VEGETABLE PHYSIOLOGY. 85 



as the whole of the leaf. The sap in consequence stagnates, 

 and at last the bond of union between the leaf and stem is dried 

 up, and cracks. The wound that the stem thus receives cicatrizes 

 before the petiole separates ; and the petiole separates at last in 

 consequence of the interrupted connexion between the leaf and 

 stem, which the crack has occasioned. This, it must be 

 confessed, does not make up for the deficiencies of the hypothesis 

 of Du Hamel ; for in the first place there is no proof that the 

 bond of union between the leaf and stem cracks in the manner 

 here supposed. And even upon the supposition of its being the 

 fact, it is, in the second place, extremely improbable that the 

 petiole should after the cracking of this bond of union still 

 continue attached to the stem, till the w T ound thus occasioned has 

 cicatrized ; because when the original bond of union cracks, 

 there remains no other bond of union by which the petiole is to 

 retain its hold. 



Another explication is that of Vorlick, as quoted by Willdenow ; 

 the leaf which possesses a peculiar vitality within itself, though 

 dependant upon the vitality of the plant, and generally of shorter 

 duration, dies when it reaches maturity ; and the plant, being 

 able to exist for a time without leaves, throws off the dead leaf 

 as the animal throws off the dead part from the sound part. But 

 the peculiar vitality which the leaf is here supposed to possess 

 seems to be altogether a groundless assumption, and an unphilo- 

 sophical multiplication of causes without any apparent necessity. 

 Is it not the individual vitality of the plant extended to a 

 perishable organ, and again withdrawn when that organ has 

 discharged its destined functions, or become by disease or decay 

 unfit for the purposes of vegetation ? This, w r e presume, is a 

 better founded supposition than the foregoing ; though the 

 reference to the phenomenon of the throwing ofF of the dead 

 part from the sound part in the animal subject is sufficiently well 

 adapted to the purposes of illustration ; and the analogy suffi- 

 ciently striking, at least under some of its aspects, to warrant its 

 introduction. It does not, however, seem quite evident that the 

 idea of sloughing is comprehended in the opinion of Vorlick, at 

 least as represented by Willdenow ; but if so, the analogy does 



