178 Methods in Plant Histology 



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 changing the nutrient solution whenever 

 a whitish scum appears on the surface. 



Fixing solutions which are successful with Spirogyra may prove 

 ruinous to Vaucheria. In general, fixing solutions for Vaucheria 

 should be very weak. Mount a few filaments, and with a pipette 

 add a few drops of the fixing agent. Plasmolysis is likely to take 

 place within 10 to 30 seconds if it is to take place at all. If plas- 

 molysis takes place, weaken the fixing agent. The following formula 

 has given the best results with V. geminata: 



Chromic acid 1 g. 



Glacial acetic acid 8 c.c. 



Water 800 c.c. 



This is a very weak solution. A loose mat as large as one's 

 finger requires 100 c.c. of the reagent. From 36 to 48 hours should 

 be allowed for fixing. Wash 24 hours. Use the Venetian turpentine 

 method. With Magdala red and anilin blue the oospores will 

 show various shades of red, according to the stage of development; 

 the filaments will show various mixtures of blue and red. 



In mounting, use the small scissors freely. You cannot untangle 

 a mat of Vaucheria so as to give good views. 



For the development of the oogonium and antheridium, for 

 fertilization 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. 

 The fresh-water forms are usually attached to sticks or stones in 

 quiet or running water. The mats feel rough and crisp and, even 

 under a pocket lens, show the characteristic branching by which the 

 form is easily recognized. The absence of a mucous coat makes 

 Cladophora a convenient host for numerous parasitic algae, among 

 which diatoms belonging to the genera Cocconeis and Gomphonema 

 are particularly abundant. 



