THE VEGETABLE CELL 147 



treat from light has been shewn with certainty in upward grow- 

 ing parts of plants, are the tendiils of Vitis and ATnpeiopsis 

 qvbinquefolia, as was first observed by Knight (" FMl, Trmtsf' 

 1812, 314 ; '^Physiol. Papers/' 164) ; while other tendrils which I 

 tested in this respect either gave no decided result, or turned to- 

 wards the light (" UeK das Winden der Manken u, Schlingpfian- 

 zm/' 77). 



OhsBT'o. Dutrochet as&erts that the stems of all twining plants have 

 the property of turning away from light. From very numerous obser- 

 vations oB plants with climbing or twmiog stemb, 1 must declare this to 

 be altogether incorrect, for, like other plants, they turn towards the light. 

 But I have no definite experience to enable me to decide whether the 

 hook-like curvatia^e of the end of the stem of Yitis, Coryhia, <fec., and 

 further, the downwaicl direction of the shoots of Fraxinus pendida^ ai^e 

 (as Dutrochet asserts) to be attributed to the influence of light. 



We are quite deficient of anything like a sufficient explanation of the 

 curvature of plants caused by light It is not even made out whether 

 this curvature is a result of an irritability of the cellular tissue, or of the 

 alteration of endosmotic condition of the cells tln'ough the increased eva- 

 poration caused by light. The latter hypothesis seems to be opposed by 

 the circumstance, that these movements occur just as well when the plant 

 is under water as when in air ; at any rate we have at present no evidence 

 that light causes an excretion of water from the submerged parts of 

 plants which it shines on, m in plants which are exposed to air. The 

 curvature does not appear to be any way connected with the presence or 

 absence of the green colour, since the light-avoiding tendrils of the Yine 

 are coloured quite as green as the stems of most plants, and since the 

 roots of certain plants (of Allium Gepa and Allium sativum, according to 

 Durand and Dutrochet) turn towards the light. 



An explanation must, of course, give an account as well of why par- 

 ticular parts avoid light, as of why others cui^ve towards it ; I may, there- 

 fore, pass over the earlier explanations, wliich only refer to the latter 

 point, and many of which are vague in the highest degree, as for ex- 

 ample " light attracts the plants," &c. ; but the explanation given by 

 Dutrochet ("i/^w." ii. CO ; ''Ann des. 8c. nat 3 Ser^' iv. 72) must be 

 touched upon. Dutrochet derived the curvature of the stem and root 

 ' from the supposition, that the cortical cells of the illuminated side lose a 

 portion of their sap in consequence of the known effect of light, to in- 

 crease the evaporation from plants, and the cells therefore contract. It 

 depends then on the structure of the cortical layer whether, in conse- 

 quence of such contraction it curves so as to become concave or convex on 

 the outer surface ; in the former case, the illuminated organ will curve 

 towards the source of light ; in the latter case, in the contrary direction. 

 Kow, Dutrochet asseiiis it is a general rule that the larger cells lie exter- 

 nally m rind of all those stems which curve towards the light, on which 

 account when a strip of such rind is laid in water it curves inwards ; a 

 rind possessing such a structure, in consequence of this, curves outward 

 and draws the stem with it, when it loses part of its sap through the in- 

 fluence of light. On the other hand, all stems and roots avoiding light 

 possess a rind of opposite structure. In criticising this theory we will 



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