THE GKEEN ALG.E (CHLOEOPHYCE^) 201 



special short branches are formed. In one species of Vaucheria 

 (V. sessilis) two kinds of branches arise near one another 

 (Fig. 163). One of these is short and irregularly spherical, 

 and has a beak at its free end. This branch forms one large 

 cell within it. The other branch is longer, somewhat coiled, 

 and has a terminal cell that is cut off by means of a cross 

 wall, which is much farther from the main plant than in the 

 other branch. In the terminal segment many small cells are 

 formed. Through a small opening in the tip of this coiled 

 branch these cells escape, some of them entering the beak of 

 the other branch, and one of them uniting with the large cell. 

 This union forms a spore which proceeds to develop a heavy 

 protecting wall. After a 

 period of rest this spore 

 germinates and produces 

 a new plant. 1 



If this spore had been 



- , , V, . ,, FIG. 164. A vegetative cell of a common 



formed by the union of gpecie s of ^ ogonium 



similar gametes, we then Greatly enlarged 



should have called it a 



zygospore ; but it is formed by the union of gametes that are 

 very unlike, one large gamete, the egg or oosphere, and the 

 other a small gamete, the sperm, and the resulting spore 

 is called an oospore, which means "egg spore." When similar 

 gametes unite to form a zygospore, the process is called conjuga- 

 tion, but when dissimilar gametes unite to form an oospore, the 

 process is called fertilization. The special sex organ which 

 produces the sperm is the antheridium, and that which pro- 

 duces the egg is the oogonium, which means the " egg case." 



Vaucheria has three methods of reproduction, vegetative, 

 by asexual spores (zoospores), and by sexual spores (oospores). 



One plant may use vegetative reproduction at one period of 

 growth, asexual spore reproduction at another, and sex spore re- 

 production at another, but two methods are rarely used at once. 



1 To THE TEACHER. No attempt is made to present the difficult and tech- 

 nical questions relative to alternation of generations in the thallophytes. 



