ACTION OF LIGHT ON GROWTH IN LENGTH. 
or may possibly change qualitatively the physiological and morphological nature 
of the newly-formed organs. 
The dependence of growth on light has already been spoken of in general terms 
in Sect. 8 ; and it was there especially insisted on that, in order to avoid serious 
misconceptions, this must be distinctly separated from the question of the part 
taken by light in assimilation. Here also we are concerned only with the processes 
of growth itself, since we always start from the point at which the cells or organs 
concerned have already obtained a sufficient quantit}', or even excess, of formative 
materials. 
It has been already stated that the various parts of the flower grow as readily 
in permanent darkness as in light. Most internodes, on the contrary, as has been 
explained in Sect. 18, grow more slowly when exposed to light on all sides, and 
remain shorter than when growing in the dark ; when the light reaches them from 
one side only, they curve concavely towards the source of light. Other organs how- 
ever, as root-hairs, tendrils, and some internodes, become longer on the side exposed 
to light than on that left in the dark. We have seen also that the leaves of Ferns 
and Dicotyledons soon cease growing in the dark and remain small. These observ- 
ations show clearly enough that different cells and organs are differently affected 
by light as respects their growth. Since the light itself remains the same and there 
is a supply of formative materials, any explanation of these differences must aim at 
showing how it is that the inherited organisation of the plant in each case is affected 
just in one particular way and no otherwise by the oscillations of the ether. It is 
however at present quite impossible to give such an explanation \ since far too little 
is yet known of the phenomena themselves ; the ascertained facts cannot yet even be 
reduced to a general law, especially in consequence of the obscurity which involves 
the behaviour of leaves (see infra) and of negatively heliotropic organs under the 
action of light. If these difficulties, which were referred to in Sect. 8, were solved, 
the organs of plants might be divided in respect of their behaviour towards light into 
three kinds : — (i) those the growth of whose cells is in general independent of light; 
as petals, stamens, fruits, and seeds ; (2) those whose growth is retarded by light ; 
the positively heliotropic organs which become abnormally elongated by absence of 
light ; and {3) those whose growth is promoted by light. To this last category 
would belong negatively heliotropic organs if we could be certain that negatively 
heliotropic organs grew more slowly in darkness than in light. The observations of 
Schmitz^ on Rhizomorphs show, however, that this is not usually the case, for, 
although they are negatively heliotropic, they grow, like positively heliotropic organs, 
more quickly in darkness than in light. 
The question in what manner light affects the mechanism of the growth of 
the cell-wall can therefore, in the present state of our knowledge, have a definite 
^ If Müller, in the second part of his Botanische Untersuchungen (Heidelberg 1872), gives the 
impression of having achieved this with but little difficulty, this only shows how far he is from a 
true method of investigation. 
2 Schmitz, Linnsea, 1843, p. 513. [Similar results have been obtained by Müller (Thurgau) 
with the negatively heliotropic roots of Chlorophytum and of Monstera Lennei (Ueb. Heliotropismus, 
Flora, 1876), and by F. Darwin with those of Sinapis alba (Ueb. das Wachsthum negativ helio- 
tropischer Wurzeln, Arb. d. bot. Inst, in Würzburg, II, 1880).] 
3 H 
