190 
METHODS IN PLANT HISTOLOGY 
disappear until later in the season, when some of the oospores 
germinate. 
Vaucheria sessilis is found at all seasons in the greenhouses, but 
it is usually in the vegetative condition. Klebs found that the 
formation of oogonia and antheridia can be induced in V. repens 
(a variety of V. sessilis) within 4 or 5 days by putting the material 
into a 2 to 4 per cent cane-sugar solution in bright sunlight. The sex 
organs will not be formed in weak light or in darkness. 
The formation of zoospores may be induced in the following way: 
Cultivate in a 0.1 to 0.2 per cent Knop’s solution for a week, then 
bring the material into tap water, and keep the culture in the dark. 
Zoospores may appear within 2 days. Bright light or a temperature 
higher than 15° C. will check the production of zoospores. A 2 per 
cent cane-sugar solution kept in the dark is also likely to furnish 
zoosporic material. If no zoospores are formed when the solution 
is kept in the dark, the nutrition has been too weak: strengthen the 
nutrient solution and keep the culture in the light for a few days; 
then put the culture in the dark, and zoospores should appear. The 
formation of zoospores may continue for a couple of weeks. 
Aplanospore3 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 changing 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 
fixing is not thorough enough to prevent shrinking during subsequent 
processes. 
Iron-alum haematoxylin is the best stain. Magdala red 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. 
