224 



INTRODUCTION TO BOTANY 



When Spirogyra cells divide, the division wall is at right 

 angles to the length of the plant. This results in an increase 

 in the number of cells and usually in the growth in length 



of the whole plant. 

 Growth occurs so rap- 

 idly that, in a few 

 days after the plants 



are first seen in the 

 spring, they become 

 so abundant that they 

 pollute the water in 

 which they grow, and 

 it is often necessary 

 to remove these and 

 other algae, as is seen 

 later (sect. 216). 



210. The reproduc- 

 tion of Spirogyra. It 

 is possible for a sin- 

 gle plant to become 

 broken into two or 

 more pieces, when 

 each one may grow 

 into a new plant ; this 

 is vegetative repro- 

 duction and resem- 

 bles the vegetative 

 reproduction that was 

 seen in Pleurococcus. 

 But this is not the 

 usual method of repro- 

 duction in Spirogyra. 

 The cells of two plants that lie near one another may unite 

 in pairs by means of tubes growing out from the walls of both 

 of the uniting cells (fig. 175). These tubes meet and their 

 end walls are absorbed, so that there is a continuous tube 



FIG. 175. Spirogyra 



A, a vegetative cell, showing the form of the cell 

 and of the spiral chloroplast (ch), also the nucleus 

 (n) and cytoplasm (cy) ; B, beginning of conjuga- 

 tion (a and b), and tubes which failed to conjugate 

 (c and d) ; C, completed zygospores (z) 



