122 ANATOMY OF STEM, ROOT AND LEAF 



B. The perennial woody stems of dicotyledons. 



(a) Division of the cambium-cells. — In the earliest stages of 

 the stems of shrubs and trees the arrangement and constitution 

 of the tissues are essentially the same as in simple short-lived 

 herbaceous stems. With an increase in age there is, however, a 

 steady increase in thickness from year to year, and in transverse 

 sections of such thickened stems the isolated small vascular 

 bundles, so obvious when the stems are very young and soft, 

 are no longer visible. 



The greatest part of the increased bulk of tissues in such stems 

 as these, is brought about by divi- 

 sion of the initial cells of the 

 cambium-ring. 



Each initial cambium-cell (a. 

 Fig. 56) divides in two by a wall 

 parallel to the surface of the stem ; 

 one of these two daughter-cells 

 remains permanently capable of 

 division while the other is either 

 directly converted into a permanent 

 cell, or divides once or twice, after 

 which the cells produced become 

 gradually changed into permanent 

 elements. The change into a per- 

 manent cell or cells may happen 

 to either of the two produced by division of the initial cell ; 

 if the inner one is modified it is added to the wood i^v), if the 

 outer one is altered it goes to increase the bast {U) 



Division of the cambium-cells, and the growth and develop- 

 ment of the products continue from spring to autumn ; in winter, 

 cell-division ceases. Since the cambium extends in the form of 

 a continuous cyHnder within the stem, a new cylinder of wood 

 is added every growing season to the outside of that already 



Fig. 56. — Transverse section through 

 a small portion of the cambium-ring 

 in a young black currant shoot. £: Cam- 

 bium; a initial cell ; 7(/wood ; ^bast; 

 m medullary ray. (Enlarged about 450 

 diameters.) 



