838 MECHANICS OF GROWTH. 



heliotropism, as 1 think, of the thallus of Marchantia and the prothallia of Ferns, as 

 well as of other decidedly bilateral organs. Among organs which are not green must be 

 ' especially mentioned the negatively heliotropic aerial roots of Aroideae and epidendral 

 Orchids ^ ; but, beyond all others, the roots of Chlorophytum guayanum, which are ex- 

 tremely sensitive to light coming from one side. Negative heliotropism has, in addition, 

 been stated to occur in the roots of seedlings of Cichoriaceae, Cruciferae, &c., and has 

 recently been certainly determined by WolkofF in the case of Brasska Napus and Sinap'ts 

 alha. Among unicellular organs destitute of chlorophyll the only ones known at present 

 with certainty to be negatively heliotropic are the root-hairs of Marchantia (Pfeffer). 



The observation that a number of organs destitute of chlorophyll and endowed with 

 negative heliotropism, and in particular the highly sensitive roots of Chlorophytum, are 

 very transparent, led Wolkoff to the hypothesis that the rays of light may be refracted 

 by their cylindrico-conical shape, so as to produce a more intense illumination of the 

 tissue on the side removed from the source of light than on that exposed to it ; and that 

 therefore the concave curvature on the former side is in fact a form of positive helio- 

 tropism. The apices of roots, when separated by a transverse section, if illuminated 

 from one side and viewed from above, exhibit exactly the optical conditions which are 

 assumed by this hypothesis. It must however not be forgotten that the apices of roots 

 which are by no means negatively but at an earlier period even positively heliotropic, 

 like those of Vicia Faba, manifest the same phenomenon, though perhaps to a, lesser 

 degree. Whether, on the other hand, it is possible to suppose a similar refraction of 

 light in the case of the very thin-walled negatively heliotropic root-hairs of Marchantia, 

 is still in doubt. Further researches must show whether Wolkoff 's suggestion is tenable 

 or not. The Rhizomorphs would probably afford good material for observations on 

 this subject, since, according to the researches of Schmitz, they are distinctly negatively 

 heliotropic, and yet they grow more slowly in light than in darkness. 



In the cases of the older internodes of the Ivy, which are only very slightly trans- 

 parent, the older and lower parts of tendrils, &c., the existence of an active focal line 

 on the shaded side cannot be admitted, because this would evidently imply that it 

 included more intense blue and violet light than, from the fact that the tissue which is 

 penetrated by the light contains chlorophyll, it is probable it does. The negatively 

 heliotropic curvature takes place however, at least in the Ivy as well as in the roots of 

 Chlorophytum, only in highly refrangible light (after passing through an ammoniacal 

 solution of copper oacide), not in yellow light (which has passed through potassium 

 bichromate). If, as Wolkoff at one time supposed, the more vigorous nourishment, 

 i.e. accumulation of assimilated substances, were the cause of the more rapid growth 

 on the side exposed to light in this class of negatively heliotropic -organs, they ought to 

 curve much more strongly in the less refrangible (red, orange, or yellow) than in the 

 more refrangible rays. This hypothesis would moreover fail to explain why the same 

 internodes which when young showed decided positive heliotropism, at a later period 

 when their growth has almost ceased manifest the opposite behaviour towards light. 



The experiments which Wolkoff is now (1873) carrying on in the botanical laboratory 

 at Wiirzburg, and which are not yet completed, lead at present to the conclusion that 

 there are two kinds of negatively heliotropic organs. In one kind are included roots, in 

 which the negatively heliotropic curvature takes place near the apex at the spot where 

 growth is most rapid ; to the other kind belong internodes where the negatively helio- 

 tropic curvature takes place only at the older parts whose growth is completed, while 

 the young quickly-growing parts manifest positive heliotropism. In these latter cases 

 the additional peculiarity occurs that the older parts, after being exposed to light on 

 one side, will continue for some time to curve in the dark so that the side previously 

 exposed to light becomes still more convex. This is a property which appears to be 

 wanting in organs of the first kind as well as in those that are positively heliotropic. 



According to a great number of observations of my own and statements of others. 



