/J 



6 MECHANICAL LAWS OF GROWTH. 



of the leaves of Cucurbita Pepo and Nicot'iana Tabaciofi, taken at intervals of three 

 hours, to construct curves of growth, which in spite of adverse fluctuations of temper- 

 ature, rose from evening to morning, attained a maximum after sunrise, and then fell 

 during the day till evening ; exactly what I showed to be the case with positively helio- 

 tropic internodes. If this general law is established, it results that the broad netted - 

 veined leaves of Dicotyledons grow more quickly in the dark than in the light, and are 

 therefore hindered in their growth by light. But when such leaves remain nevertheless 

 smaller in permanent darkness because they cease growing earlier, this must be inter- 

 preted as an unhealthy condition depending on the suspension of certain processes of 

 metastasis which must precede growth and which are induced by light. In conformity 

 with this hypothesis we must suppose that in leaves which unfold under the alternate 

 influence of day and night, growth is directly hindered by light ; but that at the same 

 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 desti- 

 tute 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 experiments have yet been made as to their effect on unicellular organs which ex- 

 hibit positive heliotropism, as Vaucheria-tubes 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 therefore 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 cuticularis- 

 ation 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 Negatively 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 Trcpoeolum 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 

 heliotropism, as I think, of the thallus of Marchantia and the prothallia 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 



* Knight, Phil. Trans, 1812, p. 314. — Dutrochet, Memoires, &c„ vol.11, p. 6 et 557.— Durand 

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



^ According to a great number of observations of my own and statements of others. 



