230 GENERAL MORPHOLOGY OF PLANTS 



branching is so close and compact as to approach tissue masses 

 in the cushion-like mats or spheres of some species of Coleoch&te 

 and Ch&tophora, or to form definite leaf -like cell plates, as in the 

 sea lettuce and the shield coleochcete. 



372. Differentiation of sexual organs. In the lower green 

 algae, the one-celled forms (Protococcoideae) and the conjugating 

 algae (Conjugatae), the gametes are usually equal in size, though 

 in some forms there is a distinct difference, the male, or sperm cell, 

 being smaller than the female, or egg cell. This is true in many 

 of the filamentous algae (Confervoideae) where both gametes are 

 motile cells. In forms like (Edogonium, and Vaucheria among 

 the siphon algae (Siphonales), definite and distinct organs are 

 developed the sex of which can be recognized. The female organ, 

 the egg case (oogonium), is large and contains a single large egg 

 with an abundance of cytoplasm to furnish nutriment to the ger- 

 minating egg. The male organ, the sperm case (antheridium), 

 is smaller in size, but develops usually a very large number of 

 small sperm cells. These escape at some little distance from the 

 egg case. Of the large number of sperms formed only a very few 

 ever reach the egg to fertilize it. The production of the large 

 number of sperms is a provision of nature to make sure that some 

 by chance will reach the egg. Since the sperm case is some dis- 

 tance from the egg case it is necessary that some means shall be 

 provided to bring the sperm to the egg. The sperms have two, 

 or several, cilia and are free swimming. 



