68 ELEMENTS OF PLANT ANATOMY. 
secondary growth, bordered pores occur ; these are seldom if ever 
found in the corresponding phloem tissue or bast. 
WOOD PARENCHYMA. 
The parenchymatic tissue of xylem originates like that of 
phloem by the division of long cells cut off from the cambium. 
This may be seen from their appearance and position in a longi- 
tudinal section, where they lie in rows, parallel to the axis, 
each about the length of a cambium cell, and each end cell has 
the characteristic sharpened point of cambium tissue. It often 
happens that division does not take place, the cell cut off from 
the cambium remaining long and thin-walled, and giving rise to 
a tissue which can neither be classed as parenchyma or libriform, 
but which has received the name of intermediate tissue. Like 
the parenchyma of the phloem, that of the wood consists of 
living cells, and their position in the xylem indicates that their 
function is, in part, connected with the upward transfer of 
water. 
Russow divides all vascular bundles into three classes accord- 
ing to the arrangement and relative position of phloem and 
xylem.) Phese are, 
1. Collateral—Xylem on one side, phloem on the other. 
2. Concentric—Xylem in the middle, phloem around it, 
sometimes reversed. 
3. Radial—Xylem and phloem radially arranged and alter- 
nating. These will be treated more fully in the subsequent 
chapters on the anatomy of the various groups of plants. The 
vascular bundle is in many cases separated from the surrounding 
tissues by a layer of cells differing from those within and without. 
This layer is known as the endodermis or sheath. Its contents 
are generally starch. 
The tissues of the complete fibro-vascular bundle are tabu- 
lated by Haberlandt as follows : ‘ 
