LECTURE V. 



How does the food enter into the circulation of plants— Structure of the several parts of 

 plants— Fun ;tions of the root— Course of the sap— Cause of its ascent— Functions of 

 tlie stem— of the leaves— and of the bark — Circumstances by which the exercise of these 

 functions is modified. 



Having now taken a general view of the source from which plants 

 derive the elementary substances of which their solid parts consist, and of 

 the slates of combination in which these elements enter into the vegeta- 

 ble circulation, — the next step in our inquiry is — how are these substan- 

 ces admitted into the interior of living plants — and under what condi- 

 tions or regulations? We are thus led to study the structure and func- 

 tions of the several parts of plants, and the circumstances by which the 

 exercise of these functions is observed to be modified. 



§ 1. General structure of plants, and of their several parts. 



Plants consist essentially of three parts — the roots, the stem, and the 

 leaves. The former spread themselves in various directions through 

 the soil, as the latter do through the air, and the stem is dependent for its 

 food and increase on the rapidity with which the roots shoot out and ex- 

 tend, and on the number and luxuriance of the leaves. 



We shall obtain a clearer idea of the relative structure of these several 

 parts by first directing our attention to that of the stem. 



The stem consists apparently of four parts — the pith, the wood, the 

 bark, and the medullary rays. The pith and the medullary rays, how- 

 ever, are similarly constituted, and are only prolongations of one and 

 the same substance. The pith forms a solid cylinder of soft and spongy 

 matter, which ascends through the central part of the stem, and varies 

 in thickness with the species and with the age of the trunk or branch. 

 The wood surrounds the pith in the form of a hollow cylinder, and is itself 

 covered by another hollow cylinder of bark. In trees or branches of 

 considerable age the wood consists of two parts, the oldest or heart woody 

 often of a brownish colour, and the newer external wood or alburnum^ 

 which is generally softer and less dense than the heart wood. The bark 

 also is easily separated into two portions, the inner bark or liber, and 

 the epidermis or outer covering of the tree. The pith and the bark are 

 connected together by thin vertical columns or partitions, which inter- 

 sect the wood and divide it into triangular segments. A cross section 

 of the trunk or branch of a tree exhibits these thin columns extending 

 in the form of rays, or like the spokes of a wheel, from the centre to 

 the circumference. Though they form in reality thin and continuous 

 vertical plates, yet from the appearance they present in the cross sec- 

 tion of a piece of wood, they are distinguished by the name of medulla- 

 ry rays. 



These several parts of the stem are composed of bundles of small 

 tubes or hollow cylindrical vessels of various sizes, and of different 

 kinds, the structure of which it is unnecessary for us to study. They 



