1887-88.] Mr Fothergill on Leaves of Glimbing Plants. 811 



^limbing plants must greatly relieve the bending strain on the 

 loer part of the petiole. 



Second, since basal development brings tbe centre of 

 gravity of the lamina nearer the point of expansion of the 

 petiole, there will be a much shorter distance between the 

 point of support of the lamina and the point of action of its 

 weight than otherwise would be the case. Thus a much 

 slighter petiole will be able to keep the lamina approximately 

 horizontal, if it be cordate, than if it be ovate. Thus 

 basal development of leaves must greatly relieve bendiiig 

 strain on the petiole at its point of expansion into the lamina. 



For these two reasons, then, I suppose the leaves of most 

 climbing plants to be largely developed basally. Their 

 petioles we may expect to be weak a priori; and we see that 

 cordate and allied forms of leaves will exert less strain on 

 these petioles than would other leaves, for the two purely 

 physical reasons mentioned above. 



Where the petiole is absent, there can be no strain on it, and 

 this is simply the extreme case of a shortened petiole, so the 

 shorter the petiole, the less marked is the basal development. 

 Where the petiole is strong enough, it can also support a 

 leaf of ordinary shape; but a long petiole, if not strong, 

 must be helped by having the weight of the leaf brought 

 — first, nearer the main axis, so reducing the strain at the 

 insertion of the petiole in the axis ; and, second, nearer the 

 expansion of the petiole into the lamina, so lessening the 

 strain at that point. 



This little question appears to me deserving of the amount 

 of trouble it has demanded. I have been unable, so far, to 

 connect it with the question of cordation and peltation in 

 general ; for there are plenty of cordate and peltate leaves 

 where neither a priori reasons nor actual examination shows 

 weakness of supporting system, but I imagine that after 

 eliminating the complexities of many basally developed 

 leaves, other than those of climbing plants, the connection 

 between the present point and the more general question 

 may become visible. 



Note. — A peculiarly pretty coiifirmation of the above view is given by the 

 leaves of some Passillorie, e.g., P. rubra and P. Vespcrtilio. Here the centre 

 of gravity is shifted towards the base of the leaf, not by largely developing 

 the basal lobes, but by cutting off the apex, so that one gets a truncated or 

 even depressed leaf apex. 



