200 CHANGES OF LEAVES. 



tissues of the leaf itself, too, are gradually dried up ; and the 

 whole structure loses its vitality, and is cast off, as a dead part 

 is from the body of an animal. 



313. Trees and shrubs, which are spoken of as evergreens, 

 do not really retain their leaves for more than a year ; but they 

 are not cast off until a new crop appears ; and the exchange does 

 not take place suddenly but gradually, so that the aspect of the 

 tree never undergoes much alteration. In evergreens, the func- 

 tions of the leaves ate carried on, though with great languor, 

 during winter ; but at other parts of the year they are less active 

 than those of the species which lose their leaves in autumn. 

 There are some trees of tropical climates, which completely lose 

 their leaves two or three times in every year, appearing as bare 

 as in winter ; and these are speedily replaced by a new crop. It 

 is probable (th'ough it has not been certainly ascertained) that in 

 such trees, a new woody layer would be formed by every crop of 

 leaves. In very hot and dry summers of this country, trees 

 have been completely stripped of their foliage early in July, and 

 have had strength to put forth a new and apparently vigorous 

 crop of leaves. Such an effort, however, appears very exhausting 

 to the tree, which is seldom so vigorous the next year. 



314. The facts stated in this chapter, respecting the influence 

 of Vegetation on the surrounding air, are very interesting in con- 

 nection with a plan which has recently been practised with much 

 advantage, of growing plants under glass cases very nearly closed. 

 They are constructed of almost any form or size ; the experi- 

 ment will answer with a chamber of a foot in each direction, or 

 in one as large as a common green-house. The plants to be 

 reared are placed in a kind of trough, with a sufficient quantity 

 of light earth and water ; and the glass cover is then fitted upon 

 this in such a manner, as not to be quite air-tight, but to allow 

 of extremely little communication between the interior and the 

 surrounding air. The advantage of this system is, that the 

 plants are kept in an atmosphere thoroughly saturated with 

 moisture ; and that they can obtain the necessary supply of car- 



