THE ORIGIN OF GALLS. 551 



time no gall is produced. The eggs deposited in the tissue, or attached to it, are 

 also incapable of directly inciting gall-formation. There is no marked alteration 

 in the neighbourhood until the grub or larva leaves the egg and excretes a fluid 

 substance. Then growing cells of the most varied description are formed adjacent 

 to the larva, and these rapidly assume the peculiar forms which have just been 

 described. This, of course, applies also to cases where the larva has been hatched 

 from the egg at some distance from the spot and has had to seek out a tissue 

 suitable for its dwelling, as also to instances where adult gall-mites and leaf-lice 

 choose out a suitable place for the deposition of their eggs and then secrete a fluid 

 round them when they lay them. If the animal dies, the growth and renewal of 

 the tissue immediately ceases. The cells round the dead body turn brown and die, 

 so that we may conclude the formation of the gall to be due solely to the substance 

 excreted by living animals. 



Those who investigate galls consider that it is chiefly the acrid "saliva" 

 excreted by the larvae to liquefy their food which acts on the cell- tissue of the 

 dwelling they have selected, but there is no doubt that other excretions may also 

 take part. The chemical composition of this substance is unknown, but we shall 

 hardly be wrong if we include it in the group of nitrogenous compounds called 

 enzymes which were discussed at vol. i. p. 464. Enzymes have the power of 

 altering and decomposing substances, even through the cell-wall, and in this way 

 we can account very simply for a whole series of otherwise inexplicable phenomena 

 in the formation of galls. Moreover, urea or closely-allied nitrogenous compounds 

 may be excreted, so that there is nothing to be said against the view that some 

 of the substances diffuse into the interior of the plant-cells. It is at least certain 

 that the fluid substances excreted by the gall-producing animals, in whatever way 

 they influence the protoplasm in the plant-cells, do not kill it, but actually 

 stimulate it to an extraordinary new activity directly demonstrated by the pro- 

 duction of tissues with a definite external form. 



Observation shows that these tissues are formed and fashioned differently 

 from what they would have been without the influence of this substance. It 

 follows, therefore, that the substances excreted by the animals have the capacity 

 of afiecting in some way the specific constitution of the protoplasm which deter- 

 mines the species in the plant-cells influenced by them. It is specially interesting 

 to note in this connection that it not only is the protoplasm of the cells directly 

 acted on by the excretion which is stimulated to an altered form of constructive 

 activity, but that this stimulus is transmitted from cell to cell in ever-widening 

 circles. The spruce-fir aphis Ohermes abietis attaches itself firmly by its beak to 

 the scale of a Fir bud, and can directly influence only a few cells of the young 

 shoot hidden in the bud. Nevertheless thousands of cells on this shoot soon begin 

 to assume an altered form, a proceeding which reminds us strongly of the action 

 of a ferment (c/. vol. i. p. 505), and also brings to our mind the influence exerted 

 by the spermatoplasm on the ovary. The spermatoplasm is only directly concerned 

 with a few cells in the ovule, but these propagate the influence on all sides to 



