32 FUNCTIONS OF PLANTS. 



extent, in winter as well as in summer ; but that an extraordinary absorp- 

 tion by the roots, and consequent ascent through the alburnum, takes place 

 with the development of the buds, in consequence of the stimulus of heat in 

 spring. The swelling of the buds, and the expansion of the leaves, decom- 

 pose a quantity of sap in the same manner as the swelling of the embryo of 

 the seed (102); a portion is fixed in the plant, and a portion given off into the 

 atmosphere ; and to supply the consumption thus occasioned, the office of the 

 spongioles of the roots is called into extraordinary action, and nature, always 

 stronger than strong enough, produces a superabundant supply. 



120. The leaf of the plant is an organ of so much importance, that there 

 can be no growth beyond the first development of the seed without it. No 

 mode of treatment will compensate to a plant for the want of leaves, and the 

 most vigorous plant that exists may be destroyed in a short time by the 

 removal of all the leaves as soon as they appear. The important consequences 

 that result from this fact, are not sufficiently known to many gardeners, and 

 they require particularly to be impressed on the minds of amateurs. We 

 have seen in a preceding paragraph how trees may be weakened, and parti- 

 cular shoots killed, by the removal of leaves. The most powerful weeds, for 

 example, Perennial Thistles, Docks, Ferns, Rushes, and all similar plants, 

 may be killed in grass lands on the same principle ; that is, by the removal 

 of the leaves as soon as they appear, and before they are developed. 



121. The normal form of a leaf consists of an expanded part called the 

 disk, and a narrow prolongation called the petiole (91); but some 

 leaves are solid and cylindrical, and others are so modified as to appear like 

 scales ; for example, in bulbs, the bracts in the fruit of the pine-apple, 

 spines in the common thorn, tendrils hi the vine ; and, consequently, all these 

 organs or appendages ought to have buds, either visible or adventitious, in 

 their axils. This is accordingly found to be the case. Shoots have been 

 produced where the tendrils of a vine have been cut off; and in the fruit of 

 the pine-apple, every bracteal leaf having a " pip" or flower in its axil, has 

 produced a sucker. (Cowel.) The disk of the leaf is considered as an ex- 

 pansion of the inner bark (91) ; its veins are the continuation of the ligneous 

 fibres of the bark, and its cellular substance of the horizontal system or 

 cellular tissue of the trunk. The woody tissue which forms the veins of 

 leaves, as already observed, is arranged in two layers ; one forming the 

 upper surface of the leaf, by which the sap is elaborated ; and the other, the 

 under surface, by which the elaborated sap is returned to the inner bark. 

 The two plates of layers may be readily seen in a leaf which has been ma- 

 tured, and afterwards anatomised, by the alternate action of water and the 

 atmosphere. The upper layer has its vessels in communication with the 

 interior of the stem, while the under layer communicates only with the 

 inner bark ; the upper one maintains a connexion with the soft wood, in 

 order to receive the sap from it, while the under one is connected with the 

 inner bark, in order to return the sap through it to the stem and roots. 



122. The two plates of vessels and cellular matter which form the disk of 

 the leaf, are covered with a thin skin or epidermis. This epidermis, when 

 the leaf is beginning to expand, abounds with innumerable minute cavities 

 filled in that early stage with fluid ; but ultimately, when the leaf is fully 

 grown, these cavities become dry. In plants indigenous to moist and shady 

 places, the epidermis is thin; but in those growing naturally in hot, dry, ex- 

 posed situations, it is very hard and thick. It varies, indeed, not only with 



