230 



METHODS IN PLANT HISTOLOGY 



mount in Venetian turpentine, stained in phloxine and anilin blue, 

 shows the nucleus very clearly. 



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

 pecially 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. 53). 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 contin- 

 ues for from 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 sea- 

 sons in the greenhouses, but it is usu- 

 ally 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 2-4 

 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 an 0.1 to 0.2 per cent Knop's solu- 

 tion for a week, then bring the ma- 

 terial into tap water, and keep the cul- 

 ture 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 



Fig. 53. — • Vaucheria: A, Vaucheria 

 geminata, showing antheridium and five 

 oogonia containing fertilized eggs; from 

 a preparation fixed in formalin, acetic 

 acid, and stained in iron-alum haema- 

 toxylin. B, V. sessilis: from a prepara- 

 tion fixed in chromo-acetic acid and 

 stained in eosin and gentian violet. 

 X150. 



