XXVI INTRODUCTION TO BOTANY. 



branches, consists of cellular tissue, occupying the centre or 

 lon<?itudinal axis of the stem. 



(/>) the iiudullar// s/ieath, which surrounds the pith, abounds in 

 s])iral vessels (1-53, c), and is in direct connection, throui^h its 

 ramifications, with the leaf-buds, and the veins and nerves of 

 leaves. 



{c) the ic'ood, which lies directly on the medullary sheath, is formed 

 of woody tissue (153, b), through which, in most cases, duds 

 (153, c), variously disposed, are interspersed. A new circle of 

 wood is annually fonned, on the outside of the circle of the 

 pievious year ; the age of a stem therefore may be ascer- 

 tained, in a large number of cases, by counting the numbers of 

 its rings of wood : in some cases of tropical trees and ever- 

 green trees of temperate climates, several rings of wood are 

 formed in a year. The older and denser, comparatively sap- 

 less wood, is called heartwood or duramen, and is often 

 coloured ; the younger, living and incompletely formed, is the 

 mpifood or alburnum, and is usually white. 



{d) the medullary rays, which originate in the pith, traverse the 

 wood, and terminate in the bark, and are formed of cellular 

 tissue : they occur as vertical plates, radiating from a centre, 

 and keep up a communication between the living portion of the 

 interior of the stem and its outer surface. As the heartwood 

 is formed the inner portions of the medullary rays die. In 

 wood they are what carpenters call the silver-grain. 



{c) the bark, which lies outside the wood, and foi-ms the outer layer 

 of the stem. It is coated by the epidermis (157), and like the 

 wood, consists of concentric layers ; namely, the corky layer, or 

 dry, outer bark, formed of hard, compressed cells ; the cellular 

 or green or middle bark, formed of loose, thin-walled, pulpy 

 cells, containing chlorophyll (156, e) ; and the liber or inner 

 bark, fonned of long, tough, woody tissue, called bast-cells. The 

 liber, like the wood, is annually deposited ; the green layer is 

 a product of the first year only, being soon choked by the corky 

 envelope. 



168. The mineral food of plants, absorbed by the roots, passes upwards 

 through the younger wood of the stem, mixing with previously organized 

 matter, but not being essentially altered ; in this state it is called sap or 

 crude sap. The crude sap, as it ascends thi'ough the stem, is attracted into 

 the leaves, where it is exposed to the direct action of sunlight, under which 

 influence alone can assimilation take place. As assimilated or elaborated 

 sap, it is returned into the stem, and either used up in the processes of 

 further growth, or deposited either in the wood, in the stock, in the peren- 

 nial }iart of the stem or root, or in any other part of the plant where matter 

 is stored up for future use. 



§ 5. The Leaf . 



169. Anatomically the leaf consists of a central fibro-vascular system or 

 woody skeleton, derived fi-oni the woody system and medullary sheath of 

 the stem ; a cellular system siuTOunding the fibro-vascular, and interwoven 

 with it, and derived from the middle bark ; and an outer skin or epidermis, 

 pierced by stomates. 



170. The fibro-vascular system is arranged on two principal types : — 

 {a) the exogenous, in which the nerves and veins branch irregularly 



and usually anastomose into a sort of network. 

 (/») the endogenous, in which the prineii)al ]ierves usually extend 



