TYPES OF CRYPTOGAMS; THALLOPHYTBS 



241 



in nitric acid, the cellulose wall and its contents may be destroyed 

 and the markings of the siliceous shell more easily observed. Each 

 diatom consists of a single cell. 



272. Movements of Diatoms. Living diatoms exhibit a peculiar 

 power of movement. In. the boat-shaped species the movement is 

 much like that of a row-boat, forward or backward. 



THE STUDY OF SPIROGYRA 



273. Occurrence. Spirogyra, one of the plants commonly known 

 as pond-scum, or " frog-spit," occurs widely distributed throughout 

 the country in ponds, springs, 



and clear streams. It is of a 

 green or yellowish-green color,- 

 and in sunny weather usually 

 floats on or Dear the surface of 

 the water, buoyed up by the 

 numerous oxygen bubbles which 

 it sets free. It may be found 

 flourishing in unfrozen springs, 

 even in midwinter. 



274. Examination- with the 

 Magnifying Glass. 1 Float a 

 little of the material in a white 

 plate, using just water enough 

 to cover the bottom of the latter. 

 Study with the magnifying glass 

 and note the green color of the 

 threads and their great length 

 as compared with their thick- 

 ness. Are all the filaments about 

 equal to each other in diameter ? 



Handle a mass of the material and describe how it feels between 

 the fingers. 



275. Examination with the Microscope. Mount in water under 

 a large cover-glass and examine first with a power of about 100 



1 Consult Huxley's Biology and Spalding's Introduction to Botany. 



FIG. 176. A Group of Diatoms. 



A, Achnanthes; B,Cocconema; 

 C, Meridion; J>, Pleurosigma. 



