ORDER MUCORALES 151 



substratum usually remain nonseptate. Ayers (1935) has shown that 

 septation in Dispira cornuta van Tiegh, begins in the germ tube and that 

 numerous septa are formed in both young and old mycelium. The cell 

 wall is reported by von Wettstein (1921) to contain chitin and pectose 

 compounds and no cellulose, but Mangin (1899) reports true cellulose 

 in young sporangia of some species and Hopkins (1929) found both 

 cellulose and chitin in Mucor rouxianus (Calmette) Wehmer. It must be 

 noted that Nabel (1929) could find no cellulose in this species nor in any 

 other members of the order. 



Asexual reproduction is typically by the formation of nonmotile, en- 

 cysted spores (aplanospores) in sporangia terminal to the hyphae. These 

 sporangia are formed in the same manner as in the Saprolegniales and 

 Peronosporales, by the passage of a portion of the contents of the hypha 

 into a terminal enlargement which is then cut off from the hypha by a 

 septum. The multinuclear contents of the sporangium are divided by 

 cleavage planes into naked, at first polyhedral, cells containing one or 

 more nuclei each. These then round up and encyst and escape by the 

 rupture or dissolution of the sporangium wall. They germinate by a stout 

 germ tube. Swingle (1903) has shown that all of the protoplasm in the 

 sporangium is used up in the formation of the spores. There are very 

 interesting evolutionary modifications of this typical sporangium, leading 

 to the development of structures comparable to conidia. 



I The greater part of the mycelium may be represented by that within 

 the substratum, the aerial portion consisting of scarcely more than enough 

 to give rise to the sporangia. On the other hand in many species the aerial 

 mycelium may be very extensive, forming a large cottony mass from 

 which arise, here and there, the sporangiophores. It is usually white but 

 often the sporangiophores are dark colored. As it grows older this aerial 

 mycelium may become septate but not truly cellular for the segments 

 formed are plurinucleate coenocytes. A number of species of Mucor when 

 growing in a medium rich in nutrients and of rather high osmotic pres- 

 sure (e.g., a rather concentrated sugary medium) form a yeast-like growth 

 instead of the normal filamentous mycelium. 



The Mucorales are mainly saprophytic on vegetable matter, more 

 rarely on animal matter, and are abundant in the soil and in plant debris. 

 Many are coprophilous. Some are weak parasites on living plant tissues 

 which are rich in stored food but not active, such as the roots of the 

 sweet potato {Ipomoea batatas (L.) Lam.). A number of species are para- 

 sitic upon other fungi, even upon other Mucorales. A few have been 

 described as parasites on animal tissues. 



The probable course of sporangial evolution within the Mucorales 

 can best be followed by studying the sporangia of a selected list of genera. 

 The genera chosen may not represent direct lines of descent, since the 



