EXHALATION OE TEANSPIEATION. 123 



large leaves of Colza evolved in an hour from one to two per cent of 

 their weight of water. Experiments have shown that the mean amount 

 of water contained in the leaves of the Cherry] Laurel is 6 3 '4: per cent, 

 and of this only about 6 per cent could be easily removed by sulphuric 

 acid or chloride of calcium. In the sun leaves transpire most in a 

 saturated atmosphere. In the shade transpiration ceases when the 

 atmosphere is loaded with watery vapour. Experiments on exhalation 

 may be made by taking a fresh leaf with a long petiole, putting it 

 through a hole in a card which it exactly fits, and applying the card 

 firmly and closely to a glass tumbler, about two-thirds full of water, 

 so that the petiole is inserted into the water, then inverting an empty 

 tumbler over the leaf, and exposing the whole to the sun, the fluid 

 exhaled will be seen on the inside of the upper tumbler. The ex- 

 periment may be varied by puttihg the apparatus in darkness, when 

 little or no exhalation takes place, or in diffuse daylight, when it is less 

 than in the sun's rays. This process of exhalation imparts moisture 

 to the atmosphere, and hence the difference between the air of a 

 wooded country and that of a country deprived of forests. The cells 

 in the lower side of a leaf where stomata exist are chiefly concerned 

 in the aeration of the sap, whUst other assimilative processes go on in 

 the upper cells. 



Leaves, after performing their functions for a certain time, wither 

 and die. In doing so, they frequently change colour, and hence arise 

 the beautiful and varied tints of the autumnal foliage. This change 

 of colour is chiefly occasioned by the diminished circulation in the 

 leaves, and the higher degree of oxidation to which their chlorophyll 

 has been submitted. Leaves which are articulated with the stem, as 

 in the Walnut and Horse-chestnut, fall and leave a scar, whUe those 

 which are continuous with it remain attached for some time after 

 they have lost their vitality, as in the Beech. Most of the trees of 

 this country have deciduous leaves, their duration not extending over 

 more than a few months ; while in trees of warm climates, the leaves 

 often remain for two or more years. In tropical countries, however, 

 many trees lose their leaves in the dry season. This is seen in the 

 forests of Brazil, called Gatingas. The period of defoliation varies in 

 different countries according to the nature of their climate. Trees 

 which are called evergreen, as Pines and Evergreen-oak, are always 

 deprived of a certain number of leaves at intervals, sufficient being 

 left, however, to preserve their green [appearance. Various causes 

 have been assigned for the fall of the leaf. In cold climates, the 

 deficiency of light and heat in winter causes a cessation in the 

 functions of the cells of the leaf; its fluids disappear by evaporation; 

 its cells and vessels become contracted and diminished in their calibre ; 

 various inorganic matters accumulate in the texture ; the whole leaf 

 becomes dry*; its parts lose their adherence ; a process of disjunction 



