84 SYLVA AMERICANA. 



totally baffled the attempts of phytologists, and to have been a puzzle 

 that no one could make out. Du Hamel, one of the most sagacious 

 and industrious of all phytologists, labored hard to explicate the 

 phenomenon, but without success. He observed that leaves 

 which fall the soonest transpire the most, and are consequently 

 the soonest exhausted and rendered unfit for the discharge of 

 their functions ; so that the period of the fall of the leaves of 

 different species is probably in proportion to their capacity for 

 transpiration. Their fall is accelerated by frost, or by excessive 

 heat, followed by rain. It is also accelerated, if not actually 

 induced, by the structure of the pedicle which is very different 

 from that of the branch, having no prolongation of pith, and 

 nothing analogous in its mode of insertion, nor in its external 

 figure, which is divisible into an upper and 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 intemodia, 

 but which readily give way after a frost. The comparison, 

 however, throws but little light on the subject, as the illustration 

 is itself, 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 

 vigor a great supply is necessary, soon become dry and conse- 

 quently 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 foot stalk and stem or branch, at the point of 

 articulation; and hence the leaf loses its hold, and falls. This 

 is certainly a very plausible conjecture; though it may be doubted 

 whether the explication 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 satisfactory ; and hence 

 other explications have accordingly been offered. 



The first of these explications of which we 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 of the petiole become gradually more woody, as w r ell 



