Chlorophyceae 177 



Vaucheria.--This form can always be obtained in greenhouses, 

 especially in the fernery, where it forms a green felt on the pots. 

 The greenhouse form is likely to be Vaucheria sessilis. Another 

 species, V. geminata, is very common in the spring, when it may 

 be found in ponds and ditches (Fig. 39). Vaucheria is also found in 

 running water, but in this situation is almost certain to be sterile. 

 In the vicinity of Chicago, V. geminata appears late in March or 

 early in April and within a few weeks begins to fruit abundantly. 

 The fruiting continues for 4 to 8 weeks, and then the alga may 

 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 ma- 

 terial into a 2 to 4 per cent 

 ane-sugar solution in 



bright sunlight. The sex A 



B 



organs will not be formed F ' G - ''<"** r. aeminota; B, v. 



sessilis; a, antheridia; o, oogonia. 



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. 



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. 



