270 VATJCHERIA. 



sessilis, which can be found all the year round on the soil 

 in pots in greenhouses. Another species, Y. terrestris, is 

 sometimes found along with V. sessilis, or in similar 

 places ; in the former species the sexual organs are in 

 groups (usually an antheridium with an oogonium on 

 either side) on a common stalk, while in V. sessilis the 

 antheridia and oogonia are seated separately on ordinary 

 branches of the thallus. 



Another species, V. geminata, may be found in spring 

 in ponds and ditches ; it usually fruits during April and 

 May, and may then be lost sight of until late summer, 

 when the oospores germinate. Specimens of Vaucheria 

 found in running water are usually sterile. 



386. Culture Methods. The development of zoogo- 

 nidia may be observed in Vaucheria material which has 

 been transferred from damp soil to water, placed in 

 saucers. If no zoogonidia appear normally, their pro- 

 duction may be induced by either of the following methods, 

 each of which should be tried : 



(1) Cultivate the plant in 0'3 per cent. Knop solution 

 for a week, then transfer it to distilled or tap water. 



(2) Cultivate the plant in 0'3 per cent. Knop solution 

 for a week, then place it in darkness. 



(3) Cultivate the plant in 2 per cent, cane sugar solution 

 in darkness if no zoogonidia appear, add more sugar to 

 make the solution stronger, up to 4 or 5 per cent., and 

 after keeping in the light for a few days put the culture in 

 darkness again. 



When once induced in one or other of these ways, the 

 production of zoogonidia may continue for two or three 

 weeks. 



The formation of sexual organs in V. sessilis, which is 

 usually found in greenhouses in the vegetative condition, 

 may be induced by placing the plant in 3 per cent, cane 

 sugar solution and exposing the culture to sunlight. 

 Bright light or a temperature above 15 C. will usually 

 check the formation of zoogonidia, while sexual organs are 

 not formed in weak light or hi darkness. Again, the 



