136 



AGRICULTURAL BOTANY 



into two great classes. By far the greater number 

 of plants are outside growers ; that is, the woody part 

 of the stem is arranged in rings or layers about a cen- 

 ter, called a pith, and outside of this is a corky layer, 

 called the bark. The growth each year occurs by addi- 

 tion on the outside of the previously formed wooden 

 rings, although inside the bark. The outside growers 

 are called Exogens, or Exogenous plants. 



Note- The inner bark of the stem contains some very fine ducts, 

 or tubes, called sieve tubes. Through these tubes the larger part of 

 the liquid food prepared in the leaves is passed to parts of the plant foi 

 storage or to increase the size of the plant. The flow of sap in general 

 is, then, upward through the sapwood, and downward through the sieve 

 tubes of the inner bark. Besides this there is a considerable movement 

 of sap from one cell to another, both upward, downward, and laterally. 



Between the bark and the sapwood is a very thin 

 layer, or zone, that is the growing part of the tree. It is 



called the cambium layer. 

 It is well supplied with 

 cells filled with proto- 

 plasm. In spring this 

 layer becomes so gorged 

 with nourishment that 

 if a twig of hickory or 

 willow is pounded, the 

 cambium layer is broken 

 up and the bark may be 

 slipped off the wood. 

 Boys utilize their prac- 

 tical knowledge of this 



FIG. 61. Section of an Endogenous Stem. . , . 1-1 T 



in making whistles. It 



is the same fact that tempts goats and calves to bark 

 trees in the spring, and enables savages, in time of 



