LEAF ADJUSTMENT 119 



they lie in two rows in one plane. They reach this position, 

 however, by the inter nodes of the stem twisting through 90 

 degrees, alternately to right and left ; the short stalks of the 

 individual leaves then twist and bring the blades horizontal. 



Leaf adjustment is most striking in plants growing 

 against a wall with strictly unilateral illumination. The 

 garden nasturtium, Tropaohim majiis, is as good a case as 

 can be found. Its leaves are spirally arranged, but when the 

 plant grows against a wall the petioles curve and twist 

 so that all the leaves come to lie to the front of the stem ; 

 the angle which the peltate blade makes with the stalk, and 

 the inclination of the apical part of the stem, bring the 

 blade accurately normal to the incident light. 



Quite different is the relation of the grass-like type of 

 leaf to light. It is typically long and narrow, and it stands 

 more or less upright. In the relatively small surface exposed 

 in the upright position, and in its small powers of adjust- 

 ment, it appears to be less efficient than the broad-leaved 

 type. But two points must be kept in mind. Such plants 

 very often grow in crowded communities, and the narrow 

 leaf, allowing the penetration of light through the mass of 

 vegetation right to the ground, must be an important factor 

 in making this type of community possible. The upright 

 leaf, too, utilises horizontal rather than vertical light. The 

 anatomical structure of the grass leaf, with assimilating 

 parenchyma on both faces, and the stomata more or less 

 equal in number on the two surfaces, emphasises this 

 relation to light. An extreme case is offered by the Iris, 

 with its double rank of bifacial leaves. This arrangement 

 in a broad-leaved plant would be very inefficient ; but when 

 the light utilised is mainly horizontal the arrangement takes 

 on a different aspect. 



§ II. Mechanism of Leaf Adjustment 



To alter their primary position and come into relation 

 with the incident light, the leaves must of course carry out 

 definite movements. Unequal illumination is the stimulus 



