334 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



need only to be transferred to water to bring about the formation of these special 

 cells. They continue to be produced for some time under these conditions, but 

 finally the process ceases even in water. If the water culture is then removed 

 from light to darkness, zoospore formation begins again, and by transferring 

 such a culture back and forth, between darkness and light, it is possible to call 

 forth this reproductive response or to check it, at will. If the plant is grown in 

 water without the requisite mineral salts, the power to form zoospores is lost 

 and does not reappear, even if the culture is transferred to darkness, unless the 

 essential nutrient salts are re-supplied. 



In sexual reproduction each Vaucheria filament usually develops two lateral 

 outgrowths, one of which forms the antheridium while the other becomes the 

 oogonium. The egg cell of the mature oogonium is fertilized by one of the 

 numerous sperms liberated from an antheridium, and the oospore formed by 

 this union develops into a new individual, the whole process constituting sexual 

 reproduction. 



Sexual reproduction in Vaucheria is dependent upon external conditions. 

 Adequate light conditions and the presence of carbon dioxide in the solution or 

 in the air about the cells, are necessary for the production of sex organs, for the 

 ordinary processes of nutrition must continue during the formation of these 

 organs. No sexual organs are formed in light when carbon dioxide is lacking, 

 unless, indeed, the lack of the latter is supplied by sugar in the solution. 

 Absence of light cannot be thus counteracted by the presence of sugar, how- 

 ever. When the culture medium contains sugar and the atmosphere is with- 

 out carbon dioxide, antheridia and oogonia are formed in light and not in 

 darkness. Vaucheria filaments may be so treated that they are unable to 

 reproduce sexually, even when illuminated. If the culture is grown for a 

 comparatively long time in a sugar solution, in weak light or in darkness, the 

 cells become gorged with oil and lose the power to reproduce. 



Finally, the quantitative relation between the number of oogonia and the 

 number of antheridia may be modified by altering the external conditions. 

 There is generally .one oogonium for each antheridium in Vaucheria repens, for 

 example, though less frequently there may be one antheridium for each two 

 oogonia. The number of oogonia formed may be reduced, while the number of 

 antheridia may be greatly increased, by subjecting the plants to high tempera- 

 ture or to much reduced atmospheric pressure. As many as five antheridia in 

 a group, without any oogonia at all, may sometimes be formed with this 

 treatment. 



In connection with the study of sexual reproduction the question arises as 

 to what may be the conditions determining the entrance of the sperms into 

 archegonia or oogonia. To attack this problem experimentally, very fine 

 capillary glass tubes filled with various solutions are laid in a drop of water 

 containing the sperms to be studied. According to the nature of the solutions 

 diffusing from the open ends of the tubes and according to the kind of sperms 

 present, the latter are either attracted in large, numbers and swim into the tubes, 

 or they are not affected at all. It appears that each species of sperm is attracted 



