754 MECHANICAL LAWS OF GROWTH. 



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 

 tubes, 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 hypothesis would appear on the contrary to be worth more attention 

 whether the reason why light retards the superficial growth of positively heliotropic 

 cells is not because it first of all promotes increase of thickness, and therefore 

 diminishes the extensibility of the cell- wall under the influence of the pressure of 

 the sap on the side exposed to the strongest light. This hypothesis w^ould be 

 confirmed by Kraus's observations, according to which the cuticularising of the 

 epidermis as well as the thickening of the walls of the cortical and bast-cells is in 

 fact materially hindered in etiolated internodes, and the extensibility of these cell- 

 walls consequently increased by the want of light. This explanation would apply 

 not only in the case of the shaded side of a multicellular internode which curves 

 towards the light, but also in that of a Vaucheria-tube or internode of Nitella ; since 

 it may be supposed that the w^all is in the first place more strongly thickened on the 

 side exposed to light and hence becomes less extensible, and therefore yields less 

 to the pressure of the sap, and, in consequence, grows more slowly. We have at 

 present no observations on heliotropic unicellular tubes. 



If then it is proved, as the recent researches of Wolkoff give ground for be- 

 lieving, that the negative heliotropism of organs which contain chlorophyll depends 

 as little as that of roots on the stronger power of assimilation possessed by the 

 side exposed to the source of light, it must be assumed that all the actions 

 which have been mentioned as possible in one direction may take place also in an 

 opposite direction ; and this will show the great difficulty of the investigation. 



A complete account of the mode in which growth depends on light is scarcely 

 possible at present ; what has now been said will call the attention of the reader to the 

 most important questions involved in the investigation. It may be desirable however to 

 collect some of the more important facts at present known, and to add some critical 

 remarks. 



(a) Organs ^vbose growth is retarded by light. To take first the case of those inter- 

 nodes (including, according to Hofmeister, the unicellular ones of Nitella) which, when 

 the light is unequal on the two sides, curve so that the side facing the source of light is 

 concave while the other side is convex, or in other words are positively heliotropic. 

 These exhibit a periodicity in their longitudinal growth corresponding to the alternation 

 of day and night, when the temperature is sufficiently constant. The growth is more rapid 

 from evening to morning, and less so from morning to evening. Both these facts are 

 however consistent with the phenomenon that the same internodes often grow longer, 

 and even considerably so, in permanent darkness than they would under normal conditions. 

 These three results lead naturally to the conclusion that it is the direct action of light 

 (and only in fact of its more refrangible rays, see Sect. 8), which retards the growth of 



See Dutrochet, Memoires pour servir, Paris 1S37. vol. II, p. 60 et seq. 



