THE FRESH-WATER ALGAE 1 19 



through which the spores gradually press their way. In some 

 instances the membrane splits and the spores are thus liberated. 

 If the conditions be not favorable to the liberation of the spores, 

 however, they may move for a time within the mother membrane, 

 or may never come into motion at all, but may germinate immedi- 

 ately without being liberated, and become invested with membranes 

 of their own. They soon increase in size so that the zoosporangium 

 wall is broken, but they often remain adhering to each other for a 

 long time, thus forming a cluster of cells like the parent individual. 



Sexual reproduction is always preceded by fertilization. This 

 process consists in a union of two cells which may be either alike 

 or unlike, and are known as gametes If the cells are alike they 

 are called isogametes, but if unlike, heterogametes. The simplest 

 form of fertilization is seen when two isogametes unite or conjugate 

 to form a zygospore. These gametes may be two motile cells 

 resembling zoospores, as n Protosiphon, or they may be non- 

 motile cells, either distinct individuals as in the Desmids, or as 

 parts of filaments, as in Spirogyra. Frequently a sKght difference 

 in size may be detected between these two cells, and undoubtedly 

 this is a beginning of sex differentiation. In all of the higher 

 algae this differentiation has advanced much farther and a great 

 difference exists between the gametes: the female cell, the oosphere 

 or 'egg cell, being large and non-motile, while the male cell, the 

 antherozoid or spermatozoid, is endowed with independent motion. 

 Only in the Florideae does the male cell, the spermatium, lack 

 motion, and remain dependent upon the action of the water to 

 transfer it to the egg cell. 



The female organ which bears the egg cell is called the oogo- 

 nium, the male organ which bears the antherozoid is the antherid- 

 ium. The result of fertilization of an egg by an antherozoid is an 

 oospore, which is resistant to unfavorable conditions and is usually 

 dormant for a period before germination. 



The female organ of the Florideae is called the procarp. It is 

 flask-shaped and made up of two parts, the enlarged basal por- 

 tion, the carpogonium, which contains the egg cell, and a projecting 

 neck, the trichogyne, for conducting the spermatium to the egg. 

 The influence of fertiUzation is manifested by a rapid production 



