468 TRANSFORMATION OF ENERGY 



{Acer, Abies), or photogenic torsions may be set up in the stem {Cornus 

 mas, &c.), which serve the same purpose, i. e. of bringing the point of inser- 

 tion of the leaf on to the flank. 



Our summary of hehotropic phenomena has been brief and that for two 

 reasons. It would be quite superfluous to treat of orthotropic organs in great 

 detail, because in their case the phenomena are generally speaking perfectly 

 obvious, but that is not the case with the plagiotropic organs. The phenomena 

 exhibited by such organs are very complicated, and investigations in individual 

 cases leave much to be desired. What we have said, however, may serve as 

 an introduction to the more important general problems we have yet to study. 

 It was pointed out at the commencement of this lecture that the old 

 explanation of heliotropic curvature was applicable only to orthotropic, positively 

 heliotropic organs and that on that account it must be rejected. We must 

 now investigate more closely what exactly takes place in heliotropic movements, 

 so as to obtain if possible some insight into the mode of operation of the stimulus. 

 Our knowledge of stimulus action has been greatly extended since Darwin 

 (1881) showed that, in certain cases, the heliotropic movement could make itself 

 apparent at a considerable distance from the place which was unilaterally 

 illuminated. Since then Rothert (1894) has repeated and extended Darwin's 

 experiments, and nowadays this field of research is perhaps the most accurately 



studied in the whole range of plant physiology. With 

 /? the view of obtaining some conception of the charac- 



f^ ^^ teristic relations which exist let us observe the behaviour 



of seedlings of Setaria or some other member of the 

 Paniceae group of the Gramineae. As in all members 

 of that order, a primary sheathing leaf is formed above 



I If JII the scutellum, and this leaf we will, for the sake of 



Fig. 148. Seedlings of Se- simplicity, term the cotyledon. It is of a tapering form, 



e'ro?ated'7^diing \i meS and in a short time reaches its definite length of 376 mm., 



age. //, the same curved whilc thc Underlying, somcwhat narrower region, the 



after'bXJ'"onge^^exp''o^sldTo hypocotyl, grows much lougcr and may become 



unilateral illumination. After e_6 cm. in length. In scedliugs of a certain age the 



Rothert, 1894. (About nat. "^ , i j • t ^^ i4.i, u -n, i, * i ^.-n 



size.) cotyledon is full-grown although the hypocotyl still 



goes on growing vigorously. While, at first, growth 

 in the hypocotyl is general through its entire length, later on the base 

 ceases to exhibit any signs of increase, the most vigorous growth being con- 

 fined to a region just below the apex. If such a seedling be illuminated on 

 one side, a sharp heliotropic curving takes place at the apex of the hypo- 

 cotyl, which creeps gradually backwards as far as growth will permit (Fig. 

 148). This curvature makes itself apparent only if the cotyledon be illumi- 

 nated from one side whether the hypocotyl be exposed to light or not. If the 

 cotyledon be shaded and the light be permitted to fall on one side of the hypo- 

 cotyl no heliotropic curving takes place. Hence we may conclude that it is 

 only the cotyledon that is sensitive to the light stimulus, and that it is only 

 the hypocotyl which can carry out the movement. The excitation which the 

 light effects in the cotyledon must be transmitted to the hypocotyl, and 

 the curvature takes place only from such a transmitted excitation. We have 

 thus in this case a definite organ for the perception of the stimulus of light, 

 viz. the cotyledon, and, as Rothert has shown, it is more especially the apex 

 of that organ that is the sensitive part ; on the other hand the motile organ, 

 the hypocotyl, is some distance away from the sensitive organ, and in it the 

 power of perception is entirely absent. From the behaviour of these organs 

 we may draw the further conclusion that perception and heliotropic excita- 

 tion are two distinct phenomena — a point to which we have already drawn 

 attention in speaking of geotropism (p. 444) — which depend on different pro- 

 perties of the protoplasm and which are independent of each other in so far 



