322 A TEXTBOOK OF GENERAL BOTANY 
slow-running streams, ponds, etc. The vegetative plant consists 
of a row of cells which are similar except that the lower one 
serves to attach the plant to the substratum. Each cell contains 
a single nucleus and a single large chloroplastid (Fig. 327). 
Reproduction. Asexual reproduction takes place by means of 
zoospores with two or four cilia (Fig. 828). Each zodspore has 

Fig. 326. Protococcus, a green alga which is fFyq.327. Cell from fila- 
common as a green coating on tree trunks and ment of Ulothrix 
stone walls Note that each cell con- 
Protococcus consists of either a single cell or a tains a single nucleus and 
colony of cells. Each cell contains a nucleus and a band-shaped chloroplas- 
a single chloroplastid. (x 2470) tid. (x 462) 
a red eyespot and a chloroplastid. The zodspores are formed in 
ordinary vegetative cells by the division of the protoplasm into 
a number of separate parts which acquire the characters of 
zoospores. The zodspores escape through an opening in the 
cell wall and, after swimming for a time, come to rest and 
grow into new plants (Fig. 328). 
Sexual reproduction is by means of gametes. These are pro- 
duced in the same way as the zodspores and in structure are 
like the zoospores except that the gametes never possess more 
