ACTION OF LIGHT ON GROWTH. 75^ 



of the phenomena themselves ; the ascertained facts cannot yet even be reduced to 

 a general law, especially in consequence of the obscurity which involves the action 

 of light on leaves and on negatively heliotropic organs. If these difficulties, which 

 were referred to in Sect. 8, were solved, the organs of plants might be divided in 

 respect of their behaviour towards light into three kinds: — (i) those the growth of 

 whose cells is in general independent of light ; as petals, stamens, fruits, and seeds ; 

 (2) those whose growth is retarded by light; the positively heliotropic organs which 

 become abnormally elongated by absence of light; and (3) those whose growth is 

 promoted by light. To this last category would belong negatively heliotropic organs 

 if we could be certain of the relation in which negative stands to positive heliotropism ; 

 whether, as has elsewhere been mentioned, it is not, at least in many cases, a modi- 

 fication of the positive form depending on the chemical action of light which 

 is essential to growth ; although recent researches render this very improbable. 



The question in what manner light affects the mechanical laws of growth of 

 the cell-wall can therefore, in the present state of our knowledge, have a definite 

 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 

 incidence is oblique 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 supposiUon that rays of light which penetrate 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 tube. 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 quesdons 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 indirectly 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 living protoplasm, and that the protoplasm itself is set in 

 motion by light, in consequence 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 efi'ects 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 stems of Claviceps, and in many 

 roots of seedhngs ; 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, 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 



3 c 



