148 THE LOWER FUNGI— PHYCOMYCETES 



II. Thallus in some forms consisting of cylindrical l)rancliing hyphae 

 definitely constricted at intervals; in others differentiated into a 

 prominent stout main axis from which usually arise relatively more 

 slender branches, with or without definite constrictions. 



2. Leptomitaceae, p. 171 



Saprolegniaceae 



The hyphae composing the floating external mycehum in this 

 family are of greatest diameter near the point of attachment 

 to the substratum, and decrease slowly toward their outer 

 ends. They are, however, nearly cylindrical. Their branching 

 is usually of the monopodial type, though rarely dichotomous. 

 Septa, other than those cutting off the reproductive cells, are 

 absent ; the lumen of the filament consisting of a single continuous 

 branching cavity containing many nuclei. The mycelium is, 

 therefore, coenocytic, as in the majority of the lower Phyco- 

 mycetes. The hyphal walls are unlike those of higher fungi in 

 that they give the chemical reaction for pure cellulose {i.e., turn 

 blue with chloriodide of zinc). Different species differ greatly 

 in the diameter of their hyphae, the entire range extending 

 approximately from five to one hundred microns. The length 

 of the filaments is determined to an extent by the amount of 

 available nutriment. In soil inhabiting species the mycelium is 

 relatively depauperate, and in culture forms a dense mat of 

 limited extent. 



The asexual spores are borne in cylindrical, clavate to globose 

 sporangia which are usually terminal, being cut off by transverse 

 septa at the tips of the hyphal branches. Intercalary sporangia 

 sometimes occur. In Dictyuchus they may be cut off terminally 

 in basipetal succession. 



The protoplasm of the sporangium is much denser than that 

 in the remainder of the hypha, and is filled with numerous nuclei 

 which flow in from the thread before the septum is formed. 

 Cleavage planes appear in the protoplasm, and by their branching 

 and intersecting the content of the sporangium is split up into 

 a number of small irregularly polyhedral masses each containing 

 a single nucleus. These are the spore origins, each of which 

 finally functions as an independent spore. The phenomena 

 associated with the escape of these spores from the sporangium 

 and their germination differ in different species of the group, and 

 to a considerable extent afford bases for separation of genera. 



