272 PLANT PHYSIOLOGY 



have flowered and seeded. In every way they behave alike. 

 It would seem then that these plants require, as the stim- 

 ulus to form flowering branches, an intensity of light which 

 is more than sufficient to maintain the vegetative activi- 

 ties at a perfectly healthy pitch. 



Vochting further shows that various degrees of illumina- 

 tion above what is sufficient for the vegetative processes, 

 but insufficient for those connected with sexual reproduc- 

 tion, will produce effects exactly corresponding to the de- 

 gree of illumination. Light enough to induce the formation 

 of an erect branch and its growth to a height of several 

 centimetres may be insufficient to induce the formation of 

 any flower-buds upon it. Still stronger illumination will in- 

 sure the formation of flower-buds and their attaining a very 

 considerable size, but will not induce them to open. The 

 bearing of this fact on the formation of cleistogamous 

 flowers Yochting considers very important. Such a degree 

 of illumination, instead of inducing the perfect development 

 and opening of flower-buds, will stimulate the vegetative 

 buds in the axils of the pairs of leaves, just below the 

 flowering part of the erect branch, to develop. Still stronger 

 light, inducing the opening of the flower-buds, if still in- 

 sufficient, is indicated by the small size of the flowers or 

 by the rudimentary condition of some of the floral organs. 

 The corolla, and in two-lipped flowers the upper lip, give 

 evidence by their imperfect development of slightly in- 

 sufficient light, while to suppress or to reduce the cal^yx 

 still less light must fall upon the plant. The essential 

 organs, stamens and pistils, are least dependent upon light. 

 However, the flower as a whole, all of its parts, and even 

 the branch which bears it, are dependent for their forma- 

 tion upon illumination of sufficient intensity and duration. 



The light evidently acts as a stimulus, inducing certain 

 effects, for if the formation of a flowering branch has be- 

 gun, its growth will continue for a time even in darkness. 

 If the formation of flowers on the branch has begun, they 

 too will continue to grow in darkness. But to insure the 

 perfect development of the flowers of most plants, suffi- 

 ciently intense illumination, repeated with sufficient fre- 



