GROUP i. THALLOPHYTA: ALGLE. 257 



and bearing four cilia and a pigment-spot : the planogametes resemble the 

 zoospores but are proportionately smaller and have only two cilia. When 

 the zoospores come to rest, they secrete a cell-wall, and become attached by 

 the colourless end which forms the root-cell of the developing filament. 

 The planogametes conjugate to form zygospores, but if they fail to con- 

 jugate they may germinate independently, and they do so in the same 

 manner as the zoospores, only the resulting filament is smaller. The 

 zygospore grows and attaches itself by its hyaline portion which de- 

 velopes into a root ; after a period of quiescence its contents divide and 

 are set free as 2-8 zoospores Fresh water and marine. Principal genera : 

 Ulothrix, Conferva. 



Order 3. Ulvaceae. The membranous body consists of a single flat 

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

 a cavity (Enteromorpha). or of two layers in close contact (Ulva) ; the 

 body is attached, at least when young, by a root, and is sometimes 

 branched (esp. Enteromorpha) ; the growth of the body is intercalary, all 

 the cells being concerned in it. 



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

 zoospores have four cilia, the planogametes two ; conjugation of piano- 

 gametes has been observed in the three above-mentioned genera; the 

 zygospore, on germination, develops 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 4. QEdogoniaceae. Filaments unbranched (except Bulbochsete), 

 attached by a root ; growth intercalai - y. 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-ring 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 repeatedly 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 zoosporangium, setting free its proto- 

 plasmic contents as a single zoospore with a circlet of cilia round its more 

 pointed colourless end. On coming to rest, the zoospore 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. 62). 



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

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

 protoplasm in each oogonium contracts away from the wall to constitute 

 the single oosphere. Access to the oosphere is afforded either by the per- 

 foration 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 transverse 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 spermatozoid. The sper- 



M.B. S 



