GROWTH 39 



entirely submerged is due to lack of an adequate supply of oxygen. The 

 efficiency of Sordaria fimicola in converting the constituents of the 

 medium into mycelium decreased as the depth of the medium increased. 

 This fungus was less than half as efficient when grown in 50 ml. of medium 

 as when grown in 6.25 ml. 



EFFECT OF EXTERNAL FACTORS ON MORPHOLOGY 



While the study of morphology, as such, is not within the province of 

 physiology, there is a close connection between these two aspects of 

 mycology. Form and function are the two ways in which the poten- 

 tialities of organisms come to expression. The morphology of a fungus 

 may be modified by environmental factors to such a degree as to be 

 unrecognizable. These changes in morphology may be microscopic as 

 well as grossly visible. 



Pasteur (1879) noted that species of Mucor, when grown submerged in 

 liquid and in the absence of air, assumed a yeast-like form. Not only did 

 they resemble yeasts, but under these conditions they fermented sugar to 

 alcohol. Under aerobic conditions no detectable amounts of alcohol were 

 formed. Reproductions of Pasteur's drawings have been published by 

 Foster (1949). 



When yeasts are cultured in liquid media and allowed to age undis- 

 turbed, a film or membrane frequently covers the surface of the liquid. 

 Film formation frequently starts as a ring of cells on the wall of the flask 

 at the air-liquid interface. The morphology of the yeast cells in such 

 films is unusual in that the cells are joined together in filaments. The 

 supply of oxygen must play an important role in the formation of fila- 

 ments. The temperature range within which film formation occurs varies 

 with the species of yeast and is usually considerably less than the tempera- 

 ture range for growth. Most species of yeasts forms films only between 

 6 and 30°C., although Zikes (1919) found Monilia Candida and Mycoderma 

 cerevisiae to form films at 37°C. The early literature on this subject has 

 been summarized by La Far (1911). 



Nickerson and Van Rij (1949) have reviewed the mechanisms of fila- 

 ment formation in yeast and conclude that the processes of cell elongation 

 and cell division are controlled by different enzyme systems. Appar- 

 ently, the sulfhydryl enzymes which regulate the process of cell division 

 may be inhibited without greatly interfering with cell elongation. Among 

 the agents which inhibit cell division are cobalt, iodoacetate, and peni- 

 cillin. The effect of penicillin on Saccharomyces cerevisiae is shown in 

 Fig. 6. Camphor and other narcotizing agents produce somewhat the 

 same changes in morphology of yeast cells (Levan, 1947). 



Many pathogenic fungi which cause disease in man are dimorphic. 

 These fungi are usually yeast-like in the host but frequently form myce- 



