[Chap. XLVII THE ALGAE 617 



cytoplasm open on one side. It appears to be pressed against the cell wall 

 and is frequently confined to the middle section of the cylindrical cell. 

 Its form in cross section resembles that of a horseshoe. Starches and 

 proteins accumulate in vegetative cells, but in the spores one finds mainly 

 oils. The life history of a Ulothrix is far more complex than that of 

 Pleurococciis. 



After vegetative development the protoplast in each of many cells may 

 become subdivided by a succession of mitotic divisions into 2, 4, or 8 

 small protoplasts, each of which escapes through an opening in the wall 

 of the parent cell and becomes a motile spore^ with four flagella. Both 

 big and little motile spores may be formed in different cells of the same 

 filament. The smaller ones are formed in groups of 16 or 32. After swim- 

 ming about for a short period, a motile spore may become attached to 

 some object and germinate by elongating and then dividing transversely. 

 Subsequent cell divisions all in the same plane result in a new filament. 



In still other filaments, or occasionally in some cells of the same fila- 

 ment, the protoplast may become subdivided by successive mitotic 

 divisions into 8, 16, 32, 64, 128, or rarely more cells. Each of these cells 

 may pass through the wall of the parent cell in the same manner as the 

 motile spore, which it resembles superficially except for its smaller size 

 and its single pair of flagella. Such motile structures do not germinate, 

 and new filaments do not develop from them. They are termed gametes. 

 One gamete unites with another gamete forming a zvgote, which eventu- 

 ally becomes thick-walled and sinks into the mud at the bottom of the 

 pond where it remains dormant for several months. The union of the 

 two gametes is termed conjugation. When the zvgote germinates, its 

 protoplast becomes subdivided into 4 to 16 parts, each of which becomes 

 a motile spore from which a new filament grows. The two gametes that 

 unite appear to differ chemically and are referred to as + and — 

 gametes. Since they are similar in appearance they are regarded as the 

 simplest kind of sex cells. 



The special method of vegetative multiplication by means of either 

 motile or non-motile spores which are formed without a previous union 

 of gametes is often termed asexual reproduction. The related series of 

 processes including the fomiation of gametes, their subsequent union, 

 and the development of the resulting zygote into motile spores from 

 which new filaments develop are referred to as the sexual reproduction 



^ Motile spores have flagella and are frequently termed "zoospores" ( animal spores ) be- 

 cause they swim about in the water. 



