ON FORMATIVE INDUCTION AND PECULIARITIES OF CELLS 143 



tion of their generalized embryonic properties are said to be specifically 

 determined or differentiated. Such post-embryonic primordia may be 

 distinguished as specifically foliar, radicular, or caulicular (leaf, root, stem). 

 Naturally all specific leaf-primordia or stem-meristems are equipotential 

 among themselves, although they may undergo dissimilar differentiation 

 under varied conditions. 



Every phase of development is the inevitable result of the existent and 

 pre-existent conditions, and even although factors of entirely obscure internal 

 origin are among the most important of these, there is no need to invoke 

 the aid of a mystical vital force in explanation of the phenomena of life and 

 growth. Nor are the superficial morphological explanations derived from 

 a kind of abstract idealism of any value, though often of tempting 

 simplicity. 



The living organism is able, in virtue of its peculiar structure and 

 properties, to carry out reactions or give responses which are not normally 

 produced, and may never have occurred in any previous ancestor. This 

 would be the case if an introduced insect produced a special kind of gall 

 upon a plant, although the two organisms had never before come into 

 contact. Many plants are also able to respond extremely well to chemical 

 substances which do not occur in nature, and which therefore may never 

 have been presented to any ancestor. 



Deformations and abnormal growths indicate, therefore, the reactive 

 powers of the organism, but afford no sure guide in determining problems of 

 phylogeny 1 . Although from a phylogenetic standpoint a foliage leaf may 

 be regarded as a sterile sporophyll, it does not necessarily follow that the 

 early stages in the development of a foliage leaf or even of a petal are such 

 as would lead, if continued, to the production of a fertile sporophyll. It is 

 indeed hardly to be expected that the ontogeny of an organ or of an 

 individual should precisely recapitulate all the winding deviations of its 

 phylogenetic development. Moreover the attainment of any advantageous 

 peculiarity by selection and survival may involve the appearance of new 

 properties which played no part in the progress of phylogeny, since they 

 appear only when it is completed. 



In the progress of development the conditions for the formation of new 

 organs are created according to definite laws. Thus secondary roots are formed 

 after the differentiation of the tissues in the primary root, and appear in definite 

 relation to the positions of the vascular bundles. Similarly, the leaf-primordia 

 develop in some cases immediately at the growing apex, but in Elodea and in 

 other plants at a slight distance from it. The growing apex of a root remains 



1 Goebel, Vergl. Entwickelungsgesch. d. Pflanzenorgane, 1883^.424; Organography, 1900, 

 I, p. 177 ; Sachs, Flora, 1893, p. 233. 



