PHENOMENA OF THE LIFE OF PLANTS. 
EXHALATION, RESPIRATION, CIRCULATION. 
Havine acquired some knowledge of the external arrangements 
and of the internal structure of the roots, stem, and leaves, we 
may now consider the essential phenomena in the life of plants, 
namely, vegetable physiology, in some of its most important points. 
Vegetation presents in exhalation, respiration, and circulation of 
the sap in the interior of the tissues, three functions only secondary 
in their importance to reproduction. 
EXHALATION. , 
Exhalation in plants is performed by the leaves and branches. 
Plants exhale water or vapour by their leaves; it is retarded by 
the presence on the surface of the leaves of a coating of wa* 
which gives them their greyish-blue glossy appearance, and tt 
varies according as the epidermis is thick or thin. 
The medium in which the plant is placed also greatly influences 
the function of exhalation. If the air is very dry, the exhalatio? 
is abundant and rapid. It is less active where the air is cha ‘ 
with humidity ; it increases as the temperature rises ; it is dimim- 
ished during the night. 
It is not alone by the stomates of the leaves, but also through 
means of the epidermic membrane itself, that exhalation takes 
place. The result of the perfect equilibrium which exists between 
the absorption of the roots and the foliaceous exhalation, is proof 
of a normal state of healthy vigour in the plant. If exhalation 
exceeds its absorbing powers the plant must fade. 
