TISSUE-SYSTEMS. 173 



tissues. As a rule, these tissues are capable of division 

 into two groups, in one of which the wood is developed, 

 and in the other the bast. To the former of these groups 

 the general term xylem is applicable, and to the latter the 

 term phloem. The xylem is made up of the elongated 

 woody cells with pointed and overlapping ends, already 

 referred to as fibrous tissue, the wide tubes (vessels) 

 with variously marked walls, formed by the disappearance 

 of the cross-partitions between cells placed end to end, 

 and more or less short-celled tissue or parenchyma. The 

 phloem is likewise made up of three constituents : the 

 long, thick-walled, flexible cells called bast-cells, which 

 correspond to the fibrous tissue of the xylem ; the wide 

 thin-walled sieve-cells, corresponding to the vessels; and a 

 certain amount of thin-walled parenchyma. 



296. The fibro-vascular bundles, as they are called, 

 have their origin in the meristem of the growing point. 

 This meristem is at first uniform, but soon groups of long 

 cells arise in it, and these are then 

 known as procambiitm, to distinguish 

 them from the surrounding ground- 

 tissue. This procambium is gradually 

 converted into the fibro-vascular 

 bundles. 



297. In dicotyledonous plants, the 



Fig. 228. , . , , 



fibro-vascular bundles are more or less 

 wedge-shaped, as shown in Fig. 228. The inner part of 

 each bundle consists of xylem and the outer of phloem, 

 and between the xylem and the phloem there is a layer 

 of meristem, known as the cambium. The soft cells of 



Fig. 228. Cross-section of a young dicotyledonous stem, showing six bundles. 



