DIV. I 



MORPHOLOGY 



143 



tangential walls, cells continue to be cut off towards the inside, and 

 later some are formed to the outside. The latter become secondary 

 cortical tissue ; the cells to 

 the inside develop into con- 

 centric vascular bundles, in 

 which the xylem surrounds 

 the phloem, and parenehy- 

 matous tissue with thickened 

 and lignified walls (Fig. 166). ? h ~ 



The meristematic cells have a 

 rectangular shape in transverse 

 and radial section^, while in tan- 

 gential section they are polygonal ; 

 they are thus tangentially-placed 

 flattened prisms (cf. Fig. 169 A, IT], 

 So long as the meristem is only 

 forming new tissues on the one 

 side, the initial cells can be re- 

 placed at the expense of the inner 

 permanent cells of the cortex. 

 When, however, the meristem is 

 active on both sides the initial layer 

 persists. 



True secondary thickening of 

 the root in Monocotyledons is only 

 known in the case of the genus 

 Dracaena. The cambial ring arises 

 in the cortex of the root just out- 

 side the endodermis. 



Secondary Thickening of 

 Gymnosperms and Dicotyle- 

 dons. 1. Formation, Struc- 

 ture, and Activity of the 



Cambium in Stems. In the FlG . ice. Transverse section of the stem of Cordyline 



(Dracaena) rubra. f, Primary vascular bundles ; 

 /", secondary vascular bundles ; /"', leaf - trace 

 bundle within the primary cortex ; m, parenchy- 

 matous fundamental tissue ; s, bundle-sheath ; t. 

 tracheides ; c, cambium ring ; cr, cortex, the outer 

 portion being primary, the inner secondary cortex ; 

 ph, cork cambium ; I, cork ; r, bundles of raphides. 

 ( x 30. After STRASBURGER.) 



open vascular bundles of the 

 Gymnosperms and Dicotyle- 

 dons the formation of second- 

 ary tissues may take place as 

 soon as the primary tissues 

 have matured, or may even 



begin before this. Only the 



former case need be considered here. The primary meristem remain- 

 ing between the xylem and phloem of the bundle becomes the 

 cambium and commences again to divide actively. The vascular 

 bundles are usually arranged in a circle. When the cambial activity 

 has commenced in the bundles, cambium also forms across the 

 medullary rays, by parenchymatous cells dividing tangentially. This 



