Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/443

Rh VEGETABLE.] REPRODUCTION 425 fcn- i of u 01 a. The protophytic Algee are reproduced by asexually developed spores, but in some forms an indication is already given of the differentiation of these spores into sexual reproductive cells which takes place in the higher forms of the group. In Protococcus, for instance, zoospores are produced, but the zoospores are not all precisely similar. In some cases the protoplasm of the cell divides only once or twice, the result being the formation of two or four relatively large zoospores, macrozoospores ; in other cases the protoplasm divides a greater number of times so that a considerable number of relatively small zoospores, microzoospores, are pro- duced. Functionally these zoospores are alike ; they all come to rest, and form new Protococci. Amongst the Confervoidae, which are more highly organized plants than the protophytic Algae, we find forms, of which Ulothrix may be taken as the type, which likewise produce macrozoospores and mierozoospores in their cells. The macrozoospores of Ulothrix simply come to rest and germinate ; they are distinctly asexual spores. The microzoospores may also do this, but not uufrequently they coalesce in pairs ; the product of fusion, the zygospore, as it is termed, then develops into a Ulothrix filament. This fusion of two similar reproductive cells this conjugation, as it is termed is one of the simplest forms of the sexual process ; the zygospore is then a sexually produced spore, and the two cells which conjugate to form it are spoken of as gametes, planogametes when they possess cilia, aplanogamctes when they do not. Comparing Ulothrix with Protococcus, we see that in both the macrozoospores are asexual reproductive cells, whereas the micro- zoospores of Ulothrix exhibit an imperfect sexuality, inasmuch as they may germinate without previous conjugation. The piano- gametes of Ulothrix are, however, to be directly connected with the microzoospores of Protococcus ; that is to say, the gametes are to be traced back to asexual spores. This is a point of fundamental importance. Similarly, in Botrydium, one of the Siphoneas, there are two kinds of zoospores, some of which are asexual and others sexual ; the visible difference between them is, in this case, not one of size, but the gametes have two cilia and the zoospores only one. In the conjugation which takes place in the above-mentioned plants the gametes are quite similar in form and size, and take an equal part in the formation of the zygospore. The first indication of the differentiation of sexual gametes is afforded by Edo- carpus siliculofius and Scytosiphon, belonging to the Phseosporeae. The zoospores of these plants are produced in well-defined sporangia, some of which are multilocular and others unilocular. Inasmuch as only the zoospores developed in the multilocular sporangia have been observed to conjugate, that is, to be gametes, those developed in the unilocular sporangia are probably asexual. This being so, the multilocular sporangia are to be regarded, not as mere sporangia, but as sexual organs (Gametangia) producing sexual reproductive cells. The process of conjugation of the gametes is, according to Berthold, as follows : the gametes are at first quite similar in every respect; some of them, however, soon withdraw their cilia and come to rest, whereas others remain actively motile ; one of the still motile gametes then coalesces with one which has come to rest to form a zygospore. The gametes in this case behave differently in the process of conjugation : the one is passive, the other active ; the former is to be regarded as the female, and the latter as the male reproductive cell. But even in this case the gametes, if they fail to conjugate, can germinate independently. In Cutleria the sexual differentiation of the reproductive cells is more marked ; the male and female cells are developed in distinct sporangia, which may be termed respectively antheridia and oogonia, two male cells being formed in each antheridium, and one female cell in each oogonium ; the female cell is considerably larger than the male, but they are both planogametes, that is, conjugating cells which swim by means of cilia ; the female cell, however, soon withdraws its cilia and comes to rest, and then conjugation takes place, with the formation of a zygospore, much in the same way as in Ectocarpus described above. The next stage in the sexual differentiation of the Algae is to be found in such forms as Volvox, Vaucheria, (Edogonium, Fucus, and the Characeae. In these plants, as in Cutleria, the reproductive cells are entirely incapable of independent germination ; they have lost altogether that characteristic property of spores ; as in Cutleria also they are developed in two kinds of sporangia antheridia and oogouia and they are very different from each other in their form, size, and behaviour. In most cases the oogbniiuu gives rise to a single relatively large cell, the oosphere, which is at no time provided with cilia, and is not set free from the oogouium ; in some of the Fucacese, there may, however, be two (Pelvetia), four (Ozothallia [Ascophyllum]), or eight (Fucus) oospheres produced in each oogouium, and in all the Fucaceae the oospheres are set free from the oogonium. The antheridium gives rise to a large number of small ciliated cells, the antherozoids, one of which subsequently fuses with the oosphere. The coalescence of two such highly differen- tiated sexual cells is termed fertilization, to