52 



Principles of Plant Culture. 



since the individual cells overlap and are in intimate 

 contact, they form threads or fibres. These fibres serve 

 the double purpose of giving strength to the plant and 

 conducting water, with its dissolved food materials, to 

 the different parts. By the absorption of the ends of 

 some of the cells, tubes (duets), of 

 very considerable length are formed. 

 In other cells of vascular bundles, 

 the walls are much thickened and 

 strengthened by woody deposits. 

 These groups or bundles of fibres 

 and ducts divide and subdivide in 

 the leaves, forming the so-called 

 veins and veinlets. In the roots 

 they divide in a similar manner, ex- 

 tending lengthwise through all the 

 branches and branchlets. 



Fig. 20 shows a longitudinal sec- 

 tion of a vascular bundle of the rye 

 plant and Ftig. 21 shows a cross-sec- 

 tion of a vascular bundle of the sunflower. 



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

 stem of the plantain* 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,t a 

 cylinder extending through the center of the stem is 

 free from vascular bundles, forming the pith. The 

 young stems of asparagus, the ball of the kohl-rabi and 



Prosen- 



from 



Fig. 

 chyma cells 

 stem of rye. Highly 

 magnifled. (After 



Frank and Tschirch.) 



' Plantago. 



Sambucus. 



