HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 105 



the primary medullary rays ; (5) the hypodermal cortex- 

 layer has produced a zone of periderm, lying within the 

 epidermis and consisting of flattened cells arranged in 

 regular radial rows. 



Some of these points may be seen better in sections from 

 the older regions, to which we shall now pass. 



118. T. S. Three or Four Year Old Lime Stem. 



Note (1) the disorganised and torn epidermis ; (2) the 

 periderm ; (3) the cortex ; (4) the triangular phloem 

 masses, consisting of the alternating tangential bands of 

 thick- walled (fibrous) and thin- walled tissue ; (5) the 

 cambium, a narrow zone of flat thin-walled cells in 

 radial rows as usual ; (6) the arrangement of the xylem 

 in three or four layers (annual rings) which may vary 

 a good deal in thickness ; (7) the pith, and (8) the 

 medullary rays. 



Starting from the centre, note the following details : The pith 

 shows large empty (air-containing) cells tending to be arranged in 

 rosettes around small cells, which may contain tannin or crystals ; 

 in the outer part there are large mucilage sacs ; and the outermost 

 pith tissue, into which project the primary masses of wood containing 

 the protoxylem vessels, consists of small cells with tannin or starch. 



In each annual wood-ring large vessels are produced at first, but 

 later in the year the cambium produces only narrow xylem elements 

 the abrupt change from the close-textured autumn wood to the 

 open spring 1 wood of next year produces the ringed appearance 

 of the secondary wood. The wider xylem elements are pitted 

 vessels; the narrower ones are either tracheids (resembling 

 vessels in having, where in contact with other tracheids or vessels, 

 bordered pits on their walls) or fibres (with a few fine pits), or 

 parenchyma cells with protoplasm and sometimes starch. 



The primary medullary rays are two or more cells broad 

 tangentially, and in their widened fan-like outer portions (between 

 the phloem wedges) there are obvious signs of tangential elongation 

 and radial division of the cells, to keep pace with the expansion of 

 the stem as secondary thickening proceeds. The secondary rays 

 are usually only one cell wide ; some of them run through from 

 pith to cortex (interrupting the sclerenchyma bands in the phloem), 

 while others can only be traced from the cambium through part of 

 the xylem and part of the phloem all the secondary rays of course 

 pass through the cambium in both directions, since they are formed 

 by the cambium. 



During the first year several alternating bands of hard and soft 

 tissue may be formed in the phloem, but later on the cambium 



