508 Armstrong . — The Function of Hormones in 
It is obvious that the leaf, like the seed, must be provided with a pro- 
tective mechanism preventing both escape and entry of all but a very 
limited number of substances. The invaluable series of observations made 
by Adrian J. Brown 1 with blue barley, Hordeum vulgar e , var. caerulescens , 
have shown that the grain is surrounded with a membrane of extreme 
tenuity, which is penetrable by but few mineral substances and also not by 
sugars, &c. ; our observations show that the leaf is protected in a precisely 
similar manner. The protection is obviously afforded by what is perhaps 
best termed a differential septum , the term semipermeable membrane being 
far too narrow and misleading as an indication of the functional significance 
of the membrane. As it appeared to be desirable to give a special name to 
the class of substances that pass through differential septa such as are met 
with in the barley grain and laurel leaf, exciting activity within the cells 
when they have thus gained an entry, we have proposed that the term 
Hormone — given by Starling to certain excitants of functional activity in 
the animal organism (including carbon dioxide)— should be applied to all 
such substances. The hormones as a class are substances which have but 
slight attraction for water and may therefore be spoken of as 4 anhydro - 
philic ’ ; hydrophilic substances generally fail to penetrate septa which are 
selective to the degree manifest in cereal grains and the laurel leaf. We 
believe this criterion to be one of special importance. 
In all our experiments freshly-picked leaves were either exposed to 
the vapour of the substance to be tried in corked tubes or nearly immersed 
in solutions. The tests were made both at room temperatures and at 37° 
with leaves picked at different periods throughout last year (1910). Young 
leaves picked in the spring responded far more rapidly to stimulation than 
older leaves collected in the late summer and autumn. 
As in the case of the laurel leaf, the substances found to be inactive 
were : weak solutions of mineral acids, caustic soda and most metallic salts ; 
the active salts were mercuric chloride (not nitrate nor sulphate), cadmium 
iodide (not chloride), sodium and potassium fluoride. Iodine, carbon dioxide 
and hydrogen sulphide act slowly, ammonia very rapidly. 
Acids of the acetic series were found to be effective in the order 
observed by Loeb and others, activity being most marked in the case of 
the least soluble acids of highest molecular weight. 
The same may be said of the alcohols of the methylic series. Lactic, 
benzoic, picric and salicylic acids act slowly, oxalic, tartaric and citric 
acids are inert. 
Volatile hydrocarbons, especially benzene and toluene, carbon bisul- 
phide, chloroform, ethers and ethereal salts, aldehydes, acetone, camphor, 
piperidine and phenols are all very active excitants ; paraldehyde in aqueous 
solution is also very active. 
1 Annals of Botany, vol. xxi, 1907, p. 790; Roy. Soc. ProC., B., Ixxxi, 1909, p. 82. 
