Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/142

Rh 126 VEGETABLE KINGDOM female cell (oospliere) is fertilized by a small motile male cell (antlierozoid), when it is said to be oogamous. The isogamous pro cess usually takes place between free-swimming ciliated cells (2)lano- gametes) ; but in the group of the Conjugates it takes place between gametes which are not set free into the water and are not ciliated (see REPRODUCTION). In those Chloropliywse, in which both sexual and asexual spore-formation occurs, there is generally a distinction of sexual and asexual forms, so that the lii e-history presents an alternation of generations. Thus, in Acetabularia the plant is the sporophyte, producing asexually non-motile spores, which on ger mination give rise to sexual reproductive cells or gametes. Each of these spores represents a sexual form or gametophyte. Again, in ColcochfKte the plant is the gametophyte, producing the sexual organs, the sporophyte being represented by a small flattened plate of cells, developed from the fertilized female cell or oospore, which never produces any sexual organs, but only spores. The Chloropliyccaz may be classified as follows : Order 1. Protococcoidcse-. Unicellular plants ; the body frequently spherical and unattached, but presenting in some forms a distinc tion of base and apex, and then it is attached by the base. Re production hy vegetative division, or by zoospores, or by a sexual process, the sexual cells being similar plauogametes. This order may be divided into two families, the Protococcaccae. and the Palmellaccai, the distinction being that in the latter the cell- walls are swollen and mucilaginous, so that the cells are held together, whereas in the former the cells are free. Vegetative multiplication by division is universal in the Palmellaccee, but is cjommonly wanting in the Protococcacefe. Order 2. Volvocincse. Unicellular or multicellular plants ; body free-swimming by means of cilia, either spherical or a flat plate. a. Isogamous forms : Pandorina, Stephanospkssra (spherical), Gonium (flattened). b. Oogamous forms : Volvox, Eudorina, Chlamydomonas (uni cellular). Order 3. IlydrodictyesR. Multicellular plants ; body unattached, a net (Hydrodictyon) or a flat plate (Pcdiastrum), formed by the combination of originally separate cells (a ccenobiuni). The sexual process is isogamous. Order 4- Sijihonete. Unseptate multinucleate plants ; body vesi cular and unbranched, or filamentous and branched, assuming most various forms, presenting distinction of base and apex, at tached by base; sometimes (Caulerpa) presenting differentiation into root, stem, and leaf. The sexual process is isogamous or oogamous. a. Isogamous forms : Codicx, Dasycladcse, Caulerpcee, Botrydiese. b. Oogamo us forms : Vaucheriaccaz. OrderS. Confervoidese. Septate multicellular plants (body uni cellular in some Desmids) ; cells uni- or multi-nucleate ; body fila mentous, branched or unbranched, sometimes presenting distinc tion of root and shoot, and then attached by the root, or a flat plate or hollow tube of cells. The sexual process is isogamous or oogamous. a. Isogamous forms- a. Gametes not free-swimming or ciliated : Conjugates (including Desmidiese,, Mcsocarpeee, Zygncmcee). p. Gametes free-swimming and ciliated. Body filamentous, unbranched : Ulothricaccse. Body filamentous, branched : Cladophoreas, Chaeto- phorefe. Body a flat or tubular layer of cells : Ulvaccte. b. Oogamous forms : body filamentous a. Sexual organs undifferentiated : Sphseroplese. /3. Sexual organs differentiated. (Edogoniese, ; body unbranched (except Bulbochsete] ; oogonium without trichogyne ; sporophyte, a single cell (oospore). Coleochietcfe ; body branched ; oogonium with tricho gyne ; sporophyte multicellular. Order 6. Characeai. Multicellular plants; body presenting dif ferentiation into leaf, stem, and root. There is no asexual produc tion of spores ; the sexual process is oogamous, the sexual organs being highly differentiated. The distinction of the genera, and to some extent also that of the orders, is rendered difficult in many cases by polymorphism. For instance, it has been ascertained that many plants which have been referred to the Protococcoidefe are simply forms of some of the Confervoidete, Siplwncx,, &c. ; and similarly some of the Ulvacete have Ulothricoid forms. Still, there can be no doubt that each of the above orders includes some autonomous forms. With regard to the affinities and phylogeny of the orders of Clilorophycesv, the Protoccoidcaz must be taken as the starting-point from which the other orders have sprung, their evolution having taken place in various directions. Beginning with the simple spherical Protococcaccas, the origin of the Folvodnese from these by cell-division can be readily traced through Chlamydomonas, the higher forms being simply motile multicellular Protococcacefe. Closely allied to the Volwcineae are the Hydrodictyete. The Siphoncx can be readily traced back to the simplest Protococcacefe through the attached Protococcacefe, such as Characium. The Cmifcrvoidcai are probably also derived from the Protococcaccie ; and a certain relationship between some members of this order the Cladopliorcx, and the Siphoneaz is indicated by the multinucleate cells of the