SIMPLER PLANTS AND ANIMALS 



sphere, and each one is provided with microscopic lashing 

 threads whose joint action turns the whole sphere over and 

 over in a slow swimming movement. In midsummer and 

 autumn a smaller green alga, Euglena (Fig. 44), covers the 

 surface of pasture ponds and slow streams with a vivid green 

 velvet-like film. Microscopic examination of the smallest 

 particle of this film will reveal hundreds of little whale-shaped 

 Euglenas, each with a flagellum waving from its front 

 end. 



Diatoms. — Diatoms (Fig. 45) are minute single-celled plants, 

 estimated to be about one-thousandth of an inch thick. They 

 float free in the water or are attached to leaves and stems; 

 growing in groups or solitary and in myriad shapes and pat- 

 terns (PI. IV). They are the most abundant of all organisms 

 in both fresh and salt water. They cover the stems of sub- 

 merged plants, and most aquatic insects carry hundreds of 

 them hanging upon the hairs of their bodies. In spring they 

 reproduce in enormous numbers, floating upon the surfaces of 

 lakes and covering the stones of stream beds with a film of 

 golden brown. 



Fig. 45. — Diatoms: i, Meridion; 2, Tabellaria. 



All diatoms are enclosed in shells of silica, transparent, 

 colorless, finely lined and patterned. A diatom shell has two 

 parts or valves, the upper valve fitting over the lower one just 

 as a cover slips over a box. The diatoms figured are common 

 ones selected because they suggest the variety and beauty of 

 those which grow in the most ordinary pool (Figs, i and 45). 



51 



