5<=>9 
regulating Metabolism . 
Not only does the colour change when the leaf is exposed to these 
various substances but the leaf becomes flaccid and the amount of reducing 
sugar in it is largely increased. 
Mirande, in a recent communication to the French Academy, 1 has 
given a long list of organic substances of various classes which will pene- 
trate into the leaf but has been unable to draw any definite conclusion as 
to the existence of a relationship between structure and activity. In our 
opinion, the difference is not one of structure, except in so far as structure 
determines the degree of affinity of the substance to water and its solubility 
relationships. 
The picture that we have formed of the mode in which hormones gain 
an entry involves the conception that the surfaces of the intermolecular 
spaces in the differential septa are coated with a protective sheath of 
molecules of ‘ hydrone ’, 0 H 2 — the fundamental molecule of water. 
Anhydrophilic substances are able to escape the gauntlet and pass 
through almost without hindrance, whilst those to which the molecules 
cling are unable to run the blockade, because the hydrone molecules 
attached to them are so much attracted by and attractive of those attached 
to the passages in the septum, so much under the influence of the water in 
which they are dissolved, perhaps also too heavily laden with hydrone and 
therefore too big to get through, although not too big in themselves. 
The hypothesis on which this conclusion is based has been developed 
in a communication made by one of us to the Royal Society 2 and also in 
two articles in ‘Science Progress’, Nos. n and 12, January and April, 
1909. The chief contention, on which stress is laid, is that the unit mole- 
cule 0 H 2 is eminently unsaturated and active; consequently that water 
itself is probably a mixture of complex molecules formed by the union of 
hydrone molecules in various proportions — perhaps in twos, threes, fours 
and fives — this mixture being saturated with hydrone in proportions 
which must be supposed to vary with the temperature ; hence, perhaps, the 
increase in the rate of change in aqueous solutions as the temperature rises. 
Foreign molecules, when dissolved in water, are supposed to have the effect 
of interfering with the formation of the various polyhydrones, the con- 
sequence being that solutions are richer than water in hydrone the larger 
the proportion of the solute : the attraction which the hydrone molecules 
have for each other is supposed to condition the so-called osmotic pressure 
effects. 
The main effect produced by hormones when they gain entry into the 
living cell is the stimulation of enzymic activity. In our communication to 
the Royal Society we have pointed out that it is to be supposed that they 
exercise a determining influence in regulating metabolism in plants as well 
as in animals ; as the section dealing very briefly with their influence on 
1 Comptes rend., t. cli, 1910, p. 481. 2 Roy. Soc. Proc., B., lxxxi, 1908, p. 80. 
