ZYGOMYCETES 



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of the sori, are differentiated into individual sporangia which, because 

 of the transition to terrestrial habitats, germinate with germ tubes 

 instead of zoospores. According to this explanation the import of 

 columella formation remains obscure. 



The original sporangial type discussed above, represented by Mucor, 

 Sporodinia, etc., in the higher forms is modified in two directions, either 

 the individualization of oospores is retarded without being entirely 

 suppressed as in the Oomycetes, or the sporangia decrease successively in 

 differentiation, size and spore number and finally sink to conidia in 

 appearance and function. 



In the direction of retardation of spore formation, undoubtedly the 

 Choanephora-Piptocephalis series must be indicated. In poorly nourished 

 Choanephora cucurbitarum sporangiophores and sporangia similar to those 

 of Mucor are formed. The ends of the brown, smooth sporangiospores 



Fig. 60. — Cunninghamella echinulata. Development of sporangiophores. (X740; after 



Moreau, 1914.) 



have two to three hyaline processes from each of which up to 20 hairs 

 arise. With liberal food, on swollen tips of vertical hyphae or eventually 

 on the short secondary branches, the spores arise, not endogenously by 

 cleavage, but exogenously by budding. Spore formation is ontogeneti- 

 cally retarded and is transferred from the interior of the sporangium to its 

 upper surfaces. These exogenous spores, in contrast to the endogenous, 

 are longitudinally striate and not ciliate (Wolf, 1917). 



This exogenous spore formation may be seen in Cunninghamella 

 cytologically investigated by Moreau (1913). In C. echinulata and C. 

 Bertholletiae, the ends of the sporophores swell, like sporangia, into sacs 

 (Fig. 60, 1), whose content is differentiated into a watery inner and a 

 rich outer zone. The peripheral layer splits, not into spore initials, as 

 Sporodinia, but allows the protoplasm to pass out into small spherical 

 sacs, sessile on short sterigmata, with three to eight nuclei each. These 

 sacs are cut off and transformed to spores (Fig. 60, 2) corresponding in 

 size and form to the typical Mucoraceous spores but borne on the outer 

 surface of the sporangia instead of within them. 



