Page:EB1911 - Volume 01.djvu/628

 and variety of form of the chlorophyll-bodies. In Ulva and Mesocarpus the chromatophore is a single plate, which in the latter genus places its edge towards the incident light; in Spirogyra they are spiral bands embedded in the primordial utricle; in Zygnema they are a pair of stellate masses, the rays of which branch peripherally; in Oedogonium they are longitudinally-disposed anastomosing bands; in Desmids plates with irregular margins; in Cladophora polyhedral plates; in Vaucheria minute elliptical bodies occurring in immense numbers. Embedded in the chromatophore, much in the same way as the nucleus is embedded in the cytoplasm, are the pyrenoids. Unknown in Cyanophyceae and Phoeophyceae, known only in Bangiaceae and Nemalion among Rhodophyceae, they are of frequent occurrence among Chlorophyceae, excepting Characeae. Sometimes several pyrenoids occur in each chloroplast, as in Mesocarpus and Spirogyra; sometimes only an occasional chloroplast contains pyrenoid at all, as in Cladophora. The pyrenoid seems to be of proteid nature and gelatinous consistency, and to arise as a new formation or by division of pre-existing pyrenoids. When carbon-assimilation is active, starch-granules crowd upon the surface of the pyrenoid and completely obscure it from view.

Special provision for vegetative multiplication is not common among Chlorophyceae. Valonia and Caulerpa among Siphonales detach portions of their thallus, which are capable of independent growth. In Caulerpa no other means of multiplication is as yet known. In Characeae no fewer than four methods of vegetative reproduction have been described, and the facility with which buds and branches are in these cases detached has been adduced as an evidence of affinity with Bryophyta, which, as a class, are distinguished by their ready resort to vegetative reproduction.

With regard to true reproduction, which is characterized by the formation of special cells, the group Euchlorophyceae is characterized by the production of zoospores (Gr. , animal,  , seed); that is to say, cells capable of motility through the agency of cilia. Such ciliary motion is known in the adult condition of the cells of Volvocaceae, but where this is not the case the reproductive cells are endowed with motility for a brief period. The zoospore is usually a pyriform mass of naked protoplasm, the beaked end of which where the cilia arise is devoid of colouring matter. A reddish-brown body, known as the eyespot, is usually situated near the limits of the hyaline portion, and in the protoplasm contractile vacuoles similar to those of lower animals have been occasionally detected. The movement of the zoospore is effected by the lashing of the cilia and is in the direction of the beak, while the zoospore slowly rotates on its long axis at the same time. Usually two cilia are present; in Botrydium and Hydrodictyon only one is present; in certain species of Cladophora four; in Dasycladus a chaplet, and in Oedogonium a ring of many cilia. The so-called zoospore of Vaucheria is a coenocyte covered over with paired cilia corresponding in position to nuclei lying below. In all other cases, zoospores are uninucleate bodies. Zoospores arise in cells of ordinary size and form termed zoosporangia. In unicellular forms (Sphaerella) the thallus becomes transformed into a zoosporangium at the reproductive stage. In the zoosporangia of Oedogonium, Tetraspora and Coleochaete the contents become transformed into a single zoospore. In most cases repeated division seems to take place, and the final number is represented by some power of two. In coenocytic forms the zoospores would seem to arise simultaneously, probably because many nuclei are already present. The escape of zoospores is effected by the degeneration of the sporangial wall (Chaetophora), or by a pore (Cladophora), a slit (Pediastrum), or a circular fracture (Oedogonium). Zoospores are of two kinds: (1) Those which come to rest and germinate to form a new plant; these are asexual and are zoospores proper. (2) Those which are unable to germinate of themselves, but fuse with another cell, the product giving rise to a new individual; these are sexual and are zoogametes (Gr. , animal, and  , husband, wife). When two similar zoogametes fuse, the process is conjugation, and the product a zygospore (Gr. , yoke). Usually, however, only one of the fusing cells is a zoogamete, the other gamete being a much larger resting cell. In such a case the zoogamete is male, is called an antherozoid or spermatozoid, and arises in an antheridium; the larger gamete is an oosphere and arises in an oogonium. The fusion is now known as fertilization and the product is an oospore. Reproduction by conjugation is also known as isogamy, by fertilization as oogamy. When zoospores come to rest, a new cell is formed and germination ensues at once. When zygospores and oospores are produced a new cell-wall is also formed, but a long period of rest ensues. All investigation goes to show that an essential part of sexual union is the fusion of the two nuclei concerned. It is interesting to know, on the authority of Oltmanns, that when the oosphere is forming in the oogonium of Vaucheria, there is a retrocession of all the included nuclei but one. That the antherozoid of Vaucheria contains a single nucleus had been inferred before.

