62 



BOTANY OF THE L1YIXG PLANT 



from the cambium, and thus facilitate radial communication. All 

 the arrangements in the woody trunk are such as to admit of develop- 

 ment being indefinitely continued. The woody column increases by 

 successive additions from without : the bast by additions from within. 

 The former presents no mechanical difficulties : but the continued 

 addition of tissue from within will tend to stretch the outer-lying 



M.R*$-r-^ 



Fig. 40. 



Transverse section of stem of Lime, cut in the spring of its fifth year. Drawn 

 by Dr. Thompson. P = pith. XyJ-Xy.v xylem of first to fifth season. M.R.= 

 medullary ray. Sp.W. =spring wood. A u.W. = autumn wood. P/t/.= phloem. 

 Cfc. = cambium. Cor. = cortex. C/c. = cork. (xn.) The small outlined square of 

 tissue is represented on higher scale in Fig. 41. 



layers of the bast, cortex, and epidermis. The effect of this is seen 

 in the Lime, where the medullary rays widen in the phloem into 

 broad parenchymatous masses, which intervene between the wedges 

 of bast. Thus the stretching of the outer layers is indirectly provided 

 for (Figs. 40, 41. M.R.). But in other trees the difficulty may be 

 overcome in other ways (see p. 65). 



In temperate climates the activity of the cambium is interrupted 

 each winter. This leaves its mark on the woody column in the form 



