60 BOTANY OF THE LIVING PLANT 



the cambial cells widen and undergo divisions, transverse and some- 

 times longitudinal, into a number of square or oblong cells. Their 

 walls become thick, woody, and pitted, but the cells retain their 

 cytoplasm and nucleus. They are living cells, and are often stored 

 with starch (iii.). Analogous changes occur in the maturing of tissues 

 of the phloem. When sieve-tubes are being formed one or more sieve- 

 plates appear on the oblique terminal walls of the cambial cells ; the 

 cytoplasm is continuous through these, and they act as bast-vessels 

 for transit of plastic materials (ix.). The formation and function 

 of bast-fibres, if present, and of bast-parenchyma, corresponds in 

 essentials to that of the fibres and parenchyma of the secondary 

 wood (viii. x.). 



One other component of the vascular column remains to be de- 

 scribed, viz. the medullary ray. The name is derived from the fact 

 that in transverse section radial lines of tissue, which look structurally 

 like the medulla or pith, run part or the whole way from the cambium 

 inwards through the wood, and are also continued outwards through 

 the bast. Such rays are narrow plates of tissue, and though they 

 extend far in a radial direction, they are continued only a short distance 

 up or down (Fig. 39). They are composed of brick-shaped cells with 

 their longer axis horizontal ; these cells often have thick pitted walls, 

 and many of them retain their protoplasm and nucleus. They link up 

 with the parenchyma of wood and bast, forming a connected system 

 of living tissue extending inwards and outwards from the cambium 

 (Figs. 40 and 41). The rays appear as bright streaks in the mature 

 wood, and are the silver grain of carpenters. Two types of rays may 

 be distinguished : primary rays, which intervene between the original 

 vascular strands, and extend the whole way from cortex to pith ; 

 and secondary rays, which are initiated subsequently in the cambium, 

 as the stem increases in bulk. These extend only part way through 

 the vascular ring. The tissue of the rays is derived from special 

 cambial cells, which are readily recognised in transverse sections by 

 the fact that they are not so narrow radially as the cells of the ordinary 

 cambium (Fig. 41). The form of the rays, and of the cells com- 

 posing them, should also be observed in tangential sections (Fig. 36), 

 in which it may be seen that minute triangular intercellular spaces 

 occur where three cell-walls meet. Thus a ventilation-system extends 

 through the rays inwards into the vascular trunk. Examined in 

 radial longitudinal sections it is apparent that the rays are, as their 

 name implies, narrow plates of tissue extending far radially, but 

 only a short distance vertically (Fig. 39). They serve as means of 



