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TRANSFORMATION OF ENERGY 



and visited by insects for purposes ot cross-pollination, whilst it is preferable 

 that the fruits should be sheltered from sunlight. Without proving it in detail 

 it may be generally accepted that heliotropic movements have in each case 

 some definite purpose to fulfil. 



So far we have considered only unilateral illumination. In nature such a 

 condition of affairs is best seen in the case of plants which grow against a wall or 

 under trees or bushes, and in these heliotropic curvatures are specially observable. 

 At the same time freely exposed plants may be expected to exhibit heliotropic 

 curvatures also, since, as the sun circles round them, the north side of the plant 

 will receive markedly less light than the south side (at least in our latitudes). 

 We might expect that such plants would curve towards the brightest light, 

 i. e. towards the south, or that they would take up different positions during 

 the course of the day, following the course of the sun. WIESNER (1901) has, 

 as a matter of fact, shown experimentally that heliotropic curvatures may 

 appear with sufficient rapidity to enable the plants to follow the sun's course 

 in the heavens. When he allowed seedlings of Vicia 

 sativa to revolve once in twenty-four hours, the plants 

 being unilaterally illuminated through a slit in an 

 apparatus designed for the purpose, he found that, not- 

 withstanding the rotation, the seedlings bent continu- 

 ously towards the slit. In nature, however, we find 

 that the plant neither follows the sun's course nor does 

 it bend towards the south. The reason for this is that 

 even direct sunlight is too bright to bring about helio- 

 tropic curvature ; only diffuse, not direct, sunlight has 

 the power of inducing heliotropic movements, as 

 WIESNER has again and again proved. 



It was already noted that in WIESNER'S ex- 

 periments (1878) the apices of the seedlings arranged 

 themselves exactly parallel with the direction of the 

 incident ray only when the intensity of the light reached 

 a certain degree, and so, as we may assume, attained 

 the position of heliotropic equilibrium ; in other cases 

 they formed larger or smaller angles with the horizontal, 

 and we cannot say whether this deviation from the 

 position of equilibrium is due to autotropism or whether 

 it is really the resultant of the combined action of geo- 

 tropic and heliotropic stimuli. Both views are possible, 

 though the former seems to us to be the more probable ; this at least is certain, 

 that autotropism acts in opposition to heliotropism, and can be neutralized 

 only when the stimulus reaches a definite amount. There is a third possibility, 

 the structure in question may not be orthotropic but plagiotropic. This is 

 certainly not the case with the principal axis, but many lateral branches, 

 although radial in structure, are plagiotropic. Since, however, there are no 

 investigations recorded on this subject which enable us to settle the question, 

 we may turn at once to dorsiventral organs whose plagiotropism undoubtedly 

 bears a definite relation to light. The leaves of the higher plants will serve 

 as an illustration of this relationship. 



Foliage leaves have, in the great majority of cases, a quite distinct dorsiven- 

 tral structure, since theupperside normally receives a greater amount of light than 

 the lower. The light position of the leaf -blade demonstrates itself very clearly when 

 we examine a simple case, where diffuse light plays upon one side only or at least 

 affects one side more than the other. If the strongest diffuse light is directly 

 overhead, as is the case in a wood (WIESNER, 1899), the leaf -surfaces are exactly 

 horizontal. If, on the other hand, it be directed horizontally, the leaves must 

 behave very differently according to their positions on the axis, if they .are to 



Fig. 147. Heliotropic curva- 

 ture exhibited by the leaves of 

 Polygonumfagopyrum. The 

 arrows indicate the direction 

 of incidence of the light rays 

 After FRANK (Lehrbuch der 

 Botanik). 



