422 Hit. P. DAE WITT ON THE POSITION OF 



play an important part in the formation of the balance between 

 opposite tendencies just alluded to, These forms of growth are 

 epinasty and hyponasty. An organ is said to be epinastic, in 

 De Vries's sense, when the longitudinal growth of the upper 

 half exceeds the growth in length of the lower half; epinasty, if 

 unopposed, will therefore result in the organ becoming convex 

 above and curving downwards. Hyponasty is the reverse of 

 epinasty ; that is to say, the organ curves upwards, owing to the 

 lower increasing more than the upper half in length. 



Frank subsequently replied to De Vries's criticism, and was 

 again answered by De Vries. I shall not attempt to give any 

 abstract of this discussion, though some points in it will be 

 touched on later. 



I have purposely given De Vries's theory in the simplest and 

 crudest form, because this suffices to show the contrast between 

 it and Frank's views ; and this kind of exposition is enough to 

 give an idea of the point from which I began my work. De Vries 

 is careful to state that his views are only put forward as possible 

 solutions of the problems. He points out the coexistence of a 

 number of opposing growth-tendencies, and suggests how they 

 might produce equilibrium. He does not intend his views to be 

 applied to any special cases. He points ■■ out that a quantitative 

 estimate of the opposing tendencies would he necessary before 

 any special explanations could be possible. 



The same principle which De Vries has formulated finds a 

 prominent place in Sachs's paper, " Ueber orthotrope und pla- 

 giotrope Pflanzentheile " *. Thus, in explaining the power which 

 Marchantia has of placing its thallus-lobes at right angles to 

 incident light, he says (p. 239) that the plagiotropism (here equi- 

 valent to obliquity) of Marchantia may be regarded as a posi- 

 tion resulting from negative geotropism, heliotropism (on the 

 underside), and epinasty on the upper or light side. In the 

 same way (p. 276), speaking more generally, he describes an organ 

 as plagiutropic when it is negatively heliotropic, and at the same 

 time negatively geotropic. The negative heliotropism seeks to 

 bend it downwards, the negative geotropism upwards; thus, 

 according to the specific sensitiveness to gravitation and light, 

 and according to the intensity and direction of the latter, the 

 organ will take up an oblique or horizontal position of equili- 



* 'Arbeiten des bot. Institute in Wurzburg,' Bd. ii. Hft. ii., 1879. 



