SECT. III. NATURAL DECAY. 



under surface resembling the figure of the leaf. 

 He compares the union of the leaf and stem to that 

 of the joints of the Vine-twig, which at a certain 

 period of its growth are stronger than the inter- 

 nodia, but which readily give way after a frost. 

 The comparison, however, throws but little light 

 on the subject, as, the illustration is itself to the 

 full as dark as the thing to be illustrated. But he 

 offers an additional conjecture which is considerably 

 more luminous ; when the sap begins to flow less 

 plentifully, the leaves, to whose vigour a great sup 

 ply is necessary, soon become dry and consequently 

 less fit to convey it. But it is known that the 

 branches grow in thickness after they have ceased 

 to grow in length, which must necessarily occasion, 

 in some degree, a disruption of the fibres of the 

 footstalk and stem, or branch, at the point of arti- 

 culation ; and hence the leaf loses its hold, and 

 falls. * This is certainly a very plausible conjec- 

 ture ; though it may be doubted whether the ex- 

 plication will apply to the case of Evergreens, or of 

 plants in warm climates, that retain their leaves for 

 several years. It is not therefore, altogether satis- 

 factory ; and hence other explications have accord- 

 ingly been offered. 



The first of these explications of which I shall 

 now take any notice is that of Willdenow; it is 

 as follows : As the sap is conveyed to the leaves 

 in greater abundance during the summer, the vessels 



* Phys. des Arbres, liv. ii. chap. ii. 

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