268 MODIFIED CIROUMNUTATION. Chap. V 



along the lower side of a part, which is thus caused to 

 bend downwards; and hyponasty is used for the reversed 

 process, by which the part is made' to bend upwards. 

 These actions come into play so frequently that the 

 use of the above two terms is highly convenient. The 

 movements thus induced result from a modified form 

 of circumnutation ; for, as we shall immediately see, 

 an organ under the influence of epinasty does not 

 generally move in a straight line downwards, or under 

 that of hyponasty upwards, but oscillates up and down 

 with some lateral movement : it moves, however, in a 

 preponderant manner in one direction. This shows 

 that there is some growth on all sides of the part, but 

 more on the upper side in the case of epinasty, and 

 more on the lower side in that of hyponasty, than on 

 the other sides. At the same time there may be in 

 addition, as De Vries insists, increased growth on one 

 side due to geotropism, and on another side due to 

 heliotropism ; and thus the effects of epinasty or of 

 hyponasty may be either increased or lessened. 



He who likes, may speak of ordinary circumnutation 

 as being combined with epinasty, hyponasty, the effecta 

 of gravitation, light, &c. ; but it seems to us, from 

 reasons hereafter to be given, to be more correct to 

 say that circumnutation is modified by these several 

 agencies. We will therefore speak of circumnutation, 

 which is always in progress, as modified by epinasty, 

 hyponasty, geotropism, or other agencies, whether 

 internal or external. 



One of the commonest and simplest cases of epinasty is that 

 offered by leaves, wMoli at an early age are crowded together 

 round the buds, and diverge as they grow older. Sachs first 

 remarked that this was due to increased growth along theuppBi.' 

 side of the petiole and blade ; and De Vries has now shown in 

 more detail that the movement is thus caused, aided slightij M 



