THE ANGIOSPERMAE : STEMS 901 



The distribution of the xylem parenchyma is highly characteristic of 

 particular species of wood and is one of the features by which they may be 

 identified. There are two main types : — 



1. Apotracheal, which includes those occurring in tracts which are inde- 



pendent of the vessels, i.e., metatracheal, and also the rarer cases 

 where they are diffuse or dispersed. 



2. Paratracheal, which includes all cases where they are grouped around 



the vessels. 



In Conifers the xylem parenchyma usually occur only in the last row of 

 elements in each annual ring. This arrangement, called terminal, is very 

 rare in Angiosperms, being known only in Magnolia and Salix. 



10. The Medullary Rays. The majority of the medullary rays in secondary 

 wood are uniseriate, that is one cell in breadth as seen in transverse sections 

 of the wood, but broader, compound rays occur in some species, while 

 aggregate rays, formed by the close grouping of a number of uniseriate rays, 

 also occur, especially in association with leaf or branch gaps in the woody 

 cylinder. All rays which originate from cambial cells are classed as secondary 

 rays, and new rays, intercalated between the previously existing ones, are 

 commenced in each year of growth, so that the total number steadily increases 

 with the age and girth of the tree, the average distance between rays, 

 tangentially. remaining roughly uniform in successive rings of growth. Once 

 a ray has been started it is invariably carried on through all subsequent 

 growth rings. Outside the cambium the ray is continued in the phloem, 

 so that each ray has a xylem and a phloem portion. The ray in the phloem 

 may be both longer and wider than in the xylem and in plants such as Tilia, 

 where the phloem rays are V-shaped, the widening is due to serial cell 

 divisions resembling cambial growth. 



^Nledullarv rav initials, which are much shorter than other cambial cells, 

 cut off cells both inwards and outwards by successive tangential divisions, 

 though manv more are added to the xylem rays than to the phloem rays, 

 corresponding to the more rapid formation of xylem. The cells cut off are 

 directlv converted into parenchyma and added to the rays without further 

 divisions. The xylem ray cells become lignified like the other xylem 

 parenchvma, the phloem ray cells are thin-walled and pitted, like phloem 

 parenchvma. During the winter, when the cambium is dormant, the ray 

 initials in the cambium take on the character of phloem ray cells, so that 

 there is then no break in the radial continuity of the two portions of the ray. 

 New initial cells are formed in spring by the tangential division of the inner- 

 most phloem ray cell. 



Most rays are comparatively short in the vertical direction, some no 

 more than one cell in height, and the tallest are not usually more than twenty 

 cells (Figs. 884 and 885). The cells are, with few exceptions, uniform in 

 character, and all contain living protoplasm, nuclei, and reserves of food 

 material. They remain living without growth or division for as long as the 

 plant exists, and the ray cells of heart wood in old trees are the oldest living 



