BLASTOCLADIALES 611 



Resting-spore formation in Blastocladia pringsheimii was induced in 

 pure culture by Emerson and Cantino (1948) when the fungus was 

 grown in an atmosphere high in C0 2 (99.5 per cent) or in a medium 

 containing carbonate. 



Machlis and Ossia (1953a) showed that maturation of resting spores 

 in Allomyces was markedly affected by the conditions under which the 

 fungus was cultured. In nutrient broth the maturation period was re- 

 duced by 70 per cent over that necessary when the fungus was grown 

 on agar slants. They also (1953b) present evidence indicating that cer- 

 tain auxins, particularly IAA under certain cultural conditions, in- 

 fluence the development and maturation of the resting spores in two 

 ways: (1) by a reduction in the time necessary for chromosphere dis- 

 appearance (as an indicator of maturation, see p. 621) and (2) a marked 

 increase in the proportion of resting spores which will germinate. 



The germination of the resting spores among members of the order 

 has long presented a puzzling situation. Dormancy of several weeks is 

 evidently necessary in some, whereas in others immediate germination 

 can occur. Cantino (1951a) studied this process in blastocladiaceous 

 fungi growing in pure culture under controlled conditions. In Blasto- 

 cladiella, when precautions were taken to eliminate in culture inhibitory 

 factors, salts for example, all of the resting spores in a population of 

 same age would germinate immediately after maturation and within 

 a few moments of one another. Cantino (op. eit.) further points out 

 that resting-spore germination is a two-step mechanism, the first step 

 terminating with the cracking of the wall, the second with the discharge 

 of the zoospores. Relatively low concentrations of anions as well as 

 different temperatures exert a pronounced differential influence on these 

 two processes. 



Sexual Reproduction and Alternation of Generations 



Sexual reproduction occurs in certain species of Catenaria, Blasto- 

 cladiella, and Allomyces (in which it was first observed), but it has not 

 thus far been convincingly demonstrated in Blastocladia (Lloyd, 1938; 

 Blackwell, 1940: see, however, E. A. Bessey, 1939: Blackwell, 1939). 

 In Blastocladiella (see Fig. 39, diagram) and Catenaria (Fig. 44 C-F. 

 p. 651) fusion of isogamous posteriorly uniflagellate planogametes has 



