6 STRUCTURE AND CLASSIFICATION OF FUNGI 



periods of dormancy. They remain intact and viable after the re- 

 mainder of the mycehum has died and disintegrated (see Fig. 36). 



Like the oidia and the yeast-like cells, chlamydospores are not 

 peculiar to any class of fungi, but may be formed by quite diverse 

 species. Moreover, they are particularly likely to develop in just 

 those species that frequently form free vegetative cells, and one may 

 often find all transitions between the actively vegetative yeast-like 

 cells and the dormant chlamydospores. Similarly, one may find all 

 transitions between the vegetative oidia formed in the submersed 

 mycelium of an organism and the conidia or true spores formed by its 

 aerial mycelium. These transitions have led to much confusion in 

 classification and nomenclature. These transitional types of growth 

 or reproduction are particularly frequent in many of the lower forms 

 of fungi important to the bacteriologist. 



Spores. The spores proper are very constant in their characters 

 and mode of formation and are therefore largely relied upon for 

 classification and identification. They may be either sexual, i.e., 

 formed either directly or indirectly after the fusion of nuclei from 

 two similar or dissimilar cells, or asexual, i.e., by the division of a 

 single cell without fusion of nuclei. Some fungi reproduce by only 

 one method, some by both, the latter forming non-sexual spores at 

 one stage of their life history and sexual ones at another stage. In 

 some parasitic fungi, the rusts, two different hosts may be necessary 

 for the life cycle, and several kinds of spores may be produced in 

 succession in each host, so that the cycle becomes complicated. 



Asexual Spores. Asexual spores are usually formed in great abun- 

 dance, are ordinarily capable of dormancy, and serve to disseminate 

 the species. At times they are enclosed in a slimy fluid, which may 

 attract insects that carry them to a new habitat. More often they 

 are dry and, being of light weight and small size, may be spread 

 widely by the wind. Mold spores are abundant in the atmosphere. 

 Asexual spores may be divided into two groups according to their 

 mode of formation: endogenous and exogenous. 



In one large class of fungi, the Phycomycetes, the asexual spores 

 are surrounded by a membrane during their formation. A cell at the 

 tip of a filament of mycelium is cut off from the rest of the filament 

 by a crosswalk The multinucleate protoplasm in this cell now sepa- 

 rates into a number of small portions each of which develops a mem- 

 brane. These small bodies are sporangiospores and the original cell 

 wall forms a sac, the sporangium, to contain them. The filament 

 which bears this sporangium is a sporangiophore. When the spo- 

 rangiospores are mature, the sporangium either ruptures from internal 



