376 REPRODUCTION 



released by their death and reaches a threshold concentration high 

 enough to induce speculation. This possibility also relates to the 

 stimulation of reproduction by ultraviolet radiation; it has been sug- 

 gested (286) that cells killed by the radiation release into the medium 

 substances which favor perithecial development. This line of investi- 

 gation deserves more attention than it has so far received. 



Other metabolic changes are often coincident with the onset of 

 reproduction, e.g., changes in respiratory rate (118), appearance of 

 tyrosinase (153) or other enzymes (100), and formation of carotenoids 

 (57a, 150a, 288, 289). The correlation between tyrosinase formation 

 and protoperithecium development in Nenrospora crassa is the most 

 striking, in that a wide variety of environmental factors suppress both 

 enzyme activity and reproductive capacity, but it cannot yet be said 

 that the relationship is causal (304). 



One obvious physiological necessity in sporulation is the transport of 

 materials — presumably in a soluble form— from the mycelium to the 

 developing spores. In Aspergillus flavus the appearance of conidia is 

 coincident with a marked fall in mycelial nitrogen, easily interpreted 

 as resulting from transport (224). Phosphate transfer is even more 

 dramatic in A. niger; during spore formation mycelial phosphorus de- 

 clines as much as 80 per cent, and at least some of this phosphorus 

 appears in the spores (12). 



The transport of materials into fruiting structures may be more 

 complete than it is usually thought to be. Thus, microscopic observa- 

 tion of Ascochyta pisi in an infected leaf shows the mycelium ap- 

 parently devoid of protoplasm at the time of formation of pyenidia 

 (43). 



Blastocladia pringsheimii in its natural habitat forms two types of 

 sporangia: thin-walled zoosporangia and thick-walled resting sporangia. 

 In pure culture, resting sporangia form only at very high carbon di- 

 oxide pressure, specifically in 99.5 per cent C0 2 at pH 5.5 (63). This 

 discovery initiated an important series of investigations by Cantino 

 and collaborators of members of the related genus Blastocladiella. 

 These investigations have been reviewed by Cantino (64), and a brief 

 summary is in order at this point. 



Blastocladiella emersonii zoospores develop into a two-celled thallus 

 consisting of a basal rhizoidal cell and an apical sporangium. The 

 sporangium is either a thin-walled papillate zoosporangium or a thick- 

 walled resistant sporangium. In pure culture the type of sporangium 

 formed is controlled by bicarbonate; virtually all thalli develop resist- 

 ant sporangia if 0.01 M bicarbonate is supplied to the medium. The 

 problem, thus, is not that of initiation of reproduction but of the type 



