22 2 THE GERM-PLASM 



outgrowths, which doubtless serve as a protection : these, when 

 they gain access to the soil, become branched like ordinary 

 roots. The assumption that under certain circumstances the 

 idioplasm of certain somatic cells becomes modified in response 

 to the stimulus produced by the parasite, so as to give rise to a 

 structure similar to that of another tissue or even organ of the 

 same plant, seems to me by no means to prove that the primary 

 constituent of this tissue must previously have been contained 

 in these cells. In animal tissues transformations of this kind 

 are certainly not known to occur. Pathological anatomists are 

 now of the opinion that only those kinds of cells occur in 

 tumours which actually belong to the sort of tissue from which 

 the tumour arises. This is not surprising, for tissues of animals 

 are far more highly differentiated than those of plants, and 

 corresponding elements in the idioplasm must also differ in a 

 corresponding degree ; and consequently, in spite of the displace- 

 ments and re-arrangements which may be produced by stimuli, 

 they can never form precisely the same combinations as those 

 which occur in the various other tissues of the body. 



I shall not discuss the case of the gall of Neinatiis, as Beyer- 

 inck's observations with regard to it are not yet complete. If it 

 should be shown that a complete willow can be produced from 

 the leaf-gall of the plant, as de Vries considers probable, it will 

 then certainly have been proved that the cells in the leaf contain 

 germ-plasm, just as in the case of the leaves of Begonia. At 

 present, however, it is only known that the gall can give rise to 

 roots, and although normal roots are always capable of forming 

 adventitious buds, it cannot be said at present whether these 

 abnormal roots are able to do so. In the willow, in any case, 

 the primary constituents of roots are distributed throughout the 

 stem in the form of invisible determinants, contained within 

 visible cells, and this accounts for the fact that the production 

 of new individuals by means of cuttings takes place exceptionally 

 easily in this plant. This may perhaps be accounted for by a 

 wider distribution than usual of the merely ' unalterable ■ group 

 of determinants for roots taking place in the plant, in connec- 

 tion with the wide distribution of the corresponding primary 

 constituents. But I do not by any means imagine that in all 

 these cases in which the cells of a plant possess inactive germ- 

 plasm, its presence is actually useful at the present day. If the 

 distribution of unalterable germ-plasm once took place in an 



