472 TRANSFORMATION OF ENERGY 



We may rather inquire as to the origin of the stimulus which brings about the 

 perception, and here we meet with two alternatives, ' Does the plant perceive 

 the direction of the light rays or the difference between the illumination on either 

 side of its body ? ' As a matter of fact the side of the plant turned towards 

 the source of light must be more brightly illuminated than the side turned away 

 from it, and if the long axis of the plant be parallel with the light ray ah 1 sides 

 will be equally illuminated. For simplicity let us limit ourselves to ortho- 

 tropic organs only. SACHS'S hypothesis has never been proved, and the 

 facts which are recorded in MULLER-THURGAU'S memoir (1876), and on which, 

 according to SACHS, these views are founded, appear to us to support equally 

 well the other view. The heliotropic curvature exhibited by unicellular 

 and very translucent plants or plant organs (such as Fungi or roothairs) appears 

 at first sight most in harmony with the facts on which SACHS'S theory is based, 

 for in these cases it may be affirmed that there is practically no difference in 

 the light intensity on the concave and convex sides, seeing that the amount 

 of light lost by absorption in the cell is not worth considering. The difference 

 has not, however, been measured, nor do we know how great it must be in 

 order to lead to perception in the plant. 



More recently SACHS'S views have been vigorously attacked by OLTMANNS 

 (1892). This author placed the plants on which his experiments were made 

 in a box into which light entered from one side only, the light being direct 



sunlight. The light was made to pass first of 

 all through a hollow glass prism filled with 

 gelatine tinted with indian ink. When the 



apparatus was arranged as is shown at Fig. 151, 

 the sunlight struck the 



prism at right angles, 



-. and hence the light rays passed (in the direc- 

 tion of the arrows) into the space below, 

 parallel to each other, while the intensity of the 

 Fig. 151. OLTMANNS' apparatus. GI, light obviously diminished from left to right, 

 glass vessel containing the plants to be Behind the prism straight -growing filaments 



experimented on ; TK. prism. 1 he direction /. T7 -, . f ijj-j. t j 



of the light and its intensity are indicated by of Vauchena Were placed, and it Was found 



the arrows. after several hours that those which were sub- 



jected to light of medium intensity had re- 

 mained quite straight, whilst those to right and left had curved apically towards 

 them. The curvatures took place in a plane parallel to the outer wall of the 

 box, at right angles to the direction of the rays, positive or negative according 

 to the intensity of the light. Unfortunately the distribution of the light 

 intensity and the course of the rays were as simple as that described only 

 in a few of OLTMANNS' experiments, and even in this one it is still a matter 

 of doubt whether the rays do not also travel in the same directions as those 

 which the curvatures followed. Although, doubtless, a decrease in the intensity 

 of light may theoretically be obtained independently of the direction of the 

 rays, in practice, in the most carefully planned experiment, owing to the 

 reflection of the light from the walls of the vessel, the dust particles, and 

 from the plant itself, deviations must occur. 



DARWIN (1881) also carried out a research, which aimed at proving that 

 heliotropic curvature does not take place in the direction of the rays. In order to 

 carry out this experiment most appropriately a seedling of Setaria is illuminated 

 on two sides, which we shall term ' right ' and ' left ', by parallel beams of light 

 of equal intensity, under which conditions naturally the seedling remains 

 uncurved. If the whole of the posterior half of the photosensitive cotyledon 

 be darkened by means of a tinfoil cap while light is permitted to penetrate 

 the anterior half as before, one would expect, if SACHS'S view were correct, 

 that no heliotropic curvature would take place. As a result of DARWIN'S 



