MOLDS. 13 



variety. In the simplest forms, the reproductive cells are scarcely if 

 at all distinguishable from the vegetative cells. In some species whole 

 hyphae break up so that each cell forms the starting-point of a new colony. 

 Other forms develop special branches bearing reproductive cells. From 

 these it is but a step to the production of fruiting branches, characteristic 

 in form, called conidiophores, bearing cells markedly specialized as re- 

 productive by form and frequently also by color, called conidia. These 

 conidia are entirely asexual in origin and capable of growing directly into 

 new colonies, although in many cases they are provided with resistant 

 walls which enable them to live for long periods if conditions are unfavor- 

 able to growth at once. In other species, specialized resting cells with 

 resistant walls are formed to enable the plant to survive unfavorable 

 conditions. These are called chlamydospores or sometimes cysts. The 

 name gemma is sometimes applied to similar structures, preferably to 

 such as grow at once. The same end is reached in still other groups by 

 the formation of sclerotia which are hard masses or balls of thick-walled 

 cells filled with concentrated food materials. These sclerotia are fre- 

 quently distinctive of the species producing them by size and appearance. 

 They sometimes resemble the sexual fruiting masses. Resting structures 

 of either type, especially when large, commonly produce typical spore- 

 bearing structures at once after germinating. Many very complex 

 fruit bodies such as the mushrooms appear to be entirely asexual in origin. 

 The systems of classification used are largely based upon the types of 

 sexual fruit bodies produced. Where such fruit bodies are not known, the 

 method of formation of the asexual spores furnishes the most satisfactory 

 basis for grouping. In classifying fungi, certain types of spore formation 

 are found to be characteristic of particular groups. Since within these 

 groups various accessory types of fruiting occur, so that some species 

 show three or even more forms of spores, that type of spore formation 

 which is regarded as characteristic of the group is known as the perfect 

 stage. If sexual fruits are found, these constitute the perfect stage of the 

 group; if no such fruit is found, the most characteristic asexual form is 

 used, as for example the common mushroom of commerce which is asex- 

 ually produced as far as we know, yet represents the most perfect and most 

 constant fruiting form produced by a very large group. Between the 

 typical forms are many gradations resulting in many families whose re- 

 lationship to one or the other group is difficult to determine. Probably 

 the ancestral history (phylogeny) of the fungi, if known, would show several 



