Chlorophyceae 



175 



phores do not stain as readily as those of Spirogyra, and consequently 

 it is necessary to use stronger stains or more prolonged periods. Use 

 the Venetian turpentine method (Fig. 37). 



For a detailed study, imbed in paraffin and cut thin sections. 

 After washing in water, arrange the filaments so that most of them 

 will have the same general direction; then, in running up through 

 the alcohols, keep the filaments from spreading too much by placing 

 a slide on the material. After imbedding, the material can be cut 

 into blocks about a centimeter square. If sections thinner than 5 fji 

 are wanted, cut out smaller paraffin blocks. 



Spirogyra. Probably no alga has been more studied by pupils, 

 teachers, and investigators than Spirogyra (Fig. 38). Nearly all of 



\ 



FIG. 38. -Spirogyra: photomicrograph of zygospores (elliptical in outline) formed 

 by lateral conjugation; the spore with circular outline has been formed without conjuga- 

 tion and is called a "parthenogenetic" spore or an "azygospore"; fixed in 5 per cent 

 formalin and stained in iron-alum haematoxylin. X275. 



the numerous species belong to the low, quiet waters of ponds and 

 ditches, where they often form large, flocculent green mats nearly 

 covering the surface of the water. A few species occur in running 

 water. The mats are very slippery to the touch a character which 

 assists in recognizing the genus in the field. In the larger species 

 the characteristic spiral chromatophore can be seen with a good 

 pocket lens, thus completing the identification, as far as the genus is 

 concerned. Mats in which zygospores have been formed are likely 



