154 



XI. THE DICOTYLEDONOUS STEM WOODY TYPE. 



vessels (ducts) are first produced in close succession, and this it is- 

 which especially marks out the limits of the annual growth. Later 

 on in the year's growth the broad ducts either arise singly or in 

 isolated groups ; in the last phase of the year's vegetation the 

 cambium produces only elements with narrow cavity. Outside 

 the cambium we see the keel-shaped masses of the bast, thinning 

 off outwards. In these can be seen an alternation of tangentially- 

 arranged bright and darker layers. The shining white layers are 

 composed of numerous closely-united bast fibres, the walls of which 

 are thickened almost to the disappearance of the cavity, the cavity 

 of each element showing only as a dark dot. The layers have 

 an irregular outline, and may be interrupted here and there. The 

 darker layers between the white consist of (1) narrow-cavitied 

 starch-containing bast parenchyma cells, which are especially in 

 contact with the bast fibres, and of (2) wide-cavitied elements, 

 sieve-tubes, with companion-cells, which occupy the more median 

 portions of the layers. About twice as many layers of secondary 

 bast fibres can be counted as there are annual rings in the wood, 

 which arises from the fact that, apart from the first two years, two 

 such layers are pretty regularly formed in each year. The primary 

 medullary rays in the wood are mostly two cells thick, but now 



and again even more ; the 

 secondary medullary rays 

 are but one cell broad. They 

 may be traced either right 

 through the cambium and 

 bast into the primary cor- 

 tex, or only to a position in 

 the bast equivalent to that 

 in which the wood end of 

 the ray is situated. The 

 outer ends of the larger 

 primary medullary rays 

 broaden out funnel - wise, 

 and separate two keel- 

 shaped masses of bast. 

 The central cylinder is sur- 

 rounded by the living green 

 primary cortex. In the outer parts of the medullary rays, and in 

 the primal y cortex, are numerous crystallogenous sacs. Still more 

 externally is a layer of collenchyma, chlorophyll-containing, and 



FIG. 58. Cross-section through a stein of Tilia 

 parvifolia in its fourth year's growth, pr, prim- 

 ary cortex ; c, cambium ring ; cr, bast ; pm, 

 primary medullary rays ; pm', outer broadened 

 (funnel-shaped) end of a primary medullary ray ; 

 SHI, secondary medullary ray ; g, inner limits of 

 the current year's ring (x 6). 



