124 LIVING ELEMENTS OF SECONDARY WOOD 



of the cells simulating the " bond " of the bricks. The com- 

 ponent cells show plainly the radial elongation referred to above, 

 but sometimes those at the upper and lower margins are short 

 and not markedly lengthened (e.g. Willows). The radial, like 

 the vertical, extent of the secondary rays is seen to be limited 

 (cf. p. 120), the rays penetrating inwards from the cambium for 

 varying distances. Should the radial section pass through one 

 of the primary rays, the latter will be seen to show a far greater 

 development, both in the vertical and radial directions. 



Apart from the occasional presence of fibres with protoplasmic 

 contents, the wood-parenchyma and the medullary rays are the 

 only living constituents of the secondary wood. The living 

 cylinder constituted by the phloem and cortex is thus connected 

 with numerous inwardly directed plates of living tissue, the 

 medullary rays. Of these, however, only the primary ones 

 extend to the pith, where they are linked up by a second cylinder 

 of living cells, the medullary sheath (p. 79), which invests the 

 inner margins of the primary xylem strands. The radiating 

 plates formed by the secondary medullary rays are, however, 

 not isolated, even where they pass between the dead elements 

 of the secondary wood, since they are connected both vertically 

 and horizontally by bands of wood-parenchyma cells. 



The mass of vessels and fibres is thus permeated by a con- 

 tinuous system of living elements connected with the food- 

 conducting tissues. The functions of this system are twofold : 

 firstly, to conduct elaborated food-substances to the cambial 

 region, the living cells of the wood, and the medullary sheath ; 

 secondly, at certain times of the year, the cells serve for the 

 storage of food-substances, e.g. starch, as can be shown by the 

 application of iodine to a section of a twig in autumn. When 

 this starch is utilised during the sprouting of the buds in spring, 

 it is changed into sugar, and is then transferred in the water 

 ascending the vessels to the growing regions, hence the sweet 

 character of the sap which exudes in bleeding. 1 The aeration 

 of the secondary tissues is effected by narrow intercellular spaces 

 which are more particularly associated with the living elements. 



1 Cf. F. & S., pp. 109, 169. Maple-sugar is derived from the evaporated 

 sap of Acer saccharinum (United States), obtained by tapping the trees in 

 spring. 



