THE PATHOGENIC FUNGI 975 



germinates and produces a new thallus. This meaning is quite different 

 from that in which the word is used when describing bacteria. The same 

 term is, however, often loosely used for any rounded cell of a fungus, whether 

 it be a part of the body of the organism, or an encysted resting form, or 

 a true reproductive type. In fact the function of a cell in these rudimentary 

 plants is often hard to define, as one. which seems at first to be merely a 

 component unit of the thallus may if it becomes detached reproduce an entire 

 organism. We may, however, divide all these rounded cells into two classes: 

 (a.) True spores, the sole function of which is reproduction; and (b) Vegeta- 

 tive spores, remembering that the latter may also serve to reproduce the 

 species. 



Spores differ in their mode of origin and in their arrangement on or 

 within the thallus. Almost every species of fungus produces several types. 

 There are two groups of reproductive spores, one sexually produced, the other 

 asexually. Of the many names given to different forms we will attempt to 

 define only those which it will be necessary to use in this chapter. It will 

 be found that many of these are used in a somewhat varying sense by 

 different writers. 



REPRODUCTIVE SPORES. An Oospore is a sexual type produced by the 

 fertilization of a female cell by a differentiated male cell. 



A Zygospore is a sexual type produced by the fusion of two undifferen- 

 tiated cells. 



Conidium is a general term applied to all asexual reproductive spores. 

 The term is by some writers restricted to exospores or those formed by a 

 bud which protrudes, from the membrane of the parent cell. 



Endospore is a general term applied to any spore formed within the 

 membrane of the parent cell. 



Ascospores are a special class of endospores which are formed in a 

 membrane known as the ascus, the number of spores in the sack being limited 

 to two, four, or eight, and constant for the particular species producing 

 them. The parent cell from which ascospores are produced has originally 

 two nuclei which fuse into one before again dividing to form the ascospores. 

 This fusion is regarded by Dangeard as a rudimentary sexual process. It 

 has also been shown by Harper and others that in certain species the parent 

 cell is the result of the fertilization of a female cell by a differentiated male 

 cell. This is not true of some of the simpler types such as yeasts. 



Basidiospores are exospores produced on a special type of sporophore, 

 known as a basidium. The number of spores on a basidium is limited and 

 constant for a given species. Like the ascus, the basidium has in certain 

 species been found to have two nuclei which fuse before the spores are 

 produced. 



VEGETATIVE SPORES. Thallospore is a general term applied to cells 

 morphologically resembling the above types which are essentially a part of 

 the vegetative portion of the fungus, 



