94 FOUNDATIONS OF BIOLOGY 



B. GAMETE FORMATION 



In the life cycle of Sphaerella it was noted that under cer- 

 tain conditions the so-called dormant cell, instead of dividing 

 twice to form four spores, divides five or six times and forms 

 32 to 64 small cells called gametes. Now it is not the struc- 

 ture but the behavior of the gametes which particularly dis- 

 tinguishes them from spores. From the standpoint of their 

 origin, gametes may be regarded as spores which have de- 

 veloped the habit of fusing to form a zygote. Moreover, 

 the origin of gametes is the origin of SEX, so that sexuality 

 arose in plants when spores, instead of reproducing, devel- 

 oped the habit of pairing and thus became gametes. The 

 act of fusing is FERTILIZATION and the cells which unite are 

 sex cells. 



A concrete example may emphasize this important point. 

 The body of a filamentous Alga, Ulothrix, is composed of 

 a linear series of cells all of which are essentially the same in 

 structure and function. Under favorable conditions the 

 cells divide and the plant grows in length. New individuals 

 are not formed by this process, although the mechanical 

 breaking of the filament into two parts, owing to the sim- 

 plicity of the body, gives two individuals. When conditions 

 become less favorable for vegetative growth, some of the 

 cells cease to contribute to the elongation of the filament. 

 Instead, the protoplasts begin to divide within their cell 

 walls, and thus each forms from 2 to 64 or more spores of 

 different sizes, depending upon the number of divisions the 

 parent protoplast undergoes. (Fig. 49.) 



The largest spores are provided with four, and the smallest 

 with two, flagella by means of which they swim actively in 

 the water when discharged from the parent plant body. 

 .However, the number of flagella is apparently of no im- 



