204 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 



the experience of the plant with respect to the nature, intensity, and 

 direction of the rays which have impinged upon them. It would 

 hardly be justifiable to say that light has originated or caused 

 dorsiventrality in the vegetable kingdom : the causes must lie deeper 

 and be infinitely more complex. Light of course has been one of 

 the complex conditions to which dorsiventrality is a primary and basic 

 developmental adaptation. Given the capacity of dorsiventral or- 

 ganization or development however in individuals, and its occur- 

 rence is often directly subject to the determinative and inductive 

 action of illumination. It is to be said in this connection that the 

 use of the term " directive " to designate morphogenic influences 

 exerted by light, as has been done by Goebel, is withal, not in harmony 

 with current usage, this term having long been applied to the action of 

 the rays in inducing phototropic, photeolic and photolytic movements 

 of the axes of the shoot and its members or of the body in general. ^^^ 



Chief among the determinative influences exerted by light are to 

 be mentioned the anatomical differentiations which may ensue as a 

 result of its action, by which an organ may become dorsiventral and 

 the positions of the dorsal and ventral surfaces altered. It is well 

 known however that any form of dorsiventrality once assumed by the 

 body or any of its members may not be changed, or reduced by altered 

 conditions of illumination. A second phase of induced bilaterality 

 is that in which organs are induced or suppressed upon comple- 

 mentary surfaces. This form of symmetry is often directly reversible 

 by changed conditions of illumination, particularly among thethallus- 

 like forms of the lower plants. Not only does the illumination de- 

 termine the relative position of the dorsal and ventral surfaces, but it 

 may also guide the polar differentiation, the apex and base of plant- 

 axes being formed in developing spores with respect to the direction 

 of the rays. 



The association of different developmental stages of a plant with 

 various intensities of illumination and the hindrance of procedure 

 in every other instance in which the intensity of the light is beyond 

 certain limits is a somewhat more complicated and delicate mani- 

 festation of the determinative influence of light upon the induction or 

 suppression of organs. The action in question must be purely stimu- 

 lative in its character, and for every stage an optimum, maximum 

 and minimum of intensity might theoretically be established. 



1^' Goebel. Influence of Light. Organography of Plants. Eng. Ed., 227-259. 1900. 



