Fungi at various Temper atures, &c. 279 
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rapidly tangentially than in a direction normal to the surface, and therefore 
the amount of mycelium will be proportional to the diameter raised to 
a power intermediate between two and three. Thus it will be seen that 
if the foregoing results are expressed in terms of c amount of mycelium in 
the colony the effects become more marked in each case, even if we only 
assume that the amount of mycelium is proportional to the square of the 
diameter. 
The main conclusion arising from the experimental work detailed in 
this paper is that the retarding effect of a given concentration of carbon 
dioxide on germination and growth is greater at a low than at a high 
temperature. The question arises as to what causes this differential effect 
can be assigned. A similar effect was obtained by Kidd for the action of 
carbon dioxide on the germination of seeds, and the nature of the action 
briefly discussed. As is pointed out by Kidd, the important factor is 
probably the tension of carbon dioxide within the living protoplasm, and 
this is controlled to a large extent by the pressure of carbon dioxide in the 
surrounding atmosphere. For the case of a fungus growing within the 
tissue of a fruit such as an apple, the factors which influence the carbon 
dioxide content of the fungal hyphae (respiration of the fruit, respiration of 
the fungus, rate of gaseous diffusion, &c.) are so various and complicated 
that any discussion of them would be useless. However, the differential 
retarding effect under consideration was shown not only for the rate of 
attack of fruit, but also for the rate of growth of fungus colonies on 
artificial media, in which case the phenomenon is obviously simpler, and to 
which case the discussion may be confined. 
The concentration of carbon dioxide within a fungal hypha growing 
in a given atmosphere of carbon dioxide depends upon : 
1. The solubility of carbon dioxide in water. 
2. The rate of respiration within the hypha and the rate of gaseous 
diffusion across the hyphal membrane. 
The solubility of carbon dioxide in water at 5 0 is to that at 15 0 in the 
ratio 1-4:1 (according to Landolt-Bornstein). If the concentrations of 
carbon dioxide in the growing hyphae at the two temperatures stand to 
each other in this ratio, then it is difficult to see that the retarding effects 
obtained can be completely explained on this ground. On reference to 
the tables given in the present paper it will be seen that the retarding 
effect of a given concentration of carbon dioxide at 5 0 is in many cases very 
much greater than that shown at 15 0 . Furthermore, the relative retardation 
shown varies from fungus to fungus on the same medium, and is strongly 
influenced by the amount of nutrient present and by the proximity of the 
temperature under consideration to the minimum temperature for the 
particular fungus. It is difficult, therefore, to see how these various effects 
can all be explained on the grounds of this one factor. 
X 
