g8 SCIENCE PROGRESS. 



the fact that every wound caused by the saw of the insect 

 gives rise to a gall, even when no egg is laid, and further, 

 that the artificial destruction of the egg does not pre\ent 

 the formation of the gall, the development of the egg 

 determines only the size of the gall. 



In concluding this short account of the latest researches 

 into the formation of galls, two points alone need be ad- 

 verted to. The one is, that in the anatomical structure of gall 

 generally, no new tissue-components appear other than those 

 which the plant attacked already possesses, though they 

 exist under other conditions as regards situation and dis- 

 tribution. The other is, that all the more highly-dif- 

 ferentiated galls spring from young tissues still in course of 

 development, these being caused to assume abnormal 

 structure as the result of the action of the gall-insect. 



III. 



Certain general conclusions which arise out of the 

 preceding facts have already been alluded to, and we may 

 here shortly refer to the theoretical considerations to which 

 they have led. The chief of these is Sachs's theory of 

 " material and form " ^ which has been specially employed 

 with reference to malformations. 



Sachs starts with the assumption that the differences in 

 the forms of the organs of plants depend upon differences 

 in their material components, and that the variations in the 

 organic forms result from variations in the processes of 

 nutrition. Thus the substances which give rise to the 

 formation of a foliage-leaf are different from those which 

 lead to the formation of a carpellary leaf. If this be so, 

 the causative action of the so-called morphological processes 

 may be admitted, just as in the instance of the morphology 

 of a crystal. We can then form a mental picture of the 

 way in which it happens that in malformations, one organ so 

 frequently appears in the place of another, or that the 

 relative forms of the two organs are commingled in the 



^ Gesammte Abhandl. iiber Pfianzenphysiologie, J. v. Sachs, ii., 1156 



