§204> Functions of Vegetable's . 
die of winter, the vegetative process will immediately 
commence, and proceed with vigour; and if the proper 
temperature be continued, leaves, flowers, and fruit may 
be produced while every other part of the tree remains 
in a dormant state. 
Leaves.— -The leaves of plants, not only contribute to 
their beauty and elegance, but perform functions of es- 
sential importance in the process of vegetation. De- 
prived of its leaves, no tree brings its fruit to maturity, 
as is too often experienced in the ravages of the cater- 
pillar on the gooseberry ; and if by any accident a tree 
has lost its leaves, the progress of its growth is inter- 
rupted till it is again clothed with foliage. 
A leaf is composed of a double layer of the fibres, 
and vessels of the footstalk, between which is interpos- 
ed a plate or layer of the parenchyma, or cellular tex- 
ture. Leaves, it has been long observed, and proved 
by decided experiments, transpire a considerable quan- 
tity of moisture, ‘ ^.nd in some cases this quantity is 
little inferior to the moisture absorbed. It is greatest 
during sun-shine and warm weather, is much interrupt- 
ed during the night, and entirely checked by cold. In 
an experiment by Dr Hales, a plant of sun-flower, 
Heliantkus annuus, lost nearly two lbs. weight in twelve 
hours of a hot dry day ; in a dry night, the diminution 
of weight was only about three ounces ; in a moist night, 
no perceptible difference was observed ; and in a rainy 
night it gained two ounces. The matter excreted by 
the leaves of plants is of very different qualities. In 
some it is aqueous, in some of a saccharine nature, and 
in others it is glutinous, resinous, or waxy. 
- The sap of plants flows from the root towards the 
