THALLOPHYTES 25 



indicated by P=J>o F~>o P, etc., which indicates 

 that the plant produces two gametes which fuse to form an 

 oospore which produces another plant, and so on. It must 

 be remembered that in plants that produce sexual cells, all 

 three ways of producing new plants are found, so that a real 

 life-history formula for such a plant would be something as 



follows : 



-P 



This simply indicates the three methods of producing new 

 plants. 



18. Differentiation of sex. At the first appearance of 

 sex, the gametes are alike in form and behavior, as in Ulothrix 

 (Fig. 7, C, d}. They are approximately the same in size, 

 and are both swimming cells with the same arrangement of 

 cilia, so that there is no visible sex-distinction. Plants with 

 such gametes are sometimes called " unisexual plants," 

 which means plants having only one sex. The phrase is 

 misleading, for to have sex at all, there must be two sexes. 

 What the phrase really means is that the sexes cannot be 

 distinguished. 



In other plants, however, the pairing gametes begin to 

 show differences, one being larger than the other and cor- 

 respondingly less active, until finally one is relatively very 

 large and entirely passive, while the other retains its small 

 size and activity. The increased size of one of the gametes 

 means an increased nutritive power, but this gain has been 

 accompanied by a loss of swimming power. This develop- 

 ment of obvious differences between the pairing gametes 

 is the differentiation of sex, whereby the two sexes become 

 apparent. The large and passive gamete is female, and is 

 called the egg; while the small and active gamete is male, 

 and is called the sperm. For example, the illustration of 

 (Edogonium (Fig. 15, B) shows a large egg (packed full of 



