36 BOTANY 



strands varies in' different roots ; it is very common to 

 find four, but two is not an infrequent number. They 

 may extend completely to the centre and all unite there 

 to form a solid cylinder. If the number is large they 

 .generally fuse together before extending so far, leaving 

 a small-celled column as a core. This is known as a 

 -pith. In form the bundles are wedge-shaped, the apex 

 of the wedge pointing outwards. 



If we trace these conducting strands towards the tip 

 of the root they can be distinguished among the soft 

 cells of the plerome by their narrow diameters and their 

 tendency to elongation. The area of each embryonic 

 strand can be seen distinctly in a transverse section, 

 their small size and a certain density of their protoplasm 

 marking them off from their neighbours. The gradual 

 change from these cells to the mature forms can be 

 traced; the alteration of the wall and its thickening 

 appear first along the outer edge of the wedge, known 

 consequently as the protoxylem, and extending thence 

 towards the centre of the root. 



If these vascular bundles are traced along the root in 

 the direction opposite to the tip they are seen to be 

 continuous with similar structures in the stem. In this 

 way a path is made throughout the plant for the trans- 

 port of the water after its absorption. 



These strands are chiefly concerned with the func- 

 tion of the root. Others which also are traceable 

 throughout the plant can be seen to lie one between 

 each pair of them in the central cylinder. These are 

 chiefly concerned in the nutrition of the root. They 

 are equally well defined and lie side by side with the 

 wood strands, separated from them by a few packing 

 cells. They differ in texture, their walls remaining 

 cellulose. They are known as bast or phloem ; and are 

 made up of vessels known as sieve tubes from . their 

 terminal walls being somewhat thickened and perforated 



