THE INDUCTION OF DORSIVENTRALITY 149 



Insufficient attention is commonly paid to the facts that a labile induction 

 can be modified or suppressed with or without a transitional reaction, and that 

 in regeneration we have a correlative action in which the remaining parts influence 

 or direct the formative activity of the meristematic cell or tissue which reproduces 

 the missing parts. In the absence of such directive influences the embryonic 

 cells of a tissue may be incapable of reproducing a new organism. 



To indicate the fact that particular embryonic cells or primordia under the 

 same conditions always produce similar organs, it is permissible with His to 

 distinguish between the germinal tracts of different organs, or with Roux to speak 

 of them as mosaic work, even though a variety of dissimilar factors co-operate in 

 inducing the specific differentiation. 



SECTION 43. The Induction of Dorsiventrality. 



Morphological and anatomical dorsiventrality may result partly from 

 the external conditions, and partly from the internal disposition, and when 

 once induced it may either be permanent or reversible l . 



An aitionomic dorsiventrality lies before us whenever a particular 

 orienting agency induces a formation of roots, branches, or hairs more on 

 one side than on the other, as well as when the internal structure is affected. 

 Such dorsiventrality naturally becomes permanent in adult parts, whether 

 it is induced by light, gravity, contact, or mechanical agencies. A radial 

 shoot for example acquires an enforced bilaterality when the stimulus of 

 light causes the leaves to develop only on the lateral flanks of its plagiotropic 

 branches. Conversely a dorsiventral organ may become radial, as occurs 

 when the apex of a dorsiventral shoot of Selaginella develops into an erect 

 sporangiferous cone, and when, in the sympodial branches of the elm, a 

 lateral bud continues the onward growth of the radial main axis 2 . Diffuse 

 stimuli may induce or favour the development of dorsiventrality or of dorsi- 

 ventral organs, as is shown by comparing etiolated leaves and branches 

 with those grown under uniform exposure. Finally it must be remembered 

 that physiological dorsiventrality may exist without rinding expression in 

 the form of visible morphological or anatomical differentiation. 



A good instance of permanent aitionomic induction is afforded by 

 Marchantia, for the isobilateral meristem developed at the growing point 

 has the dorsiventrality of the older parts continually impressed upon it. 

 This power must reside in the cells right up to the growing point, for the 

 smallest piece retains its original dorsiventrality when removed and turned 

 upside down. That the meristem at the growing apex is isobilateral is shown 

 by the fact that the dorsal surface of a gemma is always the one that was 

 most strongly illuminated when it commenced to develop adult tissue. This 



1 Cf. Goebel, Organography, 1900, I, p. 67. 



2 Cf. Goebel, 1. c., p. 73. 