distinguish it from the conjugation of similar sexual cells, and the product of fertiliza- tion is termed an oospore, to distinguish it from the product of conjugation, the zygospore. In these plants the antherozoids still retain the essential characters of planogametes, whereas the oospheres have lost them. But it is not in all Alga? that sexual reproduction is effected by Sexual the fusion of well-defined sexual cells. In the Florideae the sexual reproduc- reproductive organs are well differentiated. The male organ is an tion of antheridium which produces antherozoids ; but these antherozoids Floridese. are peculiar in that they have no cilia, and are surrounded by a cell-wall ; they are frequently, on this account, termed spermatia. The female organ is termed a procarpium or carpogonium ; it may consist of one or many cells, but in all cases it consists of a pro- jecting filament, the trichogyne, and a more expanded basal portion. It is peculiar in that no well-defined oosphere exists within it. Fertilization is effected by the antherozoid (spermatium) being passively brought into contact with the trichogyne ; complete fusion then takes place, the contents of the spermatium passing into the trichogyne ; the trichogyne now withers, and changes take place in the basal portion of the procarpium, one or more of the cells termed carpogenous cells divide, and by a process of budding give rise to a cluster of cells which are capable of germinating, and pro- duce new plants ; these are termed carpospores. At the same time the cluster of spores frequently becomes surrounded by an up- growth of tissue ; the mature fructification is termed a cystocarp. From the female organ of the Floridese there are, then, formed, in consequence of fertilization, a number of reproductive cells, the carpospores, each of which corresponds to the oospore of plants like Fucus, Vaucheria, &c. A more detailed comparison with Fucus makes this correspondence at once apparent. In Fucus the contents of the female organ (oogonium), which is a single cell, divide into eight oospheres, which are subsequently fertilized ; in the Florideae there is, before fertilization, no differentiated oosphere, but the organ, as a whole, is fertilized by the spermatium, and it is in this case after, and not before, fertilization, as in the case of Fucus, that a process of cell -formation takes place in the female organ ; hence the reproductive cells formed by the fertilized female organ of the Florideae are at once fertile, and correspond to the fertilized oospheres (oospores) of Fucus. The peculiarities of the sexual reproduction of some of the Floridese are of sufficient general interest to be mentioned here. In the Corallines, according to Solms-Laubach, the procarpia are produced several together in a conceptacle ; it is, however, only the central procarpia of the group which are capable of being fertilized, and the peripheral procarpia which produce carpospores. After the fertilization of the central procarpia, the carpogenous cells of the whole of the procarpia fuse together to form one large cell from the periphery of which the carpospores are produced by budding. This physiological division of labour is more marked in Dud- resnaya, and a few other Florideae. In these plants some of the procarpia are destitute of a trichogyne, whereas others possess that organ. The spermatia fertilize those procarpia which possess a trichogyne, but these procarpia do not produce carpospores ; but there grow out from them filaments which fertilize the procarpia destitute of a trichogyne, and these then give rise to carpospores. The development of the carpospores in the Bangiacese (Bangia, Porphyra) is peculiar. The carpogenous cell does not in this case, as in other Florideaa, produce spores by budding, but its proto- plasm divides into eight portions ; these are set free as naked masses of protoplasm, which move about for a time in an amoeboid manner and then come to rest and surround themselves with a cell- wall. In some of the higher Algse, namely, in the Sphacelarieae and in No sexual the Laminariese, families belonging to the Phasosporeas, no sexual process process has been observed as yet ; but, as our knowledge of the observed life-history of these plants is imperfect, it cannot be definitely in some stated at present that they are entirely asexual. . Algfc. In the somewhat aberrant group of the Conjugatae the sexual Con- process is peculiar. In the Desmidieae and the Mesocarpese it is jugatse. effected in this way, that two adjacent cells, belonging usually to different filaments, throw out corresponding lateral protuberances which meet, and, the intervening walls being absorbed, form a canal placing the cavities of the two cells in direct communication ; the protoplasm of each cell contracts, forming an aplanogamete, and travels into the canal, where the two masses meet and fuse. This is clearly a process of conjugation, similar to that of piano- gametes, and the product is likewise a single cell which is termed a zygospore. In the Zygnemeae, of which Spirogyra is a familiar example, the process is slightly different. Here the protoplasm of one of the two conjugating organs contracts first and passes over into the cavity of the other, there to fuse with its protoplasm. Turning now to the Fungi, we find that in the simplest forms Fungi. (Schizomycetes, Sacchaiomycetes) there is no trace of sexual reproduction, whereas in the higher forms, with some exceptions, sexual reproductive organs are present, though they are in many cases functionless. XX 54