TRANSFER OF MATERIALS IN PLANTS 



175 



Mcdulhiry 

 ray 



cells, some consisting of series of cells from which the end 

 walls have disappeared, leaving long, continuous channels. 

 Some of these vessels are illustrated in Figs, 10 and 62. 



The water and salts 

 from the roots pass 

 through one set of tubes, 

 and the manufactured 

 food from the leaves 

 through another. The 

 fibrovascular bundles 

 conducting water from 

 the roots are called 

 xylem, or wood ; those 

 conducting food sap are 

 called pJilocm, or bast. 

 The bast vessels are 

 characterized by the 

 presence of pores in 

 their end walls ( Fig. 65), 

 These end walls with 

 their perforations are 

 called sieve plates. 



In woody plants the 

 fibrovascular bundles of 

 xylem are arranged in 

 concentric cylinders and 

 constitute the wood of 

 the tree. The phloem 



bundles are arranged around the wood, in the inner portion of 

 the bark. Between the outer layer of wood and the inner layer 

 of bast is the cambium, or growing layer, from which all the 

 new xylem and all the new phloem cells originate. 



The fibrovascular bundles branch and divide so that they 

 reach into all the twigs and leaves. In the leaf they branch 

 again and constitute the so-called veins, or nerves, of the leaf 



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Fig. 63. Structure of a woody stem 



Diagrammatic view of piece of two-year-old black- 

 berry stem, showing the central pith, surrounded by 

 the wood of the first and second years, the bai-k, and 

 the epidermis E. The wood is made up of wood 

 fibers, IF, and vessels, Pd, representing a pitted 

 duct. Between the wood and the bark is the grow- 

 ing, or cambiton, layer, and this is connected with 

 deeper layers by means of the Medullary rays, Mr. 

 Under the skin are some green cells, Gc, and running 

 through the bark cells C are bast fibers, Bf, as well 

 as bast vessels, which are not shown 



