52 Principles of Plant Culture. 



stem of the plantain * furnish examples of well-defined 

 vascular bundles; in most stems the vascular bundles 



are less clearly de- 

 fined. In woody- 

 stems they are 

 closely crowded, 

 which gives the 

 wood its firm text- 

 ure. In some woody 

 plants, as the grape 

 and the elder f a 

 cylinder extending 

 through the center 



Fig. 21. Showing cross-section ot a vascular ^ j., (. • r- 



bundle of the sunflower, (Helianthus annuus). " lU-C oiciu. lo licc 

 Highly magnlfled. (After Prantl). See also from VaSCular 



■^'^■^^' bundles, forming 



t^xepith. The young stems of asparagus, the ball of the 

 kohl-rabi and the roots of turnip are "stringy" when 

 the cells of their vascular bundles become thickened by 

 the deposit of woody material in them. 



69. The Cambium (cam'-bi-um) Layer. In most plants 

 having two or more cotyledons (-16), a layer of cells in 

 a state of division (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 occurs 

 (71). 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 re- 

 moved, is due to the ijrotoplasm from the ruptured cam- 



* Plantago. f Sanibucus. 



