44 COMMON POND-SCUM. 



in the relative position of the two would indicate some 

 connecting influence. The pyrenoids have been long 

 known and variously interpreted, but recent careful 

 studies show that their outer parts are converted bodily 

 into starch. Their chemical constitution is uncertain, 

 though they respond to tests for proteids. Their occur- 

 rence is not common throughout the plant kingdom. 



When we examine the reproduction of Spirogyra, we 

 find that in its details it is quite unlike any plant yet 

 studied. In it we have the sexual process much more 

 developed than in Ulothrix, inasmuch as the sex-cells, or 

 gametes, do not become free- swimming bodies, but by 

 means of a conjugating tube one travels directly to the 

 other and unites with it to form a spore. 



That these fusing cells are not always of the same 

 size is sometimes quite clear, and it has often been pointed 

 out that they are sometimes unlike, but they are so 

 essentially similar that they can hardly be distinguished 

 as male and female elements in spore formation. Hence 

 the reproduction is said to be isogamous, i.e. by similar 

 gametes. 



The zygospore formed thus, after developing a heavy 

 protecting wall, may pass through a long period of rest, 

 and then by germination may develop a new plant. 

 Evidently, therefore, Spirogyra is not only a larger and 

 more complex plant, and thereby able to do more work, 

 but has developed the special device of a well-protected 

 spore which is adapted to carrying the plant through 

 drouth and winter. Since these spores are dense and 

 have a heavy wall they sink to the bottom of the water 

 as soon as they are set free by the partial decay of the 



