280 Brown. — On the Germination and Growth of 
The effect of respiration would be to make the concentration of carbon 
dioxide within the hypha somewhat greater than that in the nutrient 
medium immediately surrounding the hypha. The actual difference in 
concentration would depend on the rate of production of carbon dioxide 
within the hypha and its rate of diffusion through the hyphal membrane, 
both of which are accelerated by increasing temperature. According to 
Kidd, one is probably not far wrong in assuming that the variations of the 
two factors with temperature neutralize each other, so that the carbon 
dioxide content of the growing hypha would be approximately the same at 
different temperatures. It does not appear, therefore, that the differential 
retarding effects described in this paper can be explained on the basis 
merely of differential carbon dioxide content of the hyphae under the 
different experimental conditions. 
Summarizing the above argument, we may say that the greater 
retarding effect of a given concentration of carbon dioxide at a low than 
at a high temperature can be partly explained on the basis of the greater 
solubility of carbon dioxide in water at the low temperature, but over and 
above that other physiological factors come into play, the precise nature of 
which is at present unknown. 
Whatever be the physiological explanation of the various results 
obtained, it is possible to put forward an hypothesis that brings all the 
experimental results into a coherent form. The more important factors 
influencing growth are as follows : 
1. Nutrient. The rate of growth increases with the concentration of 
the nutrient up to certain limits. 
2. Temperature. The rate of growth increases with rise of temperature 
up to a certain limit, and as growth is nil at a minimum temperature, the 
rate of growth is obviously some direct function of (t—t 0 ), where t is the 
particular temperature and t 0 is the minimum temperature. 
3. The amount of growth already made. The relation of growth to 
this factor is complex, but up to a certain point it is such that the rate of 
growth increases with the amount of growth already made. 
4. Density of spore suspension of the fungus . This factor becomes 
prominent in the case of spore sowings in weak nutrient and acts in such 
a way that growth diminishes with increasing density of spore suspension. 
5. The percentage of carbon dioxide in the atmosphere. As far as can 
be seen this is always a retarding factor in the case of fungal growth, 
except in cases where alkaline ‘ staling 5 takes place and accelerated growth 
is the result. 
Considering the first four factors which constitute the environmental 
conditions of the fungus from the point of view of the carbon dioxide 
treatment, we may say that the energy of growth is proportional (in some 
ratio) to the concentration of nutrient, to the temperature, to the amount 
