, by the loss of certain of their con- stituents, have undergone sexual differentiation, and that those sexual organs which directly take part in the sexual process without the intervention of reproductive cells are sporangia which have undergone sexual differentiation in the same way. This is finally proved by the fact that in cases in which the normal phenomena of sexual differen- tiation do not take place the reproductive cells can ger- Partheno- minate without fertilization, and the female sexual organ genesis, can produce, without fertilization, cells capable of germina- tion. These cases are examples of that form of apogamy which is known as parthenogenesis. Parthenogenesis in plants producing differentiated sexual cells has been observed in the Mucorini, the Entomophthorece, and the Saproleg- nieae among the Fungi, and in Chara crinita among the Algse. In some Mucorini (occasionally in Absidia septata, A. capillata, Muco fusiger, Sporodinia, always in Mucor tenuis) and in some Entomophthorese, namely, the conjugat- ing hyphaa remained closed, and the protoplasmic contents of each surrounds itself with a cell-wall, the cells thus pro- duced being quite similar to the normal zygospore ; these cells are termed azygospores. In the Saprolegnieas and in Chara crinita the oospheres behave like oospores and germinate in the same manner. The details of the development of these asexual sexual reproductive cells has been fully investigated by De Bary in the case of the