METHODS OF CULTIVATION O 



for Ustilago zeae and 56 mM per liter per hour for Aspergillus niger 

 (97). Rates of aeration of this magnitude require an apparatus in 

 which a sterile medium can be agitated and aerated. Various labora- 

 tory-scale devices have been described (10, 55, 142, 168, 252), and some 

 are commercially available. Theoretical and practical problems of de- 

 sign have been studied in several laboratories (9, 49, 68, 97). 



Growth on solid media other than agar is useful in special problems; 

 such materials include soil (224, 260), sawdust (6), bran (206), and 

 cotton (25). The use of plant materials sterilized by treatment with 

 propylene oxide is particularly valuable in the induction of sporula- 

 tion (Chapter 11) and may have wider application in the study of 

 physiology. 



The inoculum used for cultures is usually a population of spores 

 from an agar slant. A common occurrence in the fungi is the replace- 

 ment of the normal type by a variant arising in culture; this develop- 

 ment has been analyzed especially with regard to sporulation. Other 

 examples include the loss of biochemical capacities (63), change in 

 virulence to a host plant (202), and change in assimilatory capacity 

 (246). The ultimate recourse is to preservation of cultures in a 

 dormant state (p. 4). Methods of obtaining single-spore cultures of 

 fungi are reviewed by Hildebrand (136); such cultures may be neces- 

 sary in order to recover or purify the desired type. It must, however, 

 be remembered that some spores are multinucleate (Chapter 12) and 

 may contain genetically different nuclei. 



Three physiological aspects of the inoculation process may be men- 

 tioned. First, growth is more rapid with a heavy than with a light 

 inoculum; in addition, a heavy inoculation of a submerged culture 

 usually results in the development of small colonies, with less danger 

 that oxygen diffusion into the colony will limit growth or respiration. 

 Second, the use of old spores of low viability is to be avoided; both the 

 age and the conditions of growth of the inoculum, especially medium 

 and temperature, should be standardized. For certain fungi the "chem- 

 ostat" provides an easily standardized inoculum (221). Finally, a large 

 inoculum may be required in order to initiate growth on an un- 

 favorable medium (235). 



When rapid growth is a desideratum, the inoculum may be preger- 

 minated spores or mycelium (207). Mechanically macerated mycelium 

 can be used as inoculum (233, 259). This is often the method of choice 

 for fungi which do not sporulate (3). The treatment, to minimize 

 injury, should be as brief as possible. 



Special methods of cultivating particular fungi are too numerous 

 to mention here. Continuous-flow apparatus may have some general 



