838 
MECHANICS OF GROWTH. 
heliotropism, as I think, of the -thallus of Marchantia and the prothahia 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 Brassica Napus and Sinapis 
alba. 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 rnay 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 oxide), 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 Würzburg, 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. 
