THi: FORMATION OF GERM-CELLS 22 1 



differing from those ordinarily present in the leaf. It will be 

 shown in the chapter on Variation that changes of this kind 

 do occur, and that somatic idioplasm 7uay at times, owing to 

 known or unknown causes, become so modified as to produce a 

 deviation from the inherited form of the cells of the series. The 

 sudden appearance of such peculiarities as those exhibited by 

 the moss-rose may be taken as an instance. It is very possible, 

 therefore, that owing to the specific stimulus produced by the 

 larva, and more especially by its secretion, the idioplasm of 

 certain layers of cells in the gall becomes modified and causes 

 the cells to assume another character, such as that of woody 

 fibres. 



This view receives decided support from the circumstance 

 that the gall is by no iiieans only cotnposed of those kinds of cells 

 which occur in other parts of the plant. A similar statement to 

 this is, indeed, made by de Vries, who, however, makes an 

 exception in the case of ' the peculiar layer of sclerenchyma 

 in some Cynipid galls, which afterwards becomes modified into 

 thin-walled, nutritive tissue.'' I cannot look upon this as being 

 ^only an apparent exception' to the rule, for it seems to me to 

 be a very valuable proof that no such rule exists, and that the 

 above instance is to be explained as an apparent reversion to 

 inherited forms of cells, such as were already contained in a latent 

 condition in the cells of the leaf. I should rather be inclined to 

 regard these * exceptions '' as a proof that definite new forma- 

 tions occur in galls, and that these are due to viodifications of the 

 cells from which they arise in response to the stimulus produced 

 by the larva. It can hardly be a matter of surprise that a marked 

 resemblance exists between these cells and those occurrino- in 

 other parts of the plant, for the changes produced by the larva 

 take place in an idioplasm consisting of determinants of the 

 species in question ; these changes w'ould not therefore at first 

 result in combinations of biophors (determinants) very different 

 from those which ordinarily occur in the plant. The new com- 

 bination of the biophors in different ways results, I believe, from 

 the action of the larva, and thus modifications of the determinants 

 are produced. 



The galls of Cecidomyia pocE, which de Vries mentions when 

 contesting my views, are probably to be accounted for in the same 

 manner. In response to the stimulus produced by the larva, 

 these stalk-galls become covered with a thick felt of rootlike 



