634 THE PHTLOGENY OF THE FUNGI 



spores characteristic of most Siphonales and of the Saprolegniales. The 

 close similarity of the serum reactions by Saprolegnia and Vaucheria as 

 determined by Mez would seem to add force to this idea. (Fig. 205.) 



Phylogeny of Higher Phycomyceteae 



The phylogeny of the Mucorales and Entomophthorales is probably 

 properly tied up with that of the Zoopagales. Of the Eccrinales so much 

 still remains to be learned that their origin and their relationships to other 

 groups are very obscure. The fact that their cell walls respond to the 

 cellulose test positively with chloriodide of zinc is not sufficient to indicate 

 where their closest kinship lies. 



The Mucorales and Entomophthorales have cell walls at maturity in 

 which cellulose is not readily demonstrated although in some Mucorales 

 it can be detected in younger mycelium by the use of suitable iodine- 

 containing reagents. As the mycelium becomes older fungus chitin makes 

 up more and more of the cell wall. The fact that cellulose is sufficiently 

 abundant in proportion to the chitin to be demonstrable in the younger 

 mycelium would seem to justify the suggestion of the possibility of an- 

 cestors with little or no chitin in their cell walls. Be it remembered that in 

 Pythium no fungus chitin is demonstrable (Thomas, 1942, 1943) while it 

 occurs in measurable amounts in the very closely related Phijtophthora. 

 The more typical Mucorales are usually considered to be those in whose 

 asexual reproduction the aerial hyphae terminate in sporangia within 

 which by cleavage are produced angular, naked cells which quickly round 

 up and become encysted. Upon their escape from the sporangium by the 

 dissolution or fragmentation of the membrane the aplanospores may be 

 distributed by water or by air currents or even by insects. Except for the 

 nonflagellate condition of the spores this type of asexual reproduction is 

 found in the Blastocladiales, Monoblepharidales, Saprolegniales, and 

 some of the terrestrial and aquatic Peronosporales. The fact that the 

 mycehum of the Mucorales is mostly stout and that large numbers of 

 species are soil inhabitants would seem to exclude many of the Pythiaceae 

 and Blastocladiales and Monoblepharidales from consideration. In the 

 Saprolegniaceae we find the genus Aplanes in which the spores produced 

 in the sporangium become aplanospores, without undergoing the swim- 

 ming stage. Thus, so far as the asexual mode of reproduction is concerned, 

 there is no serious barrier to the behef that the Mucorales may have 

 evolved from some Saprolegniaceous soil fungus with aplanospores. In 

 general, however, the conjugating gametangia in the Mucorales are almost 

 equal in size while in the Saprolegniales they consist of a small antherid 

 and a large oogone, the former usually producing a conjugation tube 

 which penetrates through the oogone wall and opens at its tip when 

 nearly or quite in contact with the egg. Yet in Brevilegnia diclina no such 



