334 THE WOODY STEM 



division a vertical file of phloem parenchyma cells. 

 The xylem fibres are usually more strongly lignified 

 than the phloem fibres, and the xylem parenchyma is 

 often, though not always, thick walled and lignified. 

 The tracheids and fibres commonly elongate very much 

 in developing from the tissue mother cells, and their 

 pointed ends slide past one another, so as to overlap 

 very considerably. 



Secondary Rays. Certain cambial cells, instead of 

 forming any of the above-mentioned elements, cut off 

 cells on both sides, which then divide horizontally, 

 each cell of the file so formed sometimes elongating 

 radially, so that it corresponds on each side with 

 several elements of the secondary xylem or phloem. 

 Several cambial cells adjacent in the vertical direction 

 (sometimes also several adjacent in the tangential 

 direction) behave simultaneously in this way, and once 

 a set 'of cambial cells has begun to form them, it does 

 not produce anything but this kind of cell. In 

 this way vertical plates of tissue called secondary rays 

 are formed, running through the secondary vascular 

 tissue in a radial direction, several or many cells deep 

 (i.e. in the vertical direction) and one or several cells 

 broad (i.e. in the tangential direction). As the secondary 

 xylem increases in bulk it also of course increases in 

 circumference, and the cambium increases in circum- 

 ference with it. This it does by the radial division of 

 its cells, i.e. by putting new cells into its circumference. 

 Every year fresh secondary rays are begun, while those 

 already begun are continued, so that in proceeding from 

 the centre to the circumference of the secondary xylem 

 of a woody stem several years old, the number of rays 

 cut by successively larger circles continually increases, 

 and the secondary tissue, as seen in cross-section, is 



