INFLUENCE OF EXTERNAL STIMULI 217 



becomes the more significant when we remember our present limited 

 experimental experience. The facts that have been here shortly brought 

 together also furnish proofs in support of the view of the process of 

 transformation enunciated in the earlier pages of this book. 



II. INFLUENCE OF EXTERNAL STIMULI. 



The special character of the living substance of the egg determines the 

 kind and manner of formation of organs of the plant which springs from it. 

 I have however already pointed out when speaking of juvenile states, of 

 relations of symmetry, and of malformations, that external agencies may 

 influence the configuration ; they act as stimuli whose influence depends 

 upon the capacity of reaction of the individual plant. This capacity of 

 reaction may change in the course of the development of the individual 1 . 

 A strong growing shoot of Bryopsis behaves quite differently from a less 

 strong one when the whole plant is inverted ; the latter suffers a trans- 

 formation, the former does not. Further, the phylogenetic development 

 of the plant kingdom is, although we cannot here enter into detail on 

 this matter, evidently to be traced to intrinsic causes in the nature of 

 the living substance and to the influence of external factors upon these. 

 This external influence is in many cases direct and is still observable, 

 but in others we must assume that it has been inherited and is then 

 only to be recognized by the clue of analogy. When we see, for 

 example, that the aerial roots which contain chlorophyll of many orchids 

 become flattened upon the side directed to the light only under the 

 influence of light, whilst in other roots this may be observed although 

 they are in darkness, and again, that the dorsiventrality of many organs 

 is directly influenced by external factors, whilst in others it is inherited, 

 it is permissible to conjecture that in the latter of each series of cases the 

 formation of organs was originally influenced by external stimuli and that 

 the effect was then transmitted. The wonderful conformity which the con- 

 figuration of plants exhibits even in the most different systematic groups 

 confirms this assumption. 



The dependence of the formation of organs upon external factors 

 has often a utilitarian character. The seedlings of many liverworts and 

 ferns, for example, are threadlike in feeble light but form cell-surfaces 

 in light of greater intensity, and it is quite clear that the first peculiarity 

 enables them to reach more favourable conditions of illumination, whilst 

 the second enables them to maintain a more intense capacity of assimilation. 



1 See what was said at p. 171, upon the reversion to the juvenile form. 



