K.—BOTANY. 237 
saturated, and the germ-tubes in question are in close contact with the 
surface of the epidermal cells through which a certain amount of cuticular 
transpiration is occurring.? 
The possibility that the entry through the stomata is due to a chemo- 
tropic response to some volatile substance (such as a volatile organic acid, 
aldehyde or ester) emanating from the leaf tissue and diffusing through 
the stomata ought not to be overlooked. That volatile substances from 
plant tissues can stimulate or retard germination has been shown by 
Brown‘ and by Neger,’ and the ascription by Cooley of ‘scald’ in apples 
to the accumulation in closed chambers of acetaldehyde volatilising from the 
fruit tissue is well known. It would seem also that a positive thermotropic 
reaction ought not to be overlooked in considering the physiological 
aspects of fungal penetration of the host. Penetration of the surface 
of the leaf by a germ-tube occurs under conditions of high humidity and 
very slight air movement, conditions which would tend to reduce the heat 
losses of the leaf to a low level. In such circumstances the respiratory 
processes of the tissue might easily be responsible for a leaf temperature 
of the order of 1° C. above that of the air.6 The penetration by germ- 
tubes of such surfaces as those of gelatine and collodion show, however, 
that this cannot be a main factor. 
The question of the physiological processes concerned in the other 
method of entry, that through the epidermal cell, also requires further 
elucidation, and the conditions surrounding a germ-tube developing in a 
drop of water on a leaf may be considered. When the work of Miyoshi 
on the chemotropism of fungi and of pollen tubes appeared in 1894, it was 
naturally assumed that entry was due to a positive chemotropic response 
of the germ-tube or fungal hypha to some substance diffusing from the 
surface cells of the host into the drop containing the germinating spores. 
Considerable doubt, however, was thrown on the interpretation placed by 
Miyoshi on his results by the work of Clarke and of Fulton, whodemonstrated 
that fungi showed a marked negative chemotropism to their own waste 
products. It remained questionable then as to whether fungi exhibited 
any positive chemotropism towards nutritive substances. Graves,’ 
however, by allowing for the negative chemotropism towards staling 
products and giving it a rough quantitive measure, was able to show that, 
in addition to this negative reaction, there is a definite positive reaction 
towards such substances as cane sugar and turnip-juice. A tropism of 
* The cogency of this argument is reduced by the fact that the same difficulty arises 
in the case of all chemotropic reactions. The differences in the concentration of a 
sugar on the two sides of a hyphal tip, which responds to a diffusion gradient by a 
curvature, must in many cases be exceedingly small. It seems possible that in all such 
_ eases other factors may be at work. 
#W. Brown: ‘Studiesin the Physiology of ParasitismIX.’ Annals of Botany, xxxvi., 
285, 1922. 
_&F, W. Neger: ‘Férderung der Keimung von Pilzsporen durch Exhalationen von 
Pflanzenteilen.’ Naturw. Zeit. f. Land- u. Forstwirtschaft, ii., 484, 1904. 
6 For the latest leaf-temperature measurements of crop plants see E. C. Miller and 
_A.R. Saunders (J. Agric. Res., XX VI., 15-43, 1923), who have made 20,000 observations 
of such temperatures. Except in direct sunlight and in wilted leaves they find only 
‘slight differences between the temperature of the air and of the leaf, but, as stated 
above, the conditions suitable for infection are of a special kind. 
7A. H. Graves: ‘Chemotropism of Rhizopus nigricans,’ Botan. Gazette, lxii., 
337, 1916. 
