THE GROWING PLANT 



45 



The threads in the stalk of Indian corn and the leaf- 

 stem of the plantain (Plantago) furnish examples of well- 

 defined vascular bundles; in most stems the vascular 

 bundles are less clearly defined. In woody stems they 

 are closely crowded, which gives the wood its firm texture. 

 In some woody plants, as the grape and the elder (Sam- 

 bucus) a cylinder extending through the center of the 

 stem is free from vascular bundles, forming the pith. The 

 young stems of as- 

 paragus, the ball of 

 the kohlrabi and the 

 roots of the turnip 

 are " stringy " when 

 the cells of their vas- 

 cular bundles be- 

 come thickened by 

 the deposit of woody 

 material in them. 



68. The cambium 

 layer. — In most 

 plants having two 

 or more cotyledons 

 (45), a layer of cells 

 in a state of divi- 

 sion (15) exists between the bark and the wood, 

 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 



Fig. 21. — Showing cross-section of a vas- 

 cular bundle of the sunflower, Helian- 

 thus annuus. Highly magnified. See also 

 Fig. 22. 



