26 PHYSIOLOGY OF THE FUNGI 



time reveals that many cells are devoid of protoplasm. It is quite possi- 

 ble that some of the materials released by autolysis are used by the 

 remaining living cells. 



Filamentous fungi. With exception of the third phase of growth dis- 

 cussed above, the filamentous fungi follow the same order of development 

 as the yeasts. The most obvious difference between the filamentous 

 fungi and unicellular organisms is the failure to attain an exponential rate 

 of growth. Usually, the exponential phase is replaced by a more or less 

 linear phase of growth. Emerson (1950) found a straight-line relation 

 between the cube root of the weight of mycelium produced by Neurospora 

 crassa grown in nonagitated liquid medium and the time of incubation. 

 This relation held for three surface-volume ratios. A comparison of the 

 linear, logarithmic, and cube-root growth curves indicates that this fungus 

 has a cube-root phase of growth during the interval when the linear graph 

 is concave upward. Growth in the filamentous fungi is limited to the 

 tips of the hyphae. The influence of neighboring cells which compete for 

 nutrients is a much more important factor in the growth of filamentous 

 fungi than in submerged unicellular organisms. In unagitaged cultures a 

 portion of the mycelium is usually aerial at some stage of growth. The 

 aerial mycelium derives its nutrients from the submerged cells, which 

 involves the transport of these substances over some distance. 



RATE OF GROWTH 



To study growth, it is necessary to consider both the rate and amount of 

 production of cells formed during incubation. The average rate of 

 growth is obtained by measuring the amount of growth at two intervals of 

 incubation and dividing the difference by the time interval. If the 

 weight of a fungus colony increased from 50 to 98 mg. between the fourth 

 and sixth days of incubation, the average rate of growth is 24 mg. per day, 

 or 1 mg. per hr. In experimental work, measurements of growth should 

 be made sufficiently often during the period of incubation so that a 

 smooth graph (growth curve) can be plotted from the data. The inter- 

 vals between measurements of growth may be as short as 1 day for a 

 rapidly growing fungus and as long as a week for species which grow 

 slowly. The rate of growth at any time may be determined by finding 

 the slope (tangent) of the curve. The growth rates of fungi differ, as is 

 illustrated in Fig. 2. 



Since growth is a process which takes place in time, it can be studied 

 only by making many growth measurements during the period of incuba- 

 tion. Such a study is not complete until the phase of autolysis is attained. 

 Much of the information in the literature is incomplete because growth 

 was measured only at one time. Many of the potentialities of the fungi 

 can be discovered only by prolonged observation. 



