ELEMENTARY BIOLOGY 



probably goes from the leaves by way of another set of vessels 

 called phloem, or bast. These vessels are generally larger in 

 diameter than the wood vessels (xylem) and are characterized 

 by having pores in the end walls. These end walls, with their 

 perforations, are sometimes called sieve plates (Fig. 65). The 



toughened fibers associated with 

 the bast vessels make up the 

 bast fibers. 



In the leaves the bast and the xylem 

 vessels are closely compacted in the 

 veins. In wood we have masses of 

 xylem vessels and fibers. Bast fibers 

 are commonly used in the form of 

 linen, hemp, and jute fibers. In woody 

 plants the bast is located in the bark. 



205. The ascent of sap in trees. 



Investigators have long been puzzled 

 by this. problem, and we are not yet 

 sure that we understand it. How- 

 ever, it is certain that the water 

 does rise, and that it goes through 

 the xylem vessels. 

 206. The descent of sap. From certain common observations 

 and from the results of experiments we may reasonably infer 

 that sap descends. We know that organic food is formed in 

 the leaves, and that it is accumulated in the roots and in 

 underground stems of many plants. There must therefore 

 be a current of material passing downward from the leaves. 



A tree that is girdled, that is, one that has a ring of bark 

 removed, will continue to live for the rest of the season. This 

 shows that the removal of the bark does not interfere with the 

 ascent of water and salts from the roots to the leaves. The 

 following spring, however, when the opening of the buds with 

 the rapid expansion of leaves and twigs depends upon the food 

 accumulated during the previous season, the tree will be found 



FIG. 64. Veins of a leaf 



Leaf of apple of Sodom (Solanum 

 aculeatissirmi'ni), of the potato fam- 

 ily, showing network of veins 



