ACTION OF LIGHT ON GROWTH IN LENGTH. 837 



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-inhes 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'vvely heliotropic organs'^ only a comparatively small number 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 TropcBolum 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 grovi^th, 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 (Physiologic, II. 145, i83i) and by Wiesner (Heliotropische Erscheinungen, II. p. 5). De 

 Vries states (innere Vorgiinge bei den Wachsthumskriimmungen) 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. bot. Inst, in Wiirzburg, 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.] 



2 Knight, Phil. Trans. 1812, p. 314.— Dutrochet, Memoires, &c., vol. II. p. 6 et s^y — Durand 

 and Payer's statements.— Compare Sachs, Exper.-Phys., p. 41. 



