248 



BIOLOGY 



each other, as shown at C. This fused mass is called a zygo- 

 spore (Gr. zygon = yoke + spora), z. It enlarges, becomes 

 covered with a hard case, D, and breaks away from the plant 

 that produced it. It may then remain dormant for a long time, 

 but eventually it sprouts, E, and grows into a new plant. 

 In this case the two cells that unite are, so far as the micro- 

 scope discloses, alike, and the plants that produce them appear 

 identical. But careful study has proved that there is a differ- 

 ence in the uniting plants, shown not in their shape, but in 

 their uniting powers. It has been found that there are two 



D E 



FIG. 117. CONJUGATION OF MUCOR 



Successive stages being shown in A toE; x and y are threads from different plants, which 

 unite by conjugation; z, the zygospore; at E the zygospore has sprouted to form a new 

 plant. 



types of Mucor, differing only in their power of uniting with 

 each other. For example, in Figure A, the two different my- 

 celium threads are marked x and y. It is found that while 

 outgrowths of x can unite with outgrowths of ?/, they can 

 never unite with other outgrowths of the mycelium x. There 

 are thus two different types of plants, each capable of uniting 

 with the other, but not capable of uniting with outgrowths from 

 itself. This reminds us of sex union, where an egg will unite 

 with a sperm but not with another egg. It cannot be called 

 true sex, however, since there are no distinguishable differences 



