164 



LECTURE X. 



The whole structure of the secondary cortex is influenced by the horizontal 

 tissue of the medullary rays to an extent even greater than is the case in the 

 wood. To mention two extreme cases only; the medullary rays, when they traverse 

 the cortex in great numbers, may cause the whole of the remaining tissue to appear 

 arranged chiefly in radial rows on the transverse section {Ctnchoiia); and when bast 

 fibres are present, these also appear on the transverse section to be arranged chiefly 

 in radial rows. In the other extreme case, on the contrary, individual medullary rays 

 become much wider as they proceed from the cambium to the exterior ; since their 

 parenchyma cells grow very strongly in the tangential direction, in correspondence 

 with the increase in circumference of the stem, and become chambered by radial 

 longitudinal walls, thus constitudng a parenchymatous mass of concentrically 

 arranged layers of cells, in which lie thicker bundles of soft bast, traversed under 

 certain circumstances by bast-fibres. In the Conifers the arrangement in radial rows 



Fig. 169. — Transverse section of the stem of ^U7izperi{s conimitJtzs» jrjf Cambium ; hh 

 autumn wood of the youngest annual ring-; bb bast-fibres of the secondary cortex, in peri- 

 pheral rows and with sieve-tubes between them ; »z medullary rays (De Bary). 



as well as that in concentric layers is usually evident ; and Fig. 169 shows at once 

 how the thick-walled bast fibres are here disposed in layers within the soft bast. 



Observed on a tangential section, the secondary cortex, like the wood, 

 is composed of sinuous or undulating bundles of elongated elements connected 

 in a net-like manner ; the meshes of which, likewise elongated longitudinally, 

 are filled up by the parenchyma of the medullary rays. This structure is to be 

 seen even by the unaided eye when the bundles abound in bast-fibres. 



The activity of the cambium is generally less energetic towards the exterior 

 than on the inside; i.e. the formation of cortex proceeds in the radial direction 

 much more slowly than the formation of wood. This is obvious at once, on 

 comparing the cortex in the transverse section of a large Beech stem, where it is 

 only a few millimeters thick, with the huge mass of wood ; both of them being of the 

 same age. 



In shoot-axes which eventually proceed to perennial growth in thickness, the 



