CHAPTER VII 



THE CAUSES OF SPECIFIC SHAPE 

 SECTION 39. General. 



EVERY physiological effect is the result of the activity of the living 

 organism, and the best knowledge of the external conditions for growth 

 affords no insight into the internal mechanism or specific structure which, 

 in interaction with the. different functional activities, determines the progress 

 of development. The structure of a germ of unknown origin affords little or 

 no indication as to the character of the plant that would arise from it, since 

 we are entirely ignorant of what constitutes life, and hence scientific research 

 must avoid mere speculation and restrict itself to determining the various 

 interrelated factors which co-operate with and direct this unknown 

 mechanism 1 . 



The progress of development is primarily determined by the hereditary 

 characters impressed on the germ, and the attainment of one stage of 

 development largely determines the progress to the next, so that the shape 

 of an adult is not so much the direct consequence of the properties of the 

 embryonic germ, as the result of tendencies which arise automatically and 

 in a predetermined manner during development. It is, for example, the 

 growth of a cell which produces its division, and although in Asomatophytes 

 the daughter-cells repeat the same rhythm, in Somatophytes the originally 

 equipotential cells follow different lines of development. The commence- 

 ment of such differentiation brings new factors into play, and determines 

 a new point of departure for the further development. In this manner the 

 morphological differentiation and the functional division of labour progress 

 side by side, and lead firstly to the formation of stem and root, and 

 subsequently to the development of lateral appendages upon these. 



The primitive meristems of stem and root are alike in character, and it is 

 the influence of the parts already formed which determines the development 

 of the equipotential meristem cells into the appropriate elements of the root 

 and stem respectively. In fact few cells have perfect autonomy even in the 



1 See the admirable discussions in H. Spencer's Principles of Biology, 1878, and in the article 

 * Lebenskraft ' by Lotze in Wagner's Handworterbuch der Physiologic, 1842, Bd. I. In these 

 works are developed the fundamental principles upon which all modem discussions are based. 

 The subject is not altered by being clothed in a modern dress, which often merely reflects the 

 changeable theories current at the time. 



