134 PHYSIOLOGICAL BOTANY. 



407. The Medullary sheath immediately surrounds the pith. 

 It is a thin, delicate tissue, consisting of spiral vessels. It com- 

 municates with every bud, and sends off detachments of its ves- 

 sels to the petioles and veins of every leaf. Its tubes secrete 

 oxygen from carbonic acid or water, and convey it to the leaves. 



408. The wood consists of pleurenchyma and ducts ( 392), 

 arranged more or less distinctly in concentric zones or layers. 

 The first, or inner layer, together with the medullary sheath and 

 pith, is the product of the first year. One new layer is formed 

 each successive year, during the life of the plant. 



409. There are doubtless some exceptions to this rule. In tropical countries, where 

 there is no distinction of seasons, there may be several zones deposited annually ; or, on 

 the other hand, several or all the annual layers may be so blended by the uniform mixture 

 of the duels with the wood-tissue as to be undistingnishable. The layers of the beet-root 

 are certainly not annual. They seem to correspond with the number of leaf-cycles ( 263). 



509 



509, Cross-sections of an exogenous stem (Elm) of two years' growth 1, pith; 2, 3, annual layers 01 

 wood, next the cambium; 4, bark; 610, au Endogenous stem (Sorghum, or Millet) where there is no 

 distinction of layers. 



410. The Alburnum and Duramen the sap-wood and heart- 

 wood are well-known distinctions in the wood. The former, 

 named from albus, white, is usually of a light color and softer 

 structure. It is the living part of the wood, through whose ves- 

 sels mainly the sap ascends. The interior layers of the albur- 

 num gradually harden by the deposition of solid matter in their 

 vessels, and the thickening of the cell-walls, until fluids can no 

 longer pass through them. Thus the duramen (durus, hard) is 

 formed of a firm and durable texture the only part valued as 

 timber. Its varying colors in Cherry, Walnut, Rosewood, are 

 well known. It is of no account in vegetation, and may be con- 

 sidered as dead. Hence it often decays, leaving the trunk hollow, 

 and the tree at the same time as flourishing as ever. 



