MORPHOLOGY OF HIGHER PLANTS. 201 



changes in the primary structure (Fig. iii). Some of the cells 

 lying between the ducts and the sieve become meristematic, form- 

 ing a continuous zone or ring, known as the cambium (CA). 

 On the outside of the cambium not only sieve cells but bast fibers 

 and bast parenchyma may be developed, and these constitute the 

 PHLOEM (P). Besides the ducts upon the inside of the cambium 

 tracheids, wood fibers and wood parenchyma may develop, form- 

 ing the XYLEM (X). One group of xylem and phloem, together 



Fig. III. Section in the older part, higher up on the root of pea (Pisum) showing 

 in addition to what has been observed in Fig. no, the beginning of the change from primary 

 to secondary structitfe: CA, the development of a cambium; SX, secondary xylem, and 

 SP, secondary phloem. 



with the intervening cambium, constitutes an open collateral 

 FiBROVASCULAR BUNDLE. Another kind of cells, which are 

 nearly isodiametric or radially elongated, also arise from the 

 cambium and form radial rows, which are known as the medul- 

 lary RAYS (Fig. 112, PM, SM). The latter separate the fibro- 

 vascular bundles, and the number of cells, both as regards the 

 width and depth of the rows, is characteristic for certain genera. 



