Chap. VIU. DIAHELIOTKOPISM. 441 



above conditions of illumination. All these movements 

 evidently depend in some manner on the obliquity of 

 the Kght, but cannot be called heliotropic, as this 

 implies bending tovcards the light ; whereas the coty- 

 ledon nearest to the light bends in an opposed direc- 

 tion or downwards, and both place themselves as nearly 

 as possible at right angles to the light. The move- 

 ment, therefore, deserves a distinct name. As coty- 

 ledons and leaves are continually oscillating up and 

 down, and yet retain all day long their proper position 

 with their upper surfaces directed transversely to the 

 light, and if displaced reassume this position, dia- 

 heliotropism must be considered as a modified form of 

 circumnutation. This was often evident when the 

 movements of cotyledons standing in front of a window 

 were traced. We see something analogous in the case 

 of sleeping leaves or cotyledons, which after oscillating 

 up and down during the whole day, rise into a vertical 

 position late in the evening, and on the following 

 morning sink down again into their horizontal or dia- 

 heliotropic position, in direct opposition to heliotro- 

 pism. This return into their diurnal position, which 

 often requires an angular movement of 90°, is analo- 

 gous to the movement of leaves on displaced branches, 

 which recover their former positions. It deserves 

 notice that any force such as apogeotropism, will act 

 with different degrees of power* in the different posi- 

 tions of those leaves or cotyledons which oscillate 

 largely up and down during the day; and yet they 

 recover their horizontal or diaheliotropic position.^ 



We may therefore conclude that diaheliotropic 

 movements cannot be fully explained by the direct 

 action of light, gravitation, weight, &c., any more 



• See former note, in reference to Saclis' remaiks on tliis si bject. 



