44 THE LOWER FUNGI— PHYCOMYCETES 



of unfavorable environment they may be thick-walled, and are 

 then termed resting sporangia or resting spores. When evidence 

 is given that a sexual fusion acconipani(>s their formation the 

 terms oosporangium, oospore, zygo sporangium or zygospore 

 are often applied. 



When, in parasitic species, the summer sporangia or resting 

 spores are formed within the host cell they are termed endobiotic 

 (endophytic when the host is a plant, endozoic when it is an 

 animal). When they stand on the surface of the host, with only 

 the nutritive hyphae penetrating into its interior, they are said 

 to be epibiotic (epiphytic or epizoic). In saprophytic species 

 thej^ are either intramatrical or extramatrical depending on 

 whether they lie in or outside of the matrix or substratum. 



The summer sporangium is usually more or less globose or 

 ovoid, though in some genera it is considerably elongated. At 

 maturity its contents form one to many zoospores. These are 

 usually ciliate and escape from the sporangium through a definite 

 pore, which often terminates an elongate exit tube. More rarely 

 the zoospores are non-ciliate and amoeboid. Occasionally, 

 zoospores encyst in the sporangium and germinate by tubes 

 in situ. In some genera the protoplast passes from the spo- 

 rangium into an extruded thin-walled vesicle, which later bursts 

 freeing the zoospores. In such cases the zoospores may or may 

 not be completely delimited before passing into the vesicle. 

 Since the vesicle functions as a sporangium, it is so called by 

 many writers, while the initial cell, from which it arises, is termed 

 the prosporangium. 



When the mature thallus divides to form a group of sporangia 

 or resting spores the group is termed a sorus. In some genera 

 the entire protoplast flows from the thallus into a thin-walled 

 vesicle and there divides to form the sorus. In such cases the 

 initial thallus cell is called the prosorus. 



The resting spore corresponds essentially in its methods of 

 germination to the summer sporangium except that in some cases 

 it functions as a prosorus and emits a vesicle in which a sorus 

 of sporangia are formed. It is thicker walled than the summer 

 sporangium and, though sometimes smooth, is often character- 

 istically roughened, being tuberculate, echinulate, or reticulate. 



In this order the zoospores are usually ciliate, and swim 

 actively for a time before coming to rest and retracting their 

 cilia. In most genera the zoospore, on becoming quiescent, 



