WHERE FOOD COMES FROM 



119 



The fibrovascular bundles conducting water from the roots are 

 called xylem, or wood. Those conducting saps from the leaves 

 are called phloem, or bast. The end walls of the bast vessels 

 have pores in them and are called sieve plates (Fig. 72). 



In woody plants the xylem bundles are arranged in concentric 

 cylinders, or tubes inside of tubes, and make up the wood. The 

 phloem bundles are ar- 

 ranged around the wood, in 

 theinnerportionof the bark. 

 Between the outer layer of 

 wood and the inner layer of 

 bast is the cambium, or 

 growing layer, from which 

 all thenewxylemandall the 

 new phloem cells originate. 



The fibrovascular bun- 

 dles branch and divide so 

 that they reach into all the 

 twigs and leaves. In the 

 leaf they branch again and 

 make up the so-called veins 

 or nerves of the leaf blade. 

 The sap-carrying vessels 

 of the root are connected 

 with similar tubes found in 

 the stem. In many plants 

 the bundles of vessels and 

 libers 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. 



97, The circulation of sap. We do not yet understand all 

 about the rise of sap in trees. It is certain, however, that the 

 water taken in by the roots does rise to the leaves, and that 

 it goes through the xylem vessels. We know also that organic 

 food is formed in the leaves, and that it accumulates in roots 



Fig. 71. Prop roots 



Near the base of the trunk the screw pine 

 (Paudamus) sends out prop roots in a man- 

 ner similar to that of the Indian corn. (From 

 photograph lent by the New York Botanical 

 Garden) 



