5*6 Armstrong. — The Function of Hormones in 
If, in the case of Drosera , thinning and repair of the leaf are possible 
alternately, the leaf must vary in 6 sensitiveness ’ according as it has been 
stimulated or not more or less recently ; this indeed appears from Darwin’s 
statements to have been the case. 
Salts generally, with few exceptions, have but little effect on leaves of 
cherry-laurel and Aucuba. It will be remembered that many salts were 
found by Darwin to cause inflexion of the tentacles of Drosera — sodium 
salts, for example; yet the corresponding potassium salts were without 
effect. It is noteworthy that the effect of sodium salts was transient and 
that recovery usually took place when the leaves were placed in water. 
We are therefore inclined to think that such salts may have produced an 
exosmotic effect. In our experience, in cases in which a substance pene- 
trates into the cell, if the action be sufficiently prolonged, necrosis inevitably 
follows ; the quick recovery of Drosera leaves in water appears therefore to 
be evidence that the salt does not penetrate into the cells of the glands. 
The greater activity of sodium salts may therefore be due to the 
greater osmotic tension of the solution. Darwin, as a rule, used a solution 
of one part of the salt to 437 parts of water ; the solutions of potassium 
salts he used were weaker therefore than those of the corresponding sodium 
salts, both because the molecular weight of the former is greater and because 
sodium salts appropriate a larger proportion of water in solution. 
Cadmium chloride, according to Darwin, caused inflexion but without 
discolouring the glands ; mercuric chloride not only acted very rapidly but 
caused discoloration. This order of difference corresponds very closely 
with that observed by us. 
Darwin has laid much stress on the extraordinary sensitiveness of 
Drosera to ammonia and many ammonium salts, especially the phosphates, 
which were the most active of all the salts he tried and much more power- 
ful even than the carbonate, which was more powerful than the nitrate. 
This result was intelligible, he thought (p. 1 7 1), from the difference of the 
amount of nitrogen in the carbonate and nitrate and from the presence 
of phosphorus in the phosphate ; the inflexion produced by other salts 
of ammonia, he suggested, was due to their nitrogen. 
We are inclined to think that in all cases the stimulating effect is due 
to ammonia and that the activity of the salts in solution is conditioned by 
the extent to which ammonia is liberated by the hydrolysis of the salt. 
If an Aucuba leaf be partly immersed in a weak solution of ammonium 
carbonate, the part immersed is somewhat less rapidly and less intensely 
coloured than that which is not in contact with the liquid, the ammonia 
penetrating more rapidly from the atmosphere than from the solution ; the 
difference is not improbably due to the fact that the film on the leaf surface 
is a more concentrated solution of ammonia than the solution itself. 
At first our results with ammonium salts appeared to be in harmony 
