650 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



The asexual generations of Ulothrix are produced during the autumn and 

 winter, gametes being formed in spring, and the zygote resting through the summer. 

 This course of events differs from that obtaining in most Algse with a similar life- 

 history. Ulothrix seems especially adapted to life in cold water. 



Hormidium is a genus whose members grow largely on damp earth, tree trunks, 

 &c. The chromatophore is solid, with radiating processes and a central pyrenoid. 

 The cells of some species divide longitudinally, so as to form threads two cells 

 broad. 



Various other genera are common in fresh water. 



Cylindrocapsacece. — Cylindrocapsa forms unbranched threads of short cells with 

 very thick walls, each thread being fixed in the young state by a cellulose foot. 

 The gametes show a considerable sexual differentiation. The males are pear-shaped, 

 elongated, yellow, with two flagella at the anterior end. They are produced two in 

 an antheridium, which is formed by the division of an ordinary cell into two or 

 four. The protoplasm of an ordinary cell rounds itself off directly to become an 

 egg, the wall swelling and bursting at one side to allow of the entrance of the 

 spermatozoids. Parthenogenesis also occurs. 



(EdogoniacecB. — (Edogoniwm has a thallus consisting of an unbranched thread, 

 of rather long cells, of which the basal one is fixed to some solid object. The 

 chromatophore often forms a continuous parietal layer containing several pyrenoids. 

 All the cells, except the basal one, are capable of division. When division is going 

 to occur a rim of cellulose is formed inside the cell close to the upper transverse 

 wall. After the nucleus has divided, and the new transverse wall is formed, the rim 

 is opened, as it were, by a circular cut from without, and the tension of the cell 

 causes a pulling-out of the substance of the rim. The result is the intercalation 

 of a new cylindrical piece of cell -wall in the upper daughter-cell. The young 

 transverse wall now moves up to the lower edge of the intercalated piece of mem- 

 brane. The latter soon acquires the ordinary thickness of a side wall, but the 

 segment of the old cell-wall above the spot at which the rim was formed remains 

 projecting beyond the new piece like the eaves of a house beyond its side walls. 

 After the cell has divided again, another projecting piece will be left in the same 

 way, and the series of eave-like projections so formed are a very characteristic 

 feature of the cells of an CBdog'onmm-thread. The zoospores of (Edogonium are 

 formed singly in the cells of a thread. The entire cell-body, with the exception of 

 the ectoplasm, rounds itself off and escapes from the cell by a split in the wall. 

 An anterior circlet of ciUa surrounds the colourless "mouth place," of the zoospore. 

 In germination, the zoospore fixes itself by the mouth place, sending out short fixing 

 processes, puts on a cell-membrane, and then grows out to form a new thread. 



The gametes of (Edogonium are sexually differentiated. 



The oogonium is formed by the swelling-up of the uppermost daughter-cell after 

 a division. The contents round off to form a single large oosphere. Either a round 

 hole appears in the wall, or a circular split is formed at the upper end of the 

 oogonium, the part of the filament above rotating through a few degrees so as to 



