672 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



150 grms. The weight was subsequently increased to 400 grms., without 

 injury.* 



One of the best illustrations is seen in climbing plants,t of which 

 Darwin gave a good instance, with figures of the altered petiole of 

 Solanum jasminoides, the leaf-stalk of which has normally a small horse- 

 shoe-shaped mass of wood below, and two isolated cords above — that is, 

 as seen in a cross-section. But, after having caught permanently hold 

 of some object^ the "horse-shoe" becomes a complete "circle," of 

 course, a cylinder of wood running up the petiole, just as in a stem.J 

 (Figs. 276, 277.) 



If a section of an Ivy stem be taken from a plant growing against a 

 wall, and another of the same diameter be cut from the part of the Ivy 

 growing freely in the air, it will be found that the amount of wood in 

 the latter is appreciably greater and the pith less than in the former, 

 since it is self-supporting and not aided by the adhering roots. 



Another interesting case is that of Wistaria. This is usually grown 

 as a climber, but some florists have cultivated it as a standard. When 



Fig. 277. — A, Trans, sect, of petiole of Solatium 

 jasminoides before ; and B, after attachment to 

 a support. (After Darwin.) 



this is the case it does not make long annual shoots as its energy is 

 expended in increasing the thickness of the trunk,, which has to support 

 the head of boughs and flowers. 



When, however, it is grown on a trellis, it will then make annual 

 shoots, sometimes of 30 feet in length in the United States. A similar 

 difference will also be found, as mentioned in the Ivy, in the thickness of 

 the wood, between a supported and a self-supporting shoot. 



Effects of Adaptation. — I have already observed, generally, that stems 

 grow upwards under the influence of light, and in so doing necessarily 

 subject themselves to the effect of gravity, which is always trying to pull 

 them down, so that they must be able to resist the strain. This they do 

 by making wood. If a tree be much exposed to wind on a hilly place it 

 will be observed that it becomes short- stemmed, and more or less rounded 

 above, while the same kind of tree will grow tall and branch freely in a 

 sheltering valley. Moreover, if the prevailing wind be, say, from the south- 

 west, the section of the stem will be more oval than circular, the long 

 diameter being also in the same direction as the wind. Again, large 



* For further illustrations the reader is referred to The Origin of Plant Structures^ 

 ch. X. p. 197. 



f Op. cit. p. 72. X Climbing Plants, p. 72. 



