REPRODUCTION IN PLANTS AND ANIMALS 365 



joined end to end. Sometimes a threadlike plant is broken 

 into two pieces, when each piece may continue to grow 

 as a new individual. 

 This process is vege- 

 tative reproduction. 



Sometimes two of 

 the pond-scum plants 

 which are near one 

 another perform quite 

 a different kind of re- 

 production (fig. 169). 

 From two cells, one 

 in each plant, there 

 grow two tubular out- 

 growths. These grow 

 toward one another 

 and unite their tips 

 in such a way as to 

 form a continuous 

 tubular passage be- cy- 

 tween the cells. When 

 this has been done 

 all the living sub- 

 stance or protoplasm 

 from one cell passes 

 through the tube into 

 the other cell. The 

 two masses of proto- 

 plasm unite, and the 

 mass thus formed be- 

 comes very compact 

 and a heavy wall 

 develops closely about it. The old cell walls decay, and the 

 new spherical or oval cell goes to the bottom and passes 

 through a resting period which may last through the winter 



FIG. 109. A pond scum 



A, two vegetative cells showing the form of the 

 cell and of the spiral chlorophyll band (c/i), also 

 the nuclei of the cells (), and the granular part 

 of the protoplasm (c?/) ; B, cells showing the be- 

 ginning of the union of the tubes (a and b), 

 and tubes which have failed to unite (c and d) ; 

 C, completed spores (2) 



