24 THE COTTON PLANT IN EGYPT CHAP. 



37*5 C. is apparently proportional to the percentage of 

 " x" which we add to the culture medium. Thus, a 

 50 per cent, solution has a stopping point in the neigh- 

 bourhood of 32 '0 C., however quickly the temperature 

 be raised. 



Were it not for the accumulation of " x " in the cells, 

 the growth-rate would go on doubling with every ten 

 degrees until the death-point was reached at about 49 C. 



This substance is formed only in growing cells of the 

 fungus, and unless the formation is very rapid it diffuses 

 out into the surrounding medium before an injurious 

 concentration can accumulate. Stale mycelium can there- 

 fore be rejuvenated by washing with fresh culture medium, 

 or with water. Conversely, when the volume of the 

 surrounding culture medium is of microscopic dimensions, 

 as in a cell of the host plant, staling takes place rapidly. 



We are now in a position to comprehend the peculiar 

 relationship to temperature of the " sore-shin " disease. 

 Owing to the minuteness of the cells of the host, a very 

 short exposure to temperatures above 30 C. will completely 

 inhibit the parasite's growth unless the tissue is washed 

 by water and the host-plant is thus given time in which 

 to form the protecting barrier of corky cell walls. 



It would seem natural to assume that the host-plant 

 itself had a different temperature-relationship, since its 

 cells would otherwise become stale at the same time as 

 those of the fungus, and high temperatures would have no 

 protecting effect. This assumption is not correct, however, 

 and a different explanation has to be tried, in view of the 

 following experimental work. 



In parallel with the experiments on the fungus, a few 

 similar measurements were made with the root of 

 germinating cotton-seeds. The seedling was immersed 

 in a double-jacketed water bath with a glass side, through 

 which the elongation of the root-tip was observed by a 

 Comparator measuring to O'Ol mm. The reserves in the 



