ACTION OF LIGHT ON GROWTH IN LENGTH. 
' time certain chemical changes take place which in general make growth possible, and 
- enable it to continue in the succeeding darkness, if it does not last too long. That this 
has nothing to do with assimilation is shown by Batalin's experiments with leaves 
destitute of chlorophyll. 
If we now enquire what are the mechanical changes which light causes in the organs 
we have been considering, and by which their growth is retarded, it is to be regretted 
that no attempt has yet been made to study them in unicellular organs which exhibit 
positive heliotropism, as Vaucheria-txxhes and internodes of Nitella, since they present 
the most simple case from a mechanical point of view. In the case of the internodes 
of Phanerogams which consist of tense layers of tissue, Kraus found in the etiolated 
state a smaller tension between the medullary and cortical layers, and that the cell- 
walls of the layers of tissue placed in a state of passive tension by the pith were less 
thickened, lignified, and cuticularised. It follows that these last are more extensible than 
in the normal internode, and therefore offer less resistance to the tendency of the pith 
. to elongate. If we suppose that in unicellular tubes light also increases the cuticularisation 
and thickening of the cell-wall, the wall will offer greater resistance to the pressure of the 
cell-sap, will become less stretched, and will therefore grow more slowly ^ 
But little can be inferred as to the mechanical influence of light on growth from the 
changes in the tension of the tissues on the convex and concave sides of internodes with 
positive heliotropic curvature. If such an internode is split lengthwise so that the side 
exposed to light is separated from the other side, the former becomes more concave, 
while the latter becomes less convex or even somewhat concave towards the shaded side. 
In other words, the tension between the outer and inner layers is greater on the concave 
- side exposed to light than on the convex shaded side. But the same phenomenon 
' occurs also in internodes with an upward geotropic curvature, and with negatively 
heliotropic internodes, as well as with twining tendrils; and could not in fact be 
otherwise. 
(b) Of Negat'fvely heliotropic organs'^ only a comparatively small num.ber are at present 
known. Among those which contain chlorophyll may be named the hypocotyledonary 
portion of the stem of the seedling of the Mistletoe, the older nearly mature internodes 
of the Ivy and Tropceolum majus, and the basal portions of the tendrils of the Vine, Vir- 
ginian Creeper, and Bignonia capreolata, I pass over at present the doubtful negative 
^ [With reference to the action of light upon growth, it is now universally admitted that the 
effects either of retardation or of curvature which it produces are the expression of modification of 
the turgidity of the growing cells. Three suggestions have been made as to the way in which this 
modification is brought about ; (i) by a change in the elasticity of the cell-wall, (2) by a change in 
the osmotic properties of the cell-sap, (3) by a change in the permeability of the protoplasm. Some 
evidence in favour of the first of these is given above in the text, and it is further supported by 
Pfeffer (Physiologie, II. 145, i88t) and by Wiesner (Heliotropische Erscheinungen, II. p. 5). De 
Vries states (innere Vorgänge bei den Wachsthumskrümmungen) that ' external and internal causes 
produce curvatures in growing multicellular organs because they promote the formation, in the cells 
of one side of the organ, of substances which are osmotically active.' For the arguments in favour 
of the third suggestion see Vines, The Influence of Light on the Growth of Unicellular Organs, Arb. 
d. hot. Inst, in Würzburg, II, 1878. 
It is impossible to enter here upon a detailed criticism of these various views. The following 
remarks must suffice. With reference to No. 1, it is difficult to understand how it can be satisfac- 
torily applied to explain the action of light upon unicellular organs, seeing that the effects are so 
rapidly produced, nor is it clear how both positive and negative heliotropism can be explained by 
means of it. No. 2 clearly cannot account for positive and negative heliotropism in the case of 
unicellular organs, and its application to the case of multicellular organs is not obvious. As to No. 3, 
we know at least that light does act upon the protoplasm of zoogonidia, chlorophyll-granules, etc., 
and there seems to be no reason why it should not act directly upon that of growing cells.] 
^ Knight, Phil. Trans. 1812, p. 314. — Dutrochet, Memoires, &c., vol. II. p. 6 et seq. — Durand 
and Payer's statements. — Compare Sachs, Exper.-Phys., p. 41. 
