MUCORALES 235 



plasm into small usually uninucleate bits. These are then 

 invested in walls and become the sporangiospores. This method 

 of spore formation is tj^pical of all sporangia, and contrasts with 

 the phenomenon of free cell formation by which ascospores are 

 formed in the Ascomycetes. The details of the two processes 

 are given by Swingle (1903) and Harper (1899). 



In the Mucorales as in the Peronosporales a gradual transition 

 from the sporangium to the conidium is evident, endogenous 

 spores being no longer developed in the higher forms. In inter- 

 mediate conditions the sporangium exists as a deciduous, few- 

 spored body to which the special name sporangiolum is applied. 

 Although it is evident that the conidium is the homologue of 

 the sporangium and has been derived from it in some cases at 

 least directly through the sporangiolum, the apphcation of the 

 three terms to the three types of structure can be accompUshed 

 without appreciable ambiguity inasmuch as in this group few 

 confusing border line conditions are encountered. The transition 

 from sporangium to sporangiolum can be demonstrated (Thaxter 

 1914: 353) in Blakeslea, while in Chaetocladium and Haplo- 

 sporangium the origin of the conidium from the monospored 

 sporangiolum is evident. 



In the majority of species the zygospore is formed as the 

 result of the fusion of two gametangia essentially identical in 

 size and shape. The conjugation process begins when two 

 hyphae lying in contact are stimulated to develop a pair of lateral 

 branches, progametangia (incorrectly termed progametes), at 

 their point of contact. These branches, which are thus from 

 the beginning adherent, swell, and a transverse septum forms in 

 each. The terminal cell, thus delimited in each, functions as a 

 gametangium, the remainder of the progametangium being 

 termed the suspensor. The wall separating the two gametangia 

 is then broken down and a fusion cell (coenozygote) results. The 

 zygospore is usually formed in this cell, the wall of the cell finally 

 forming the outer wall of the spore. In the Piptocephalaceae the 

 zygospore forms in a bud which develops from the fusion cell. 



It has long been known that in some species of the Mucorales 

 zygospores occur commonly in nature, while in others they are 

 found infrequently if at all. Their production in the latter 

 forms was assumed by earlier investigators to depend on some 

 special condition of environm-cnt, and various methods of induc- 

 ing their formation were followed, none of which could be relied 



