3l8 PLANT GROWTH SUBSTANCES 



swarmers from a germinating zygote to function as female gametes and 

 half to function as male gametes. Another question is the correlation 

 between concentration of sexual substances in the extract and the number 

 of flagella borne by swarmers from a germinating zygote. 



Algae which can be grown in vitro are the most satisfactory for study 

 of sexuality because certain of the external factors affecting the forma- 

 tion of sexual substances can be controlled. To be suitable for study in 

 pure culture the alga must be one which i) grows rapidly in culture; 

 2) is heterothallic and with both kinds of gametes motile; 3) produces 

 gametes readily; and 4) produces them in abundance. Many algae which 

 can be grown in pure culture fail to meet all of these requirements. For 

 example, Chlorella and Scenedesmus grow rapidly in culture but never 

 reproduce sexually. On the other hand, such gamete-producing algae 

 as Ulva, Enteromorpha, and Cladophora grow slowly when cultured in 

 the laboratory and do not produce gametes readily. Among the algae 

 meeting all the foregoing requirements are the siphonaceous algae 

 Protosiphon and Botrydium, and various unicellular Volvocales including 

 Chlamydomonas and Polytoma. 



The nature of sexuality has been studied most extensively in Chlamy- 

 domonas. When grown in a liquid culture-media this alga is in a motile 

 unicellular state. When grown on a .moist substratum, as on agar, 

 Chlamydomonas forms a palmella stage in which all cells of the culture 

 are without flagella and he embedded in a common gelatinous matrix. 

 When palmella cultures are flooded with water or with inorganic 

 nutrient solutions the cells develop flagella within an hour or two, escape 

 from the gelatinous matrix, and swim about in the hquid. Students of 

 sexuality in Chlamydomonas have found it far more convenient to 

 culture it in the palmella stage and then induce motiUty than to obtain 

 motile cells from liquid cultures. 



Many, if not all, species of Chlamydomonas do not have a division of 

 cell contents to form gametes. Instead, any cell can function as a gamete 

 if it contains sexual substances in sufficient concentration. In palmella 

 cultures of species isolated and grown in the laboratory at Stanford the 

 cells do not contain a sufficient amount of sexual substances until the 

 cultures have attained a certain age, and this age is not the same for all 

 species. At first cell division in these cultures is at a rapid rate, later the 

 rate becomes slower and slower. It is thought that here the sexual 

 substances accumulate at a constant rate, but that in most strains rapidly 



