310 Mr Fothergill on Leaves of Clhnhmg Plants, [sess. lii. 



deeply divided, so many other complexities are introduced 

 that it is almost impossible to use them for or against a 

 theory of this kind ; but in many cases, drawing a line round 

 the points of leaflets gave an outline which fairly showed 

 the distribution of the weight of the leaf, and this was 

 usually affected by considerable development of the basal 

 lobes. 



Where the petiole is much used in climbing, the point of 

 support of the leaf is raised so much as materially to alter 

 the relations in question, so that I prefer to exclude such 

 cases from the explanation I wish to suggest, which deals 

 with the general aspect of the case as stated above. 



Since the supporting system is one and the same throughout 

 the plant, with the pliant and yielding stem of a climbing 

 plant, we find also slight bending petioles which are unable 

 to sustain much weight, especially if it is not favourably 

 distributed. For optimum conditions of nutrition, a leaf must 

 not hang down too much so as to catch the light, or bend 

 its stalk so as to compress the conducting vascular system. 

 Sessile or shortly stalked leaves will not suffer, nor will those 

 of climbers, whose supporting system is stronger than usual ; 

 but where petioles are long, and share the weakness of a 

 feeble supporting system, some compensation must be made 

 by a favourable disposition of the weight to be borne. How 

 then are the light petioles of weak climbing plants to stand 

 erect and bear their leaves in an approximately horizontal 

 position ? 



It is clear that great development of any one part of a 

 lamina will alter the position of its centre of gravity. In 

 an ovate leaf the centre of gravity is near the centre, in 

 a cordate one it is near the insertioji of the petiole. 



Now, the moving of the centre of gravity of a lamina away 

 from the apex towards the base lessens the bending strain on 

 the petiole in two ways, or rather lessons the strain on the 

 lower part where it springs from the stalk, and on the upper 

 part where it passes into the lamina to become its 

 midrib. 



First, it biings the centre of gravity of the leaf nearer to 

 the main axis, and so lessens the moment of the force exerted 

 by the weight of the leaf about the point of insertion of the 

 petiole in the axis. Thus based dcvdo2)mcnt of the leaves of 



