228 



INTRODUCTION TO BOTANY 



Another kind of reproduction may occur at the same time 

 that zoospores are being formed, though it usually occurs at 

 other times. Upon the sides of the plant special short branches 

 are formed. Two kinds of branches arise near one another 

 (fig. 178). One is short and irregularly spherical, and has a 

 beak. 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. In the terminal segment 

 many small cells are formed. Through a small opening in 



the tip of this coiled 

 branch these cells es- 

 cape, some of them 

 entering the beak of 

 the other branch and 

 -^ one of them uniting 

 with the large cell. 

 ?'-^ This union forms a 

 tures 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 formed by the union of similar gam- 

 etes, as in Spirogyra, we should have called it a zygospore; 

 but it is formed by the union of gametes that are very unlike, 

 one large gamete (the eyy, 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 conjugation, 

 but when dissimilar gametes unite to form an oospore, the 

 process is called fertilisation. The special sex organ which 

 produces the sperm is the antheridium, or sperm case, and that 

 which produces the egg is the oogonium, or egg case. 



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

 nical questions relative to the alternation of generations in the thallophytes. 



FIG. 178. The sexual reproductive strv 

 of Vaucheria (V. sessilis) 



o, oogonia; A, antheridium. Note the opening in 



the antheridium for exit of sperms, and in the 



oogonia for their entrance to the large eggs. 



Greatly enlarged 



