HELIOTROPISM 467 



leaves of Acacia and of the Myrtaceae, which are arranged so as to be 

 vertical not to the soil but to the branch which bears them, are exposed at 

 right angles to the full midday sun. It is doubtful, therefore, whether the 

 orientation of the leaves is to be regarded in this case as a protection from 

 excessive insolation. It is possible that these leaves are in general not affected 

 at all by light. This is certainly true of many radially constructed leaves, such 

 as those of the pine, Sedum acre, &c., whose fixed light position is such that the 

 upper sides have as often the highest photic ration as the lower sides. 



It is impossible for us to enter into the wider problems connected with 

 the actual photic ration of leaves of different types, or with the relative 

 photic ration of the upper as contrasted with the under side ; we must 

 content ourselves with quoting only one fact from the elaborate researches 

 carried out by Wiesner (1899). This author showed that in addition to leaf 

 types which avoid excessive light by their orientation (articulated leaves. 

 Compass plants) there were also others that achieve the same results by 

 their form. It may be experimentally proved that more light always faUs 

 on a flat leaf than on one which is concave or convex, and hence it may be assumed 

 that the very common occurrence of leaves with uneven surfaces indicates 

 an adaptation for the protection against excessive insolation. 



It will be necessary for us to strictly hmit ourselves in the selection of 

 further illustrations of dorsiventral hehotropic organs. Omitting the consideration 

 of flowers altogether (Noll, 1885-7 ; Schwendener, 1892) let us glance only at 

 certain organs, which are not from the beginning dorsiventral but which become 

 so as the result of the action of external factors. The seedling of the cucumber 

 is orthotropic and positively heliotropic; but when it has reached a certain 

 size a sharp curvature appears above the cotyledons, which is determined by the 

 source of light, but which does not bring the epicotyl into a rest position in the 

 direction of the light, but places it horizontally. The shoot continues to grow 

 in this direction, becoming quite dorsiventral, for roots arise on its under 

 surface (Czapek, 1898 b). We have a counterpart to the cucumber in Hedera 

 helix (Sachs, 1879, p. 272). In this case the shoot is negatively heliotropic 

 and grows in an approximately horizontal direction. Thus seedlings of Hedera 

 and Cucurhita, planted side by side and exposed to unilateral illumination, 

 bend their shoots in opposite directions, and we may watch the development 

 of a perfectly plagiotropic shoot from an orthotropic, negatively or positively 

 heliotropic plumule. Aerial roots also arise on the under side of the stem of 

 Hedera, but this dorsiventrality is by no means inherent and fixed, for, as is well 

 known, it is possible, by altering the direction of the light, to make any flank 

 dorsal or ventral at will, and probably the same is true of Cucurbit a. When 

 rotated on a klinostat, both plants, if illuminated equally all round, are ortho- 

 tropic. Markedly dorsiventral shoots are generally not so, if they be taken out 

 of their rest position they do not attempt under all conditions to regain it 

 by curving or torsion, but endeavour to adapt themselves to the new position 

 by altering their structure. 



Marchantia behaves quite differently. The gemma, from which we 

 may cultivate the plant, is bilateral, and the side which is more strongly 

 illuminated becomes the upper side of the thallus ; the dorsiventrality, once 

 established, cannot be altered by altering the illumination. If the direction 

 of the light be changed, the plant behaves essentially like a foliage leaf, for Frank 

 has shown that not simple curvatures of the thallus only but torsions as well 

 are set up which bring it back to the original light position (compare Sachs, 

 1879 ; Czapek, 1898 a). We may also refer here to the behaviour of certain 

 lateral branches, whose initials are radial, but which, under the influence 

 of unilateral light, become dorsiventral, just as they do when influenced by 

 gravity (p. 453). The leaf insertions may remain unaltered, and only the 

 petioles or the leaf bases induce a twisting of the lamina towards the flanks 



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