466 TRANSFORMATION OF ENERGY 



Certainly that is not so in all cases. A not inconsiderable number of leaves, 

 as we have already seen, are able to carry out geotropic curvatures, by altera- 

 tion of turgor only, in special articulations, and without growth, and these 

 articulations are capable also of exhibiting heliotropic responses. In Robinia 

 pseudacacia, for instance, the main petiole and its articulation remain almost 

 stationary, while the leaflets exhibit continuous movements during the day. 

 In light of weak intensity the leaflets take up the same positions that we 

 have seen them assume in movements occasioned by growth ; if we imagine, 

 for the sake of simplicity, that the chief petiole stands horizontally and that the 

 strongest diffuse light falls on it from above, we shall find that all the leaflets 

 arrange themselves horizontally also ; if, however, the light strikes the leaf in 

 front in the same direction as that in which the petiole lies, the leaflets twist 

 round at their articulations through an angle of 90, so that they may adjust 

 their surfaces vertically at right angles to the incident ray. If a bright light, 

 e. g. direct sun's rays, be allowed to fall on the leaf from above, we find that 

 an entirely new phenomenon makes its appearance, for the horizontally placed 

 leaflets elevate themselves, and each forms with its opposite neighbour an 

 angle of 90 or less, instead of 180, so that the upper surfaces approxi- 

 mate. No matter what be their position the leaflets execute movements, so 

 that they make with the incident ray a very small angle or even turn their 

 edges to it. We have thus two extreme positions to consider, a surface position 

 adapted to feeble light and a profile position for strong light. These move- 

 ments must have some purpose, whether over-bright light is injurious or 

 whether the object is only to avoid excessive transpiration. The intermediate 

 positions are especially purposeful, since they obviously render possible the 

 absorption of an amount of light adapted to its intensity. 



Leaves which have no articulations are, however, unable to alter their space 

 relationships continuously, hence they assume, during their growing period, a 

 fixed light position, which is determined not by direct sunlight but by the 

 strongest diffuse light; to this light the leaf -blades place themselves at right 

 angles. Such a position insures that, on dull days at least or in shady situations, 

 a maximum amount of light shall be absorbed, while direct sunlight never 

 appears to injure these leaves, simply because it is constantly changing its 

 direction during the course of the day. In certain plants, however, the so-called 

 'Compass plants' (SxAHL, 1881), the fixed light position is determined by 

 direct sunlight. These plants, among which the indigenous Lactuca scariola 

 may be included, show the ordinary leaf orientation in shady situations, but 

 in exposed places their leaves perform certain bendings and twistings so as to 

 turn the laminae vertically and approximately north and south. The leaves 

 thus stand in profile at midday, while in the morning and evening the surfaces 

 are exposed to the incident rays. So far as the leaves are concerned which 

 arise on the north or south aspects of the stem, a torsion at the base is sufficient 

 to bring them into the profile position ; those pointing east and west, on the 

 other hand, have to carry out complicated movements in order to attain the 

 vertical N-S position, for they do not rest content with merely bending upwards 

 so as to press their upper surfaces against the stem, but exhibit curving on 

 their midribs as well, northwards or southwards. 



The leaves of the Compass plants in the mature condition are not dorsi- 

 ventrally constructed, their east and west surfaces have the same structure. A 

 bilateral construction and an accompanying vertical position of the leaf surfaces 

 occur very frequently, e. g. in Iris, and in many New Holland species of Acacia 

 and Myrtaceae. In the case of Iris, the vertical position of the leaf, although 

 the orientation is not associated with definite points of the compass, is never- 

 theless an adaptation by which the brightest rays, at least in the height of 

 summer and in southern lands, do not fall on the lamina so completely as 

 would be the case were the blade horizontal ; on the other hand, individual 



