LEPTOMITALES 855 



are lacking in Mindeniella (Fig. 72, p. 902), the holdfast and basal-cell 

 development is as pronounced as in the other three genera (Kanouse, 

 1927; Sparrow and Cutter, 1941 ; Johnson, 1951b). Why this particular 

 arborescent habit of growth should appear in wholly unrelated groups 

 of aquatic Phycomycetes is as yet little understood. It can be assumed, 

 however, that such differentiation of the thallus is well adapted to the 

 type of substratum on which all these fungi are commonly found, 

 namely, somewhat spongy decaying fruits and twigs. 



REPRODUCTION 



Nonsexual Reproduction 



In nonsexual reproduction the zoosporangia are either single or in 

 whorls or umbels. In all instances they are separated from the vege- 

 tative part of the plant by a constriction within which there is a cellulin 

 plug, the whole forming a short pedicel (Fig. 70 B, p. 890). In Lepto- 

 /??m/s (Pringsheim, 1883a; Coker, 1923), slightly modified segments of 

 the hyphae are simply transformed into sporangia, the terminal one 

 maturing first, and then others in basipetal succession (Fig. 69 A, p. 

 875). In the remaining genera, however, the sporangia are well-defined 

 broadly or narrowly ovate structures. They are smooth-walled in all 

 except Araiospora and Mindeniella. In these genera sporangia ornament- 

 ed with spines, as well as the more common smooth-walled type, are 

 formed (Fig. 70 E-F, H, p. 890). In Mindeniella (Kanouse, 1927; Spar- 

 row and Cutter, 1941), the spiny sporangia (Fig. 72 B, D, p. 902) have 

 a tendency to appear after the colonies have become well established 

 on the substratum, which suggests a possible relationship to the amount 

 of nutrition available (see also Webster, 1943), but Emerson (comm.), 

 by means of pure-culture studies, has shown their formation to be 

 related to near absence of oxygen. The spiny sporangia give rise to the 

 same kind of zoospores as do the smooth-walled ones. 



The zoospores are cleaved out within the sporangium in a manner 

 similar to that in the Saprolegniaceae. Considerable differences are 

 apparent between the members of the Leptomitaceae and the Rhipi- 

 diaceae with respect to the type of zoospore produced at discharge and 

 its subsequent behavior. In the Leptomitaceae definite evidences exist 



