248 PART III. THE CLASSIFICATION OF PLANTS. 



Chroolepus, and some others) ; the gametangia are but little, if at all, differen- 

 tiated from the vegetative cells of the body ; in Chroolepus the gametes closely 

 resemble the zoogonidia. 



The germination of the zygospore is only known in Phaeophila ; the contents 

 escape as a non-motile cell which grows out basally into a root-like organ of 

 attachment, and apically into the multicellular shoot-filament. 



Principal genera : Chsetophora, Stigeoclonium, Draparnaldia in fresh water ; 

 Chroolepus (Trentepohlia) grows on rocks and tree-trunks, and is peculiar on 

 account of its orange or violet colour. 



Order 4. Ulvaceae. The membranous body consists of a single flat layer of 

 cells (Monostroma), or of a single tubular layer of cells enclosing a cavity 

 (Euteromorpha), or of two layers in close contact (Ulva) ; the body is attached, 

 at least when young, by a root, and is sometimes branched (esp. Entero- 

 morpha); the growth of the body is intercalary, all the cells being concerned 

 in it. 



Any cell of the body may become a gonidangium or a gametangium ; the 

 zoogonidia have four cilia, the planogametes two ; conjugation of planogametes 

 has been observed in the three above-mentioned genera ; the zygospore, on 

 germination, developes directly into a new plant, producing basally the root and 

 distally a cellular filament which becomes the thalloid shoot. Inhabit both 

 fresh and salt water. 



Order 5. QEdogoniacese. Filaments nnbranched (except Bulbochaete), 

 attached by a root ; growth intercalary. The mode of growth of the individual 

 cells of (Edogonium is peculiar ; in the plane of division a ring of cellulose is 

 formed round the cell-wall ; the cell-wall then ruptures, and the cellulose-rins; 

 is stretched so as to form a membrane across the rent ; as this process takes 

 place repeatedly near the upper end of the cell, the projecting edges of the re- 

 peatedly ruptured cell-wall form a series of caps ; the transverse septa, dividing 

 the elongated cells into two, are always formed toward the lower end of the cells. 



Any cell of the body may be a gonidangium, setting free its protoplasmic 

 contents as a single zoogonidium with a circlet of cilia round its more pointed 

 colourless end. On coming to rest, the zoogonidium attaches itself by its 

 colourless end, surrounds itself with a cell-wall, and grows into a filament ; the 

 colourless portion becomes the root-cell (see Fig. 74). 



The sexual organs are differentiated. Some cells of a filament increase in 

 size and become rounded in form, each constituting an oogonium. The proto- 

 plasm in each oogonium contracts away from the wall to constitute the single 

 oosphere. Access to the oosphere is afforded either by the perforation of the 

 oogonium- wall, or by the partial breaking-away of the cell immediately above 

 the oogonium in the filament. The oosphere has a well-marked receptive spot. 

 The antheridia are formed, either in the same or another filament as the species 

 is monoecious or dioecious, by the repeated tranverse division of a cell of the 

 filament ; in some species the antheridium gives rise to a single spermatozoid, 

 but in most it undergoes division into two cells each of which produces a sper- 

 matozoid. The spermatozoids resemble the zoogonidia, but are smaller and 

 are yellow instead of green ; they are set free, and finding their way to the 

 oogonia, one enters an oogonium and fertilises the oosphere, penetrating into it 

 at the receptive spot (Fig. 178). 



