DEVELOPMENT OF PLANTS 



I5i 



pletelv 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 readilv 

 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 Cymbclla: A, valve view. B, cross-section show- 

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

 — After Pritzer. 



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 

 verv interesting - manner. Through the growth of the living 



