CAUSAL RELATIONSHIPS OF GROWTH AND DEVELOPMENT 29 



shall arise. The complex influences radiating from the parts already formed 

 exercise upon the developing tissue both a directive (autotrophic) and 

 a formative (automorphic) action. Hence the differences which appear as 

 the tissues become adult do not necessarily indicate that the primary 

 meristems of the stem and root are autonomous, and inherently different 

 from one another. As a matter of fact, a power of forming shoots lies 

 dormant in the root-cells, and indeed, in a few cases, a root apex may be 

 directly modified into the growing-point of a shoot, although, as a general 

 rule, in the normal correlation of the parts this power cannot be exercised. 



As growth and development proceed the power of morphological change 

 is gradually lost, just as clay loses its plasticity when dried and burnt. 

 Even when full powers of growth are retained by certain cells or meristematic 

 cell-complexes, the original freedom may be largely restricted by internal 

 or external inductive action. 



All these conclusions are the necessary consequence of the postulate, 

 that every vital process is determined by the interaction between inherent 

 disposition and external environment (Sect, i) ; and it is only by a thorough 

 recognition of this fundamental principle, that any sure and certain de- 

 ductions as to the nature of developmental and formative processes can 

 safely be drawn from our knowledge of the causes controlling them. This 

 caution is the more necessary since conclusions are often formed from facts of 

 morphology and ontogeny which, more especially in regard to phylogenetic 

 problems, do not correspond with the above general principles, and in which 

 the real causative relationship of the different phenomena observed is 

 neglected. 



Under the influence of a given aggregation of causes, every part of 

 a plant must necessarily assume a definite and precise form. A change in 

 the internal or external relationships can only modify this form so far as is 

 allowed by the contemporaneous specific characters of the part affected. A cell 

 complex of a primary meristem, which develops to form a leaf only when 

 particular inductive causal relationships are maintained, cannot be regarded 

 as a foliar region, but only as a spot in which is localized a potential and 

 conditional power of leaf- formation. For such a leaf-rudiment does not 

 become a leaf owing to its own inherent and inalienable power of development 

 in that direction, but owing to the directive and formative internal and 

 external influences which would be exerted upon any similar group of 

 meristematic cells occupying the same position, as is instanced by the fact 

 that a change in the external conditions may cause a callus or a shoot to be 

 produced instead of a leaf. In the same way, the region from which either 

 a leaf or a perianth segment may develop according to the external conditions 

 is in reality neither a foliar nor a floral zone, and, although it is convenient 

 in such cases to speak of indifferent regions, in the strict sense of the term 

 no such things really exist. 



