THE DYNAMICS OF GROWTH 



spp. can be successfully lyophilized (281). Haskins and Anastasiou 

 (126) report successful use of a modification of the lyophil technique. 

 Too rapid rehydration of vacuum-dried uredospores is apparently 

 injurious (265a). 



For sporulating fungi, lyophilization appears to be the best single 

 method; until more information is available, however, preservation 

 of many fungi will have to be on an experimental basis. Lyophiliza- 

 tion is comparatively easy and apparatus is commercially available. 

 Preliminary studies (197) indicate that spray drying of fungus spores 

 may also be feasible. 



3. THE DYNAMICS OF GROWTH 



Growth cannot be precisely defined; a provisional operational defini- 

 tion is increase in either mass of cells or number of cells. Mass — dry 

 weight, usually— in the filamentous fungi may be deceptive inasmuch 

 as it can represent in part the accumulation of polysaccharides, lipids, 

 or wall materials without any increase in living protoplasm. Con- 

 versely, in Neurospora crassa protein synthesis appears to continue for 

 a period of time after weight increase has ceased (7). 



The time course of growth of filamentous fungi in liquid shaken 

 cultures is exemplified by the data of Figure 1. Similar curves are ob- 

 tained with surface cultures (2, 105, 248). Typical curves are charac- 

 terized by three major phases, with transitions between them: 



1. A phase of no apparent growth. 



2. A phase of rapid and approximately linear growth. 



3. A phase of no net growth or of autolysis and decline in dry weight. 



It should not, however, be assumed that all data fit this scheme. In 

 particular, a two-phase growth curve, in which an initial growth 

 period is succeeded by a second, with a leveling off of growth inter- 

 vening, has been reported (30). The second phase may represent 

 polysaccharide synthesis only, without increase in other cell com- 

 ponents, or it may depend on a mobilization of nitrogen from older 

 hyphae and use of it for new growth. The observations of Morton and 

 Broadbent (205) on Scopulariopsis brevicaulis support the second ex- 

 planation; at least in a medium relatively high in carbon, it appears 

 that nitrogen is reutilized after exhaustion of the available exogenous 

 nitrogen. 



Probably by suitable adjustment of cultural factors, especially nu- 

 trient concentrations, a variety of growth curves very different from 

 those of Figure 1 can be obtained. Provision of calcium, for example, 



