304 PRINCIPLES OF BOTANY, ETC. 
or remain on the stem ina dry state till spring; others 
fall off when still green, and in the still milder days 
of autumn. In quite a different manner the Robi- 
nia pseudacacia parts with its leaves. The pinnate 
leaves of this tree first drop all the pinnule, and 
at last, after them the petiole to which they adhered 
drops off. 
Various reasons have been given by authors, 
why plants lose their leaves in autumn, and we 
shall now consider their various opinions on the 
subject. 
Du Hamel formed two hypotheses. He assumed; 
in the first place, a herbaceous part in the petiole, 
at the spot where its notch is, which in cold autum- 
nal nights becomes injured, and Boss the falling 
off of the leaves. 
He abandoned however this opimion, because he 
saw leaves drop off in warm autumnal days, without 
any preceding cold, and then produced the following 
explanation. ‘The moisture, which is conveyed to 
the plant by its roots, favours the growth of the 
petiole, the great transpiration of the leaves renders 
it at last quite dry, and therefore the leaves fall off, 
because the petiole has lost all its sap. | 
Mr Mustel thought that the leaves transpire less. 
during autumn. Hence the sap is accumulated in 
them, which produces a transverse fissure at their 
basis. The leaves, therefore, become separated from 
the petiole, and drop off. 
Vrolick believes that leaves possess a peculiar life, 
in which various periods may be distinctly marked. 
Their life, however, depends entirely on the life of 
: the 
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