ZYGOMYCETES 



107 



This line of development is continued through Chaetostylum Fresenii 

 (Thamnidum chaetocladioides) . Here, under unfavorable conditions of 

 nourishment, the terminal sporangia abort (Fig. 64, 6) and only with 

 adequate nourishment again bear true apical sporangia (Fig. 64, 5). 



In these two species the terminal sporangia already have declined 

 in number, as the sporangioles predominate, while in Chaetocladium 

 they disappear entirely, never to reappear. In this genus, the sporangi- 

 oles also degenerate. They become monosporous so that the spore walls 

 coalesce with the sporangial wall. In Chaetocladium Jonesii, this double 

 nature of the spore wall is evident in germination; here the sporangial 

 wall is thrown off as exospore and the spore lies free at germination 

 (Fig. 64, 8). In C. Brefeldii, however, this differentiation is suppressed 

 and the monosporous sporangium puts forth its germ tubes directly 

 (Fig. 64, 10). Thus the sporangium is here entirely transformed into a 

 conidium (Brefeld, 1872; Tieghem and Lemonnier, 1873). 



Fig. 65. — Haplosporangium bisporale. 1. Sporangiferous hypha. 2. Monosporangium 

 with a single sporangiospore. (X 720; after Thaxter.) 



Mortierella and Haplosporangium show a similar degeneration. In 

 Mortierella the sporangium, like the sporangioles of Thamnidium, is 

 separated by a basal septum from its sporangiophores; there is no dif- 

 ferentiation of its content into sterile and fertile zones and the spores 

 arise directly by cleavage of the whole protoplasm. Their number 

 decreases notably and in some species is only two to four (Tieghem and 

 Lemonnier, 1873). In Haplosporangium bisporale (Thaxter, 1914a) this 

 condition has become the rule. Here the sporangia remain very small 

 and retain only one or two spores (Fig. 65, 1). The spore wall is deli- 

 cate, the sporangial wall thick and sculptured; unrelated to Thamnidium 

 and Chaetocladium, here the same process has taken place and has led 

 to one- or two-spored sporangioles, biologically regarded as conidia. 



In a biological sense, Thamnidium-Chaetocladium and Mortierella- 

 Haplosporangium form a series parallel to Pythium-Peronospora under 

 the Oomycetes. Only in the Oomycetes, reduction of the sporangia to a 

 single spore has resulted from inhibition of zoospore formation, i.e., it 

 has remained a purely internal process which has had no direct reaction 

 on the form and size of the sporangia themselves; thus the conidia of 



