ACTION OF LIGHT ON GROWTH, y ^y 



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

 with certainty to be negatively heliotropic are the root-hairs of IVIarchantia. 



The remark 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 such differences of luminosity as 

 might be expected on 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 helio- 

 tropic, 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 Mar- 

 chantia, is still in doubt. Further researches must show whether Wolkoff's happy idea 

 is tenable or not. 



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 actual focal line 

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

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

 penetrated by the light contains chlorophyll, it is probable they do. 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 oxide), not in yellow^ light (which has passed through potassium 

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

 /. 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 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 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. 



It is evident that we are here confronted with an unsolved problem; and when all the 

 facts have been taken into consideration, the theory that there are two kinds of cells, 

 the growth of one of which (positively heliotropic) is retarded by light, whilst that of the 

 other kind (negatively heliotropic) is promoted by it, may be the simplest and most in 

 accordance with facts. This difference is the less remarkable since in the behaviour of 

 growing cells with respect to gravitation we find a precisely similar difference, but much 

 more strongly marked ^ 



* Schmitz, Linnrea, 1843, p. 513^/5^7. If, as can scarcely be doubted, Schmitz's statements 

 with regard to Rhizomorphs are confirmed, it results that no certain inference can be drawn as to 

 the positive heliotropism of an organ from the fact that its growth is more rapid in the dark. We 



