40 PLANT-LIFE 



green seaweed common in the vicinity of the high-tide 

 line. Its thallus is a branched tube, the wall being 

 composed of a single layer of cells. The class embraces 

 twelve families, but we shall confine our observations 

 to two of them the Ulotrichaceae and the (Edogoni- 

 acese. 



Ulothrix zonata occurs in ponds, but is more likely to 

 be found in fresh-water ditches and brooks. The 

 thallus in an unbranched, rather long filament composed 

 of a single row of cells, which are about xoW mcn m 

 diameter. More than 1,000 cells may be found in one 

 thread. The cells are about as long as broad. The 

 filaments are usually found attached by a colourless 

 basal cell to some object, such as a stone, in the water; 

 the attachment cell may be branched in a rootlike 

 fashion. But Ulothrix draws no food from its sub- 

 stratum, and can live free in the water; indeed, it is 

 sometimes found in floating masses. Growth is effected 

 by cross-division of the cells. Within each cell is a 

 layer of protoplasm, the primordial utricle (see p. 35), 

 containing an embedded nucleus; the bright green 

 chloroplast, with its included pyrenoids, forms a broad 

 band which, in short cells, is almost as broad as the cell 

 length, but in longer cells appears as a belt in the middle ; 

 a large vacuole, filled with sap, occupies the interior of 

 each cell. 



Reproduction in Ulothrix zonata arises either asex- 

 ually from zoospores, or sexually from gametes. Zoo- 

 spores are usually larger than gametes. They are clearly 

 distinguished by the possession of four lashes or cilia, 

 by means of which they swim; gametes have two cilia. 

 Zoospores may be formed, from one to four in number, 



