% AGRICULTURAL TEXT-BOOK. 



which in time become exhausted, leaving the older pith dry and 

 light, or mere empty cells, which are of no further use to the 

 plant. The wood consists of woody fibre, among which vessels 

 are more or less copiously mingled, capable of cany ing liquids 

 up and down between the root and the leaves. The branches 

 are only a continuation of the stem, and have a similar structure. 

 The bark consists of three portions ; (a,) the liber which lies 

 next the wood; (b,) the outer bark, composed of two parts, 1, 

 the green or cellular layer, 2 the corky layer, and (c,) the epi 

 dermis or skin which invests the whole. 



149. The root, immediately on leaving the trunk or stem, 

 has also a similar structure ; but as the root tapers away, the 

 pith gradually disappears, the bark also thins out, the wood 

 softens, till the white tendrils, of which its extremities are com 

 posed, consist only of a colorless, spongy mass, full of pores, (or 

 minute holes,) but in which no distinction of parts can be per 

 ceived. In this spongy mass, the vessels or tubes which descend 

 through the stem and root, lose themselves, and by them these 

 spongy extremities are connected with the leaves. 



150. A leaf is an expansion of the stem. Like the stem 

 therefore, the leaf is made up of two distinct parts, the cellular 



and the woody. The leaf is a highly organized structure, con 

 taining innumerable rounded globules, cells, and veins, regularly 

 arranged. It is also full of pores through which air can enter. 



151. In a growing plant, the sap enters by the extremities of 

 the roots, (spongioles,) ascends through the vessels of the wood, 

 and is passed over the inner surface of the leaf by the fibres 

 which the wood contains. Thence, by the vessels in the green 

 of the leaf, it is returned to the bark, and through the vessels of 

 the inner bark it descends to the root. 



