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ELEMENTARY BIOLOGY 



blade (Fig. 64). The sap-carrying vessels of the root are con- 

 tinuous with similar tubes found in the stem. In many plants 

 the bundles of vessels and fibers are readily pulled out from 

 among the surrounding pith cells. In celery these bundles 

 make up the "strings," and in the plantain the so-called 



" nerves " that we see sticking out 

 of the stalk when we pull up a 

 leaf. 



In the veins of leaves the phloem 

 and the xylem vessels are closely com- 

 pacted. In herbaceous stems the phloem 

 and xylem are also united ; in woody 

 plants the phloem is located in the bark. 

 Bast fibers are commonly used in the 

 form of linen, hemp, and jute fibers. 



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. 

 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 {Solaiium 

 aciilcatissimum), of the potato fam- 

 ily, showing network of veins 



206. The descent of sap. 



