200 PHARMACEUTICAL BOTANY 
differ in structure from an annual. By the close of the year a 
cork cambium (phellogen) has originated beside the epidermis. 
In the origin of the cork cambium—one of two methods: (a) either 
the epidermis may divide into an outer layer of cells that remains 
irae SGT LS: 
Po 
Fic. 130.—Portion of cross-section of four-year-old stem of Aristolochia sipho, as 
shown by the rings of growth in the wood. The letters are the same as in Fig. 129 
but new tissues have been added by the activity of the cambium; and a cork cam- 
bium has arisen from the outermost collenchyma ceils and given rise to cork. 
The new tissues are: /, cork cambium; f, cork; g, secondary phloem from the 
cambium, and just outside this is older crushed phloem; n, secondary xylem pro- 
duced by the cambium; m, secondary medullary ray made by the cambium (notice 
that this does not extend to the pith). Half of the pith is shown. Notice how 
it has been crushed almost out of existence. Compare Figs. 129 and 130, tissue for 
tissue, to find out what changes the primary tissues undergo with age, and to 
what extent new tissues are added. Photomicrograph X 20. (From Stevens.) 
epidermis and an inner layer of cells that becomes cork cambium, 
or, (b) the outermost layer of cortex cells underneath the epi- 
dermis becomes active after being passive for one year, and lays 
down walls, the inner layer becoming cork cambium, the outer 
becoming a layer of cork. The cork cuts off water and food 
supplies from the epidermis outside, and so the epidermis sepa- 
