140 PLANT PHYSIOLOGY 



If unilateral light be allowed to impinge horizontally, seedlings also take up 

 a completely horizontal position if the light be of a certain intensity (WiESNER, 

 1878). Since, under weaker illumination, they do not succeed in reaching 

 this plane, but grow obliquely upwards, we must assume that geotropism is 

 co-operating in the movement, and that the light stimulus is too weak to 

 overcome completely the stimulus of gravity. Autotropism, however, has 

 something to do with the result here as well as in geotropic curvature, and it 

 also might aid in preventing the shoot from orientating itself fully in the 

 direction of the incident ray. The case is otherwise with negative curvatures 

 induced by intense light, as OLTMANNS has observed in Phycomyces. Here 

 the plants always grow obliquely, and it is very probable that it is due to the 

 fact that they are no longer ortho tropic but plagio tropic. There are other 

 organs, however, which always behave plagiotropically, and of these we may 

 select foliage-leaves as an illustrative example. 



465, 11. 12-13, for These phenomena . . . Fig. 147. read Not all plants behave 

 in this way. In Calystegia it may be seen that the leaves placed on the stem 

 laterally and posteriorly (as regards the incident light) twist and curve their 

 petioles until the laminae succeed in regaining the same position in relation 

 to light as those which arise on the front of the stem. 

 11. 44-9, delete we need only ... p. 438). 



468, 11. 11-13, delete It was pointed out . . . rejected. 



11. 53-5, for and heliotropic ... (p. 444) read and excitation, which are 

 regarded previously as identical, are in this case two distinct phenomena 



470, 1. 6, after excitation, read What we know as to the transmission of the 

 stimulus in the Gramineae may be also applied to other relations. At present 

 it need only be noted that this transmission is doubtless carried out by living 

 parenchyma (ROTHERT, 1894 ; FITTING, 1907). One must suppose that the 

 intercellular protoplasmic threads are the media for the transference from 

 cell to cell ; the special fibrillae described by NEMEC (1901) cannot play any 

 important part in the conduction of the excitation (HABERLANDT, 1901). 



11. 36-49, for The perceptive power. . . . Neither is it true read According 

 to HABERLANDT'S (1905) researches, the movements in the petiole in other 

 plants, e.g. Tropaeolum, Humulus, &c., are to be accounted for by the per- 

 ceptive powers of the lamina. Probably (KoHL, 1894) the negative helio- 

 tropism of many roots is to be attributed to perception in their apices. It is 

 not the case, however, 



471, 11. 10-31, for in this case . . . establish the fact read i. e. purely mechanic- 

 ally, and this possibility must not be entirely excluded from consideration, 

 seeing that, according to MAXWELL, a pressure amounting to about 0-5 mg. 

 per sq. m. is induced in any medium through which a light wave is propagated, 

 in the direction of the path of propagation. The existence of this pressure 

 has been more recently confirmed experimentally by LEBEDEW. Apart altogether 

 from its limited amount, however, there are other reasons for doubting whether 

 heliotropic perception is at all dependent on this pressure. It is more probable 

 that light induces certain chemical changes ; but an action like that which 

 takes place in silver salts is inconceivable, because the red rays, which perform 

 no function in photography, were found to be active in WIESNER'S experiments. 

 It is certainly possible that heliotropic curvature in red rays is a phenomenon 

 sui generis, which may be compared with thermotropic processes, which we 

 shall consider later on (Lecture XXXVII) ; and if that be so, heliotropism 

 might be, in the restricted sense, included amongst those light effects which, 

 for the sake of brevity, may be termed photographic. On the other hand, it 

 is also conceivable that definite chemical reactions are rendered possible 



