DIV. I 



MORPHOLOGY 



161 



to form wood to the inside, and bast to the outside, the stem or root 



exhibits a secondary increase in thickness. Those permanent tissues 



which are situated externally 



to the cambial ring (the epi- 



dermis, cortex, primary phloem, 



and the bast) are naturally 



affected by this. They are 



tangentially stretched, com- 



pressed, displaced, or torn ; they 



may also grow in the tangential 



direction (DILATATION). This 



latter process^ is naturally 



limited to the living cells of 



the cortex, the phloem, and the 



bast, including those of the 



medullary rays ; in some woody 



plants even the epidermal cells 



take part in the dilatation ( 81 ). 



All these cells may grow con- 



siderably in the tangential direc- 



tion and then become divided 



by radial walls. In the bast 



such growth is frequently very 



marked in the case of the 



medullary rays ; in the Lime 



this leads to the formation of a 



secondary meristem which gives 



off rows of parenchymatous 



cells to either side in the tan- 



gential direction, so that the 



medullary rays of the bast widen 



year by year towards the OUtside FIG. 186. - Transverse section of the peripheral 



(Fig. 178 pm). The sieve- tubes 

 and their companion cells, which 

 only remain functional for a short 

 time and then die, are com- 

 pressed along with the secretory 

 cells. The sclerenchymatous 

 cells of the cortex and bast, 

 which are usually non-living 

 elements, also take no part in 

 the dilatation. 



tissues of the stem of Quercus sessiliflora. 1, 2, 3, 

 Successively formed layers of cork ; pr, primary 

 cortex, modified by subsequent growth ; in- 

 ternally to pc, pericycle; sc, sclerenchymatous 

 fibres from the ruptured ring of sclerenchy- 

 matous fibres of the pericycle ; s, subsequently 

 formed sclereides ; s 1 , sclereides, of secondary 

 growth ; er, bast fibres with accompanying crystal 

 cells ; fc, cells with aggregate crystals. All the 

 tissue external to the innermost layer of cork 

 is dead and discoloured and has become trans- 

 formed into bark, (x 225. After SCHENCK.) 



When a hollow 



cylinder of sclerenchyma is present in the cortex (Fig. 186 sc), it 

 becomes torn in the tangential direction ; the parenchymatous cells 

 grow into the spaces, and in many plants become transformed into 

 thick- walled stone cells (Fig. 186 s). Parenchyma cells, or groups 



