«34 
MECHANICS OF GROWTH. 
meaning only in reference to positively heliotropic organs ; inasmuch as it is in these 
cases certain that the growth of the cell-wall in the direction of the axis of growth of 
the organ is retarded and limited by light. But even in this case the question cannot 
at present be answered, since several others must first be solved. It must first of 
all be decided whether light acts in this manner on the cell-wall only when its 
plane of incidence is inclined to the axis of growth. A similar problem, as we shall 
see, is presented in the action of gravitation on growth. The various phenomena of 
positive heliotropism allow in fact of the supposition that rays of light which pene- 
trate the cell-wall in a direction parallel to the axis of growth of the organ do not 
hinder growth, while they do so more strongly the more nearly vertical they are to 
it, whether the organ be multicellular or a simple filament. Light therefore acts more 
intensely the more nearly the transverse vibrations of the ether are parallel to the 
surface of the cell-wall. But the solution of these questions would by no means 
explain the action of light on the growth of the cell-wall ; in the first place we must 
know whether light acts directly on the cell-wall, or if the eff'ect is produced by 
means of the protoplasm, or by chemical changes in the cell-sap. But since we 
know that the cell-wall only grows so long as it is in contact on the inside with 
hving protoplasm, and that the protoplasm itself is set in motion by light, in con- 
sequence of which it accumulates at particular parts of the cell-wall (see Sect. 8) ; 
and since this, like the growth of the cell-wall, is caused by the highly refrangible 
rays — the hypothesis must not at once be set aside. The question may moreover 
be asked whether light does not influence the growth of the cell-wall by means 
of chemical effects which it brings about in the cell-sap or the protoplasm, which 
however cannot be referred to assimilation, since they take place even in cells 
destitute of chlorophyll, as for instance in the positively heliotropic neck of the 
perithecium of Sordaria fimiseda, the stipes of Claviceps, and in many roots of seed- 
lings; and since the leaves of Dicotyledons exhibit relations to light {vide infra) 
which indicate a chemical action on assimilated substances, but not on the process 
of assimilation itself. 
So long as we take into account multicellular organs alone and merely contrast 
green and etiolated plants, great weight might be allowed to the hypothesis of a 
change in the turgidity caused by light (brought about by some chemical alteration 
in the cell-sap and the consequent change in diosmose^). But the fact that even 
unicellular tubes like those of Vaucheria and the internodal cells of Nitella are 
positively heliotropic, forbids this hypothesis, since in these cases the side exposed 
to light grows more slowly than the other, although all the parts of the cell-wall are 
subject to the same hydrostatic pressure from the sap. 
The examples already given of positive heliotropism in submerged unicellular 
filaments, as well as the heliotropic curvings of multicellular internodes under water, 
show at once that they have nothing to do with a more rapid transpiration in- 
duced by light or its results. 
The suggestion would appear on the. contrary to be worth more attention that 
the reason why light retards the superficial growth of positively heliotropic cells 
is because it first of all promotes increase of thickness, and therefore diminishes the 
^ See Dutrochet, Memoires pour servir, etc., Paris 1837, vol. II. p. 60 et seq. [This question is 
discussed in the following small print.] 
