CHLOROPHYCEAE 



231 



appear. The formation of zoospores may continue for a couple of 

 weeks. 



Aplanospores of V. geminata are formed in nature when the plant 

 is growing upon damp ground. The aplanospores may also appear in 

 a 4 per cent cane-sugar solution. 



In fresh 0.5 per cent Knop's solution in bright light, cultures remain 

 in the vegetative condition, and the result is the same in weak light 

 if the nutrient solutions are seldom changed. 

 Such cultures may be kept indefinitely by chang- 

 ing the nutrient solution whenever a whitish 

 scum appears on the surface. 



Vaucheria is not easy to fix. Solutions which 

 give fine results with Spirogyra and Zygnema 

 may be ruinous to Vaucheria. We have secured 

 the best results with a formalin-acetic solution 

 (10 c.c. formalin, 5 c.c. glacial acetic acid, and 

 90 c.c. water). Chromic-acid solutions, even 

 with 4 or 5 per cent acetic acid, cause some 

 plasmolysis. If the chromic acid is weakened 

 enough to prevent plasmolysis, the solution 

 should be allowed to act for 48 hours. The ad- 

 dition of 1 per cent osmic acid up to 6 c.c. to 

 100 c.c. of the solution does not seem to cause 

 any more shrinking, and nuclei are easier to 

 stain. 



Iron-alum haematoxylin is the best stain. 

 Phloxine and anilin blue give beautiful results, 

 occasionally, but preparations are almost sure to fade. Eosin is good 

 for topography, but will not show the nuclei. 



Use the Venetian turpentine method. In mounting, use small scis- 

 sors freely. You cannot untangle a mat of Vaucheria so as to give 

 good views. 



For the development of the oogonium and antheridium, for fertili- 

 zation and for the structure and development of the various spores, 

 thin sections are necessary. Imbed in paraffin. For nuclear details, 

 use iron-haematoxylin ; for cytoplasm, use safranin, gentian violet, 

 orange. 



Cladophora. — This genus is found in both salt and fresh water 

 (Fig. 54). The fresh-water forms are usually attached to sticks or 





Fig. 54. — Cladophora; 

 fixed in chromo-acetic acid 

 and stained in iron-alum 



haematoxylin. 



