576 Balls. — Temper attire and Growth . 
3. Amount of carbon dioxide. The culture solution is slightly alkaline, 
and remains so. 
4. Diffusion of carbon dioxide. If the diffusion rate of oxygen into 
the cell is never a limiting factor, it is still less likely that the more soluble 
carbon dioxide should ever accumulate inside the cell in sufficient amount 
to influence the protoplasm, seeing that the air of the chamber contains no 
carbon dioxide whatever. 
5. Available food materials. The culture solutions employed are fairly 
concentrated and can be diluted to at least a quarter of this concentration 
without affecting the stopping point, so that we may assume that there 
is ample food in the solution, and also (6) that this food passes into the 
cell by osmosis at a sufficiently rapid rate to satisfy all the cell’s demands. 
7. Rate of chemical change. There is some evidence in the curves 
obtained that the growth function obeys Van ’t Hoff’s law, within the 
limits imposed on it by the rapid staling which takes place at high 
temperatures. The cessation of growth is obviously not due to any 
destruction of the protoplasmic machinery, as is the case at the death- 
point ; this is shown by the aberrant result of 40-5° C., and by the normal 
behaviour of hyphae which have been heated up as far as 41 0 C. and then 
cooled down again. Some effect seems to be produced on such hyphae, 
however, for the rate of growth is noticeably slower on the second heating, 
even though the stopping point be practically unaffected h 
8. Water supply. Is never limiting. 
8, 9. The hypothesis on which these results find their only present 
explanation is that some substance (or mixture of substances) is formed 
by the protoplasm in katabolism, and inhibits growth by its accumulation. 
Some of the temperature-growth phenomena of the organism depend on 
the amount of this substance which is present in the cell, which amount 
is again dependent on its rates of production and of removal. 
The formation of this substance takes place at low temperatures 
as well as at high temperatures, but with far greater rapidity in the 
latter case. 
The hypothesis approximates to a theory in that this katabolite has 
been isolated from the organism, and some of its properties determined ; 
it has not yet been isolated chemically, nor identified. 
Throughout the rest of this discussion this body will be referred to, 
for convenience, by the designation of ‘ x\ 
Effects of previous exposure to high temperatures. 
Having considered the temperature-growth phenomena under the 
most favourable conditions obtainable, we will turn to the effect of two 
modifications of these ‘ standard conditions ’ which throw considerable 
1 Curve 2. 
