THE TISSUES OF THE STE^I 51 



their protoplasm they are dead elements. Smaller vessels develop 

 similarly, but with less disturbance of the neighbouring cells (iv. v.). 

 When wood-fibres are to be formed the cambial cells elongate, and their 

 pointed ends bore their way upwards and downwards, with a slidmg 

 readjustment among themselves. As considerable tracts of cells may 

 develop thus alike, and as the cells themselves take a more or less 

 sinuous course, they become interlocked, almost like the strands of 

 a rope. At the same time their walls become greatly thickened, and 

 woody, and their protoplasm disappears. Their function is thus not 

 vital but mechanical (vi. vii.). When wood-parenchyma is to be formed 

 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 to be 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 a radial Hne of tissue, which looks hke that 

 of the medulla or pith, runs part or the whole way from the cambium 

 inwards through the wood, and is continued outwards through the 

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

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

 distance up or down. They are composed of brick-shaped cells with 

 their longer axis horizontal; these cells 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 outv/ards from the cambium (Fig. 38). 

 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, the form of which 