former. Thus, Valonia, which is septate, and therefore really be longs to the Confervoidcx, closely resembles some of the Siphon c& in its general habit. The phylogeny of the Characcx is obscure, but it is probable that they have sprung from the Confervoidcw, possibly from the Cladophorcx. It is of interest to note that the large cells of the CharacctK become multinucleate. Sub-Class III. Phseophycese. The form of the body is very vari ous ; it may consist of a single cell ; when multicellular it may be filamentous and branched, or a flattened expansion, or cylindrical or vesicular, hollow or solid. It presents also various degrees of morphological differentiation : in some forms it is quite undifl eren- tiated ; in others it presents a differentiation of base and apex, and is then attached by the base ; in others it presents indications of differentiation into root, stem, and leaf. Vegetative multipli cation is common only in the lowest forms ; in the higher it occurs in some cases, and is effected by the abstriction of modified members of the parent, termed gcmmm. The existence of a sexual process has been ascertained in several forms ; but in many others further investigation is required to determine its presence or absence. In those forms in which it occurs it may be either isogamous or oogam ous ; the isogamous process may take place between free-swimming gametes, or between gametes which are not free-swimming or cili ated. The life-history of the plants of this group is imperfectly known ; but it has been ascertained that in some there is, and in others there is not, an alternation of generations. The Plifvophycete may be classified as follows : Order 1. Diatomaccee. Unicellular plants, either free, or con nected into filaments or masses by mucilage. Reproduction, vege tative by division or by means of asexually-produced spores (auxo- sporcs) ; or sexual isogamous by the fusion of non-ciliated gametes. The cell-wall is impregnated with silica. Order 2. Syngcncticfe. Body unicellular, the cells being held together by mucilage. Reproduction by division and by asexually- produced spores. The order includes the two forms, Chromophyton and Hydrurus. The former is distinguished by being unattached and by the motility of its spores, which have a single long cilium. Hydrurus grows attached, and the spores are not motile. Order 3. Ph&osporese. Multicellular plants ; the body is fila mentous and branched or flattened, always presenting differentiation of base and apex, and in some cases more or less well-marked dif ferentiation into root, stem, and leaf, usually attached by the base. Reproduction, vegetative by gemmse or by means of asexually-pro duced zoospores, or by a sexual process which is essentially iso gamous, the gametes being ciliated, but in the higher forms tend ing to become oogamous. The principal families of the Phtrosporefa are Ectocarpcee, Sphacclaricm, Mesoglceacese, Dcsmarcsticw, Scyto- siphoncx, Cutler iaccfe, and LaminarietK. The filamentous tj^pe of structure obtains in the Ectocarpcw, Sphacelariese, Mcsoglceacex, and Desmarcstiese. The filaments may consist of single rows of cells (monosiphonous], as in most Ectocarpess, or of several rows of cells (polysiphonous), as in the Mesogloeaceie ; or they may be polysiphon- ous with a more or less well-developed cortical layer, as in the Sphacclaricse, in which case there is also a large apical cell. The structure of the Dcsmaresticee (e.g., Desmarestia and Arthrocladia) is similar to that of the SphacclarieiK, but the growing point is not apical, as in the Sphacelariex, but intercalary, as in the Ecto- carpcie. In some of these forms (e.g., Cladostephus among the Sphacclarieee) there is an indication of the differentiation of the shoot into stem and leaf. The type of structure obtaining in the Scytosiphonex, CutlcriacefV, and Laminariese is that of a flattened expansion of parenchymatous tissue, though in the Laminar ica . there is a certain degree of histological differentiation, as is indicated by the presence of elongated cells forming structures resembling the sieve-tubes of the vascular plants. In some of the Scytosiphoncss there is a large internal cavity, so that the body presents a cylindri cal or vesicular form. Vegetative reproduction by gemma: is only known in the SphacclariefG. Reproduction by means of asexually- produced spores (zoospores) is known to occur in the majority of the Phieosporcx, but it has not yet been detected in some forms (e.g., Arthrocladia, Scytosiphon, Pliyllitis, Colpomcnia). The asexual organs are unilocular sporangia. The sexual organs (gamct- angia) are multilocular. They have been found in all the families except the LaminaricfK, but not yet in all the genera (e.g., Aspcro- coccus}. The gametangia are all quite similar, except in the Cut- leriaccee and in Tilopteris (a genus allied to Desmarestia}, in which also the gametes differ in appearance. A sexual process has only been observed in a few species of Ectocarpcee and Scytosiphonese (Ectocarpus siliculosus and E. pusillus, Giraudia sphacclarioidcs, Scytosiphon lomcntarius), and in the Cutleriaceie. A full account of the sexual process in these forms is given in the article REPRO DUCTION, vol. xx. p. 425. Order 4- Dictyotaceee. Body multicellular, thalloid, flattened,