EPINASTY AND HYPONASTY 305 



fact the correlations demand changes in position or organs in 

 different stages of the development of the axis, or of the same 

 organ under different external conditions. Thus it is to be seen 

 that leaves unfolding from the bud may hold a vertical position 

 at first, which is changed to a horizontal position during functional 

 activity, when they are also subject to the directive influence of 

 external agents such as light and gravity. Leaves formed later 

 may retain their vertical position and form a protective covering 

 from the growing point of the stem to which they are appressed. 



Such correlations prevent the interlocking and entanglement 

 of the various organs, and allow their anisotropic positions, in re- 

 sponse to the external directive factors, to be taken with least 

 effort. 



It is also probable that correlations of this kind are active in 

 regulating the movements of organs of plants placed in contiguity. 

 Recent results obtained by F. G. Smith show that the plant does 

 not allow its branches to be irregularly entangled with those of 

 neighboring individuals. 1 



Movements of correlation are most marked in dorsiventral 

 organs, and are caused by the unequal growth of the opposite 

 flanks ; the excess of growth on the upper side of an organ be- 

 ing termed epinasty and on the lower side hyponasty. 



390. Epinasty and Hyponasty. Observe the form of fronds of 

 any fern that have just emerged from the soil, and note the coiled 

 position of the apical portion. The fronds are seen to be hypo- 

 nastic and then epinastic. Make similar observations on seedlings 

 of Allium or any cucurbitaceous plant, noting the changes in the 

 growth of the two sides. The form assumed in response to epi- 

 nasty or hyponasty in these instances is for the purpose of 

 penetrating the soil without damage to the more delicate por- 

 tions of the plant. Modifications of the positions taken may be 

 induced by the cultivation of the seedlings in darkness, setting up 

 new and different stimuli. 



1 Smith, F. G. A peculiar case of contact irritability. Bull. Torr. Bot. Club. 

 27 : 190. 1900. 



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