MOVEMENTS OF IRRITATION. 513 



If growing leaf-stalks, some of them deprived of their laminas 

 the rest not, are placed in a horizontal position in the zinc box, 

 care being taken that the upper side of the organs is always 

 upwards, then the stalks provided with laminae, if their epinasty 

 is not too great, do not curve at all, or, at any rate, curve much 

 more feebly upwards than those freed from their laminae. Here, 

 then, circumstances of loading have influenced the result of the 

 experiment. Many further particulars respecting these phe- 

 nomena, and also with reference to the determination of the 

 amounts of growth taking place in connection with the curvatures 

 of shoots and leaves, will be found in a valuable memoir of H. de 

 Vries, which deals with the subject. 3 



It has already been mentioned that leaf-blades, leaf-stalks, and 

 many shoots, have the power of growing more energetically on 

 their morphologically upper surface than on their lower surface. 

 This epinastic behaviour of these organs is of great importance in 

 determining their normal plagiotropic orientation, and H. de Vries 

 advocates the view that the epinasty is the result of internal 

 growth determinants. I have been led to other results by investi- 

 gations as to the causes of epinasty in the leaves of Phaseolus 

 multiflorus and Cucurbita. 4 We grow seedlings of Phaseolus and 

 Cucurbita in flower-pots in complete darkness. At a temperature 

 of about 20 C., the hypocotyl of Cucurbita develops to a con- 

 siderable length in about ten days; the cotyledons are directed 

 straight upwards, their upper surfaces being in close approxima- 

 tion. In about fourteen days the epicotyl of Phaseolus has grown 

 to a considerable length, and the long-stalked primordial leaves 

 present a shell-like appearance owing to hyponastic growth, i.e. 

 stronger growth of their lower than their upper surface. If the 

 seedlings are now exposed to bright diffuse daylight, the primor- 

 dial leaves of the Phaseolus seedlings lose their shell-like form, 

 owing to epinastic growth. Similarly, in the cotyledons of Cucur- 

 bita, the upper side now grows more actively, so that they pass 

 from, their original orthotropic position into a plagiotropic one. 

 But it needs the light to call forth the energetic growth of the 

 upper side of the leaf ; in the dark it does not take place. The 

 epinasty is not a spontaneous but a paratonic nutation phenome- 

 non, and in future we shall not briefly speak of epinasty, but of 

 photoepinasty of the leaves. If, after keeping them in the dark 

 till they have reached the stage of development above described, 

 we bring our Phaseolus or Cucurbita seedlings into diffuse light 



P.P. L L 