From a comparison of those Euchlorophyceae which have been most closely investigated, it appears probable that sexual reproductive cells have in the course of evolution arisen as the result of specialization among asexual reproductive cells, and that in turn oogamous reproduction has arisen as the result of differentiation of the two conjugating cells into the smaller male gamete and the larger male gamete. It would further appear that oogamous reproduction has arisen independently in each of the three main groups of Euchlorophyceae, viz. Protococcales, Siphonales and Confervales. Thus among Volvocaceae, a family of Protococcales, while in some of the genera (Chloraster, Sphondylomorum) no sexual union has as yet been observed, in others (Pandorina, Chlorogonium, Stephanosphaera, Sphaerella) conjugation of similar gametes takes place, in others still (Phacotus, Eudorina, Volvox) the union is of the nature of fertilization. No other family of Protococcales has advanced beyond the stage of isogamous reproduction. Again, among Siphonales only one family (Vaucheriaceae) had reached the stage of oogamy, although an incipient heterogamy is said to occur in two other families (Codiaceae, Bryopsidaceae). Elsewhere among Siphonales, in those cases where reproductive cells are known, the reproduction is either isogamous or asexual. Among Confervales there is no family in which sexual reproduction&mdash;isogamy or oogamy&mdash;is not known to occur among some of the component species, and as many as four families (Cylindrocapsaceae, Sphaeropleaceae, Oedogoniaceae, Coleochaetaceae) are oogamous. On these, as well as other grounds, Confervales are regarded as having attained to the highest rank among Euchlorophyceae. Although the phenomena attending isogamous and oogamous reproduction respectively are essentially the same in all cases, slight variations in both instances appear in different families, attributable doubtless to the independent origin of the process in different groups. Thus, although isogamy consists in typical cases of a union of naked motile gametes by a fusion which begins at the beaked ends, and results in the formation of an immotile spherical zygote surrounded by a cell-wall, in Leptosira it is noticeable that the fusion begins at the blunt end; in a species of Chlamydomonas the two gametes are each included in a cell-wall before fusion; and in many cases the zygote retains for some time its motility with the double number of cilia. Again, in oogamous reproduction, while in general only one oosphere is differentiated in the oogonium, in Sphaeroplea several oospheres arise in each oogonium; and while the oospheres usually contract away from the oogonial wall, acquiring for themselves a new cell-wall after fertilization, in Coleochaete the oosphere remains throughout in contact with the oogonial wall. The oosphere is in all cases fertilized while still within the oogonium, the antherozoids being admitted by means of a pore. There is usually distinguishable upon the surface of the oosphere an area free from chlorophyll, known as the receptive spot, at which the fusion with the antherozoid takes place; and in many cases, before fertilization, a small mucilaginous mass has been observed to separate itself off from the oosphere at this point and to escape through the pore. In Coleochaete the oogonial wall is drawn out into a considerable tube, which is provided with an apical pore, and this tube has a somewhat similar appearance to the imperforate trichogyne of Florideae to be hereafter described. In certain species of Oedogonium minute male plantlets, known as dwarf males, become attached to the female plant in the neighbourhood of the oogonia, thus facilitating fertilization. Indeed the genus Oedogonium exhibits a high degree of specialization in its reproductive system, considering that its thallus has not advanced beyond the stage of an unbranched filament.

Many Euchlorophyceae are endowed with both asexual and sexual reproduction. Such are Coleochaete, Oedogonium, Cylindrocapsa, Ulothrix, Vaucheria, Volvox, &c. In others only the asexual method is yet known. When a species resorts to both methods, it is generally found that the asexual method prevails in the early part of the vegetative period and the sexual towards the close of that period. This is in consonance with the facts already mentioned that zoospores germinate forthwith, and that the sexually-produced cell or zygote enters upon a period of rest. It is known that zoogametes, which usually conjugate, may, when conjugation fails, germinate directly (Sphaerella). In rare cases the oosphere has been known to germinate without fertilization (Oedogonium, Cylindrocapsa). The germination of a zygospore or oospore is effected by the rupture of an outer cuticularized exosporium; then the cell may protrude an inner wall, the endosporium, and grow out into the new plant (Vaucheria), or the contents may break up into a first brood of zoospores. It is held that in Coleochaete a parenchyma results from the division of the oospore, from each cell of which a zoospore arises.

Reproduction is also effected among Euchlorophyceae by means of aplanospores and akinetes. Aplanospores would seem to represent zoospores arrested in their development; without reaching the stage of motility, they germinate within the sporangium. Akinetes are ordinary thallus cells, which on account of their acquisition of a thick wall are capable of surviving unfavourable conditions. Both aplanospores and akinetes may germinate with or without the formation of zoospores at the initial stage.

Among Conjugatae reproduction is effected solely by means of conjugation of what are literally aplanospores. Among those Desmidiaceae which live a free life, two plants become surrounded by a common mucilage, in which they lie either parallel (Closterium) or crosswise (Cosmarium). Gaps then appear in the apposed surfaces, usually at the isthmus; the entire protoplasts either pass out to melt into one another clear of the old walls, or partly pass out and fuse without complete detachment from the old walls. Among colonial Desmidiaceae, the break-up of the filament is a preliminary to this conjugation; otherwise the process is the same. The zygospore becomes surrounded with its own wall, consisting finally