294 THE LOWER FUNGI— PHYCOMYCETES 



be regarded as a one-sporcd sporangium, for after it is cut off a 

 second spore wall is formed about the protoplast. Although 

 the two walls, that of the sporangium and that of the sporangio- 

 spore, are normally in close apposition they sometimes become so 

 widely separated through the absorption of water that the spore 

 may be seen floating in the sporangium (Fig. 107). 



As the conidium approaches its mature size its contents and 

 those of the conidiophore absorb water rapidly. The osmotic 

 force exerted is greater in the conidiophore, and the septum is 

 pushed into the conidium as a definite columella. Later the 

 contents of the conidium becoming more dense, exert the greater 

 pressure, and the columella is forced back into the conidiophore, 

 its former position thus being reversed. Finally the pressure 

 exerted is so strong that the outer of the two walls enclosing the 

 conidium ruptures transversely in a circle about its base, and 

 the conidium is discharged violently into the air and carried a 

 considerable distance. The conidia of different species vary 

 greatly in size, and range in shape from spherical to ellipsoidal. 

 The wall is always smooth (never verrucose as in Massospora), 

 and is covered with an adhesive material which serves to fasten 

 the conidium to the object on which it falls. Should it fail to 

 strike an insect host it may put out a germ tube, and, on this, 

 form a secondary conidium which is in turn discharged. This 

 process may be repeated until the vitality of the protoplasm is 

 exhausted or a susceptible host encountered. Other more 

 abnormal variations, in which secondary conidia bud directly 

 from the primary or in which thicker-walled resting conidia are 

 formed, occur under unfavorable conditions. 



In most species the conidiophores emerge in great numbers 

 from the host through the less resistant portions of its surface 

 and form tufts or definite palisade layers evident to the unaided 

 eye as felt-like masses, powdery with conidia. The color 

 is usually white, but may vary to some shade of gray or green. 

 Occasionally sterile hyphae protrude beyond the layer of conidio- 

 phores. These have been termed paraphyses by some authors 

 and by others cystidia. As the host sickens and dies hyphae 

 are sometimes pushed out from its body to anchor it firmly to the 

 substratum. These are termed rhizoids. They occur chiefly in 

 species having branched conidiophores. 



True zygospores resulting from a sexual conjugation are 

 formed in some species, but in others the resting spore is devel- 



