612 SCIENCE PROGRESS 



many days without the colour changing ; in like manner, 

 strong alkalies such as potash and soda, cannot gain an entry 

 into the seed from their solutions, provided these are not so 

 strong as to exercise a solvent action on the membrane. And 

 not only are acids, alkalies and most salts kept out but sugars 

 also and the other substances which are formed during germina- 

 tion and serve as food for the growing plant. Strange to say, 

 however, the weak organic acids, carbonic acid, ammonia and 

 a large number of neutral organic substances (hydrocarbons, 

 alcohols, ethers, chloroform, iodine and a few salts such as 

 chloride of mercury or corrosive sublimate) readily pass into the 

 grain from aqueous solutions. 



Experiments which my son and I have carried out with leaves 1 

 show that these are provided with differential septa which 

 protect them in precisely the same way that the cereal grain is 

 protected. We have proposed to apply the term Hormone 

 collectively to the class of substances capable of penetrating such 

 septa, because of the stimulative effect which they produce when 

 they gain an entry. 



This is shown in the case of the common laurel by the libera- 

 tion of hydrogen cyanide, which is easily detected by the change 

 of colour it produces, from yellow to brown, when paper dipped 

 in an alkaline solution of picric acid is exposed to its action. A 

 still simpler demonstration may be given by means of the spotted 

 Japanese laurel, Aucuba japonica. When exposed to the action 

 of Hormones, this blackens more or less rapidly according to the 

 rate at which the hormone passes into the leaf. 



Such leaves contain certain complex compounds known as 

 glucosides together with corresponding enzymes or ferments 

 which can act upon the glucosides and resolve them into simpler 

 compounds ; under normal conditions the glucoside and enzyme 

 are kept apart apparently but the effect of the hormone is to 

 bring them together in some way that we do not at present 

 altogether understand, though it is probably in large measure 

 due to the fact that the entry of the hormone into the cell is 

 necessarily followed by the diffusion of water, so that the cell sap 

 is diluted, a condition which favours the action of the enzyme. 



It is more than probable, I think, that hormones are in large 

 measure the regulators of the vital mechanism. Mankind, all the 

 world over, indulges in stimulants, unconsciously if not con- 



1 Ibid. B. vol. lxxxii. p. 588(1910): Annals of Botany, vol. xxv. p. 507 (1911). 



