286 KUNKEL: FACTORS INFLUENCING TOXICITY OF 
chloride bag was suspended for the same length of time over a 
sterile medium exactly like that on which the Monilia was being 
grown* This bag took up only .39 gram of water, showing that 
most of the water taken up by the drying agent suspended over 
cultures comes from the mycelium of the fungus. These exper- 
ments show that the removal of less than one and one half grams 
of water from the atmosphere above cultures greatly decreases the 
rate of growth of the mycelium. Similar experiments were per- 
formed with Mucor Mucedo, Sporodinia grandis and Phycomyces 
nitens. In each case, drying the air above young cultures greatly 
reduces the rate of growth. Sporodinia grandis and Mucor 
Mucedo produce shorter sporangiophores when grown in a culture 
over which a drying agent is suspended than when grown in a 
moist atmosphere. In the case of Phycomyces nitens the rate of 
growth is greatly checked by drying the atmosphere but the final 
length of the sporangiophores is not decreased. 
These experiments show in a striking manner the well known 
importance of water to growth. They also suggest that the slow 
germination and the slow rate of growth in toxic solutions of 
potassium chloride may be largely due to the lowering of the vapor 
pressure above the medium. The same thing is probably true 
of all other salts that must be used in considerable concentrations 
in order to inhibit growth. But it is quite different with more 
toxic substances such as the salts of the heavy metals which 
inhibit growth when present in very small quantities. T heir 
effect on the vapor tension of the medium is, of course, insignifi- 
cant. This difference between toxic and relatively non-toxic 
substances has not been sufficiently emphasized. It seems to me 
that the above experiments offer strong evidence in support of 
the assumption that an important factor in many cases of toxicity 
is the effect of the poison on the ability of the protoplast to absorb 
water. They, at least, show that this factor is sufficient to 
account for a large part of the decrease in growth which is to be 
observed in toxic media containing relatively large amounts of 
dissolved salts. 
A salt like potassium chloride when added to a medium in 
sufficient quantities to check growth, increases the affinity of the 
medium for moisture and in this way makes it difficult for the 
poe 
BSED SPR pee ee Rees an mens © et ee 
