The Inner Structure of the Plantlet. 53 



the roots of turnip are "stringy" when the cells of 

 their vascular bundles become thickened by the de- 

 posit of woody ma- 

 terial in them. 



68. The Cam- 

 bium (cam'-bi-um) 

 Layer. In most 

 plants having two 

 or more cotyle- 

 dons (45), a layer 

 of cells in a state 

 of division (15) 



FIG. 21. Showing cross-section of a vas- . , ,, 



cular bundle of the sunflower. (Helian- CXlStS between U16 

 thus annuus). Highly magnified. (After , n , ., 



Pranti.) See also Fig. 22. bark andthewood, 



called the cambium or cambium layer (Fig. 22). It 

 is in this layer that growth in diameter of the stem oc- 

 curs (70). The bark of plants having the cambium 

 layer separates readily from the wood at times when 

 growth is rapid, because the walls of the newly-formed 

 cambium cells are extremely thin and tender. The 

 slimy surface of growing wood, whence the bark has 

 just been removed, is due to the protoplasm from the 

 ruptured cambium cells. In plants having more than 

 one cotyledon, the cambium line is usually readily dis- 

 cerned in cross-sections of the stem though it is rather 

 more distinct and the bark is more readily separable in 

 woody than in herbaceous* stems. In the latter, the 

 part within the cambium line corresponds to the wood 

 of woody stems, and that outside of it corresponds to 

 the bark. 



* Herbaceous stems are those that do not have the hard, firm 

 texture of wood, as the potato, rhubarb, etc. 



