The Dia-Heliotropic Attitude of Leaves. 



155 



After the assumption of this position, the pot containing the plant was turned 

 round through 180°. This brought about a new adjustment in the course of 

 twenty minutes, the plane of all the leaflets being once more at right angles 

 to the light. The new adjustment necessitated a complete reversal of the 

 former movements and torsions. Such perfect adjustment is brought about 

 by bright light from the sky, and not so well by direct sunlight, for reasons 

 which will be given later. 



In fig. t, b, is seen the heliotropic adjustment of the leaves of sunflower, grown 

 near a wall, the plant being exposed to light from the western sky. The 

 adjustment is essentially similar to that seen in Mimosa. The lateral leaves, 



Fig. 1. — Dia-heliotropic adjustment of leaves : (a), in Mimosa ; (b), in Helianthus annuus ; and 

 (c), in a different species of Helianthus. (From photographs.) 



1 and 3, have undergone appropriate torsions — right-handed or left-handed — 

 so that the leaf-blades placed themselves at right angles to the light. The 

 leaf numbered 2 has been raised, placing its lamina perpendicular to the light. 

 A contributing factor in this is the bending over of the stem, due to positive 

 heliotropic curvature, which accentuated the rise of the leaf number 2. The 

 same bending often causes an apparent fall of the leaf marked 4. When the 

 stem is tied to a stake, the bending over of the stem is prevented ; the leaf 

 numbered 2 is then found raised by heliotropic action ; but there is little or 

 no fall of the opposite leaf. 



I reproduce (fig. 1, c) another photograph of the heliotropic curvature and 

 adjustment of a different species of sunflower, which was grown in the open. 

 In the morning the plant bent over to the east and all the leaves exhibited 



N 2 



