DEVELOPMENT OF PLANTS 



pletely infiltrated with silica, a substance resembling glass. If 

 a diatom is burned in a flame on a strip of platinum or placed 

 in acid to remove the organic substance the appearance of the 

 valves remains unchanged. 



The glass-like valves are quite transparent and it can be readily 

 observed that the cell contents is much more highly differenti- 

 ated than in the preceding group. The chlorophyll is deposited 

 in plastids of definite form, although this color is often masked 

 by a brown pigment which causes the characteristic appearance 

 of these plants when associated in masses. The oil drops seen 

 in the cells are the product of photosynthesis starch not being 

 formed. Some species, however, can live upon decaying or- 

 ganic matter and in consequence contain colorless plastids. In 



FIG. 97. Structure of Cymbella: A, valve view. B, cross-section show- 

 ing the difference of the two sides of the diatom. C, the two girdle views. 

 After Pfitzer. 



the free swimming forms the motion consists of an irregular 

 gliding movement and this is supposed to be due to the expan- 

 sion and contraction of -minute strands of protoplasm that pro- 

 ject through the pores of the valves (Fig. 96, C, p). 



(a) Reproduction of the Diatoms. It will naturally be asked 

 how can these plants living in glass houses, grow ? As the valves 

 become changed to silica naturally any increase in size must cease. 

 Nevertheless the cells reproduce with great rapidity and in a 

 very interesting manner. Through the growth of the living 



