regulating Metabolism . 515 
Again, the attack of plants by fungoid growths may be made possible 
by the excretion of hormones by the fungus. The behaviour of Drosera 
appears to us to be significant from this point of view. Before it will be 
possible to appreciate and interpret the whole of Darwin’s wonderful obser- 
vations on this plant, it will be necessary to repeat many of the experi- 
ments, taking into consideration our modern knowledge of enzymic changes. 
It is already clear, however, that most of the substances which Darwin 
found to be capable of inciting the digestive activity of the plant are sub- 
stances that we class as hormones. Apparently the enzyme and the accom- 
panying acid are not secreted until the glands are excited by the absorption 
of some soluble matter. The fact that acids generally do not act as exci- 
tants but only certain acids appears to us to be clear proof that the glands 
are provided with a differential septum which loses the differentiating power 
and then permits of the excretion of the enzyme only after it has been 
sufficiently thinned by enzymic attack from within. Darwin himself remarks : 
4 It is strange that allied acids act very differently ; formic acid induces 
very slight inflexion and is not poisonous, whereas acetic acid of the 
same strength acts most powerfully and is poisonous. Lactic acid is also 
poisonous but causes inflexion only after a considerable lapse of time. 
Malic acid acts slightly, whereas citric and tartaric acids produce no effect.’ 1 
The order given is precisely that which we find to be the order of the 
activity of acids towards laurel and Aucuba leaves and that which Adrian 
Brown has deduced as the order of permeability in the case of the barley 
grain. Our observations as to the activity of hormones generally, it may 
be remarked, are also in complete harmony with those made by Overton 
and by Fiihner and Neubauer using blood corpuscles and with Loeb’s 
remarkable studies of artificial parthenogenesis. 
That one effect is to thin the leaf membrane and deprive it of differen- 
tiating power there can be little doubt — the enzyme could not well escape 
otherwise. We have had occular demonstration of such an effect in experi- 
ments made with young Rhododendron leaves with the object of ascertaining 
to what extent the leaves increased in weight — (a) when kept in water 
alone, (b) when kept in water to which toluene was added. In the course 
of several days a considerable increase took place only in the latter case. 
It so happened, moreover, that the bottles in which the leaves were im- 
mersed in water were left on the bench perhaps a couple of months. During 
the whole time, in the bottle in which water alone was used, the leaves 
remained green, only a few brown spots appearing here and there ; no 
appearance of colour in the water was noticeable and no fungoid growth. 
In the other bottle, in which the leaves were brown, the solution soon 
became brown in colour and full of a mycelial growth ; evidently sugar had 
passed out from the leaf in the one case but not in the other. 
1 Insectivorous Plants, 1875, p. 273. 
