STEMS, BULBS AND BUDS 



49 



which connects the fascicular p^arts and gives rise to the medvl- 

 lary rays (Fig. 23). The cambium is made up of very delicate, 

 active parenchyma cells, which 

 divide longitudinally and par- 

 allel to the surface of the stem 

 {i.e., tangentially) with occa- 

 sional longitudinal divisions 

 at right angles to the surface 

 of the stem {i.e., radially) and 

 witJi occasional cross-divisions. 

 As a result of these tangential 

 divisions new layers of cells 

 are added to the fibro-vascular 

 bundles, and the diameter of 

 the stem gradually increases. 

 Most of these layers are added 

 to the xylem, which grows 

 more rapidly than the phloem. 

 However, in this latitude the 

 xylem ceases to grow about the 

 middle of August, but the 

 phloem continues to grow until 

 l^ovember. These new layers 

 of cells are of the same charac- 

 ter, but undergo differentia- 

 tion to form the various tissues 

 of the xylem and phloem 

 which have been already de- 

 scribed. We are unable to 

 explain why some of tlieee cells 

 4' 



— Diagrammatic drawing of dico- 

 tyledonous stem showing relative location of 

 the tissues and systems. The epidermal sys- 

 tem composed of the cork (a, 6), the phel- 

 logei^ or cork cambium (c) , and the coUen- 

 chyma (d) ; the cortex composed of paren- 

 chyma (e, g) and the bast (/) ; the fibro- 

 vascular system composed of the pUoetnC/i), 

 the cambium (?) and the xylem {j)\ the 

 medullary rays ik), and the pith {I). 



