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. 



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 Fm 45 _ A HIGHLT MAGNIFIED SE0 . 



TION OF A FIBROVASCULAR BUNDLE 



a, accompanying cells; s, sieve cells; 



c, cambium layer; t, tracheids; 

 co, cortex; x, is the xylem; 



d, ducts; ph, the phloem part of 

 pa, parenchyma; the bundle; 



st, stereome cells. 



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 



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 y 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 



