THE CASTOR BEAN 



105 



known as fibrovascular bundles (Lat. fibra fiber + vas = 

 vessel). In the young stem there is a row of eight to ten of 

 these groups, arranged to form a ring a short distance beneath 

 the epidermis. The bundles do not actually touch each other, 

 but the cells of the pith and 

 the cortex are connected. v; i-.i < \9 



Structure of a Fibrovas- 

 cular Bundle.' Figure 45 

 shows a highly magnified 

 view of a cross section of 

 one of these fibrovascular 

 bundles. It consists of three 

 parts : 



1. Running across the 

 middle are several rows of 

 small thin-walled cells 

 known as the cambium 

 layer, c (Lat. cambire= to 

 exchange). These cells are 

 full of active protoplasm 

 and are the chief growing 

 cells of the stem. 



2. On the inside of this 

 layer, and therefore toward 

 the pith, is the xylem, x (Gr. 

 xylon = timber), a somewhat 



FlG. 45. A HIGHLY MAGNIFIED SEC- 

 TION OF A FIBROVASCULAR BUNDLE 



s, sieve cells; 

 t, tracheids; 

 x, is the xylem; 

 ph, the phloem part of 

 the bundle; 



a, accompanying cells; 



c, cambium layer; 

 co, cortex; 



d, ducts; 



pa, parenchyma; 



st, stereome cells. 



triangular mass of cells, the walls of which are thicker than 

 those of the cambium. Among them may be seen at least two 

 kinds of cells; one of small size but with very thick walls 

 forming the tracheids (Gr. trachea = windpipe) or wood cells, 

 t, and the other of larger size with relatively thin walls, 

 forming the ducts or vessels, d. 



3. On the outside of the cambium, and therefore toward 

 the epidermis, is a somewhat irregular mass of cells called the 

 phloem (Gr. phloios = inner bark), ph, within which may be 



