Day: position of dorsiventral leaves. 751 



fore, cannot induce epinasty or hyponasty. This is in direct 

 support of the position taken by Vines, and the results upon 

 which it is based demonstrate that the photo -epinasty of Detmer 

 does not exist as such. 



II. All dorsiventral leaves are diaheliotropic. 



III. Dorsiventral leaves may be diageotropic or apoge- 

 otropic. Radical leaves of Lactuca, Taraxacum and Nicoiiana 

 are apogeotropic, and those of Helianthus and Arisaema are 

 diageotropic, a conclusion not in agreement with the results 

 of Vines, who maintains that gravity may exert in such organs 

 a diageotropic effect only. 



IV. The ultimate position of dorsiventral leaves is a fixed 

 light position, and the geotropic or trophic tendencies find no 

 mechanical expression. The removal of the light stimulus 

 from a plant, allows the unimpeded action of the other two 

 forces. In some instances geotropism, in others, the trophic 

 tendency, predominates. In no instance, however, has a re- 

 sultant position, due to a mechanical equivalency of the two 

 reactions, been observed. 



V. The relative values of the geotropic and trophic tenden- 

 cies are such that apogeotropism is generally stronger than 

 hyponasty and epinasty, while epinasty or hyponasty are in 

 turn stronger than diageotropism when occurring in the same 

 organ. 



VI. It. is not possible to foresee the reaction of dorsiventral 

 organs with reference to their present form or function. The 

 geotropic tendency must have undergone serious alteration 

 during the period of development of the species which resulted 

 in the formation of rosettes, however. In these, as well as in 

 stem leaves, the causes lie beyond the present circle of investi- 

 gation. 



