12 



PROPERTIES OF WOOD. 



little vegetating spheroid is exposed, constructed of thin- 

 walled isodiametric cells, the meristem, or primary tissue, 

 from which all other cell-forms spring. In this bud, when the 

 growing-season commences, a central cylinder by rapid cell- 

 division lengthens the bud into a shoot, whilst the externally 

 surrounding cells (forming with the oldest vessels described 

 lower down the medullary sheath) participate in the longi- 

 tudinal growth, partly by cell-division, and partly by being 

 stretched. At different points outside this cylinder, which 

 becomes the pith (Fig. 3, A/), the cells are stretched but not 



divided, so that elongated 

 organs are formed, clearly 

 differing from the pith but 

 immediately adjoining it and 

 termed vessels or trachea. 

 They are followed by other 

 wider vessels and other 

 lignified cell-forms, so that 

 the new strand of tissue is 

 termed a vascular or woody 

 bundle (Fig. 3, /, //, etc.). 

 Each of these bundles ter- 

 minates externally with a 

 group of cells, which are not 

 lignified, the bast or phloem 

 (Fig. 3) ; between this and 

 the woody parts termed 

 xylem, adjoining the pith, a meristem, the cambium, remains 

 (Fig. 3, c', c). The cambium in the first year increases the 

 width of the woody bundles, producing bast-cells externally 

 and internally wood-cells. Between every two adjacent bundles 

 a narrow band of primary tissue unites the cortex (K) with 

 the pith. These bands are termed medullary, or pith, rays, 

 being parenchymatous groups of cells. Within each medullary 

 ray, also during the first year, and originating with the cainbia of 

 the bundles, inter-fascicular or medullary cambium connects 

 the latter into a closed zone, forming in a transverse section a 

 ring ; it consists of only a few cells in each medullary ray. 

 The shoot terminates its upward growth by a new bud, its 



Fig. il Diagram .showing the position 



f the woody bundles, J, //, etc. 

 c, c 1 Cambium. #, x Xylem or wood. 

 l>> ^ Phloem or bast. M Pith. 11 

 Cortex. J7, R Medullary ray. 



