98 CIRCULATION AND USES OF FOOD BY PLANTS 



sievelike ends. Through these cells food materials pass downward 

 from the upper part of the plant, where they are manufactured. 



In the wood will be noticed (see Figure) a number of lines radiat- 

 ing outward from the pith toward the bark. These are thin plates 

 of pith which separate the wood into a number of wedge-shaped 

 masses. These masses of wood are composed of many elongated 



cells, which-, placed end to end, 

 form thousands of little tubes 

 connecting the leaves with the 

 roots. In addition to these are 

 many thick-walled cells, which 

 give strength to the mass of 

 wood. The bundles of tubes 

 with their surrounding hard 

 walled cells are the continua- 

 tion of the bundles of tubes 

 which are found in the root. 

 In sections of wood which have 

 taken several years to grow, 

 we find so-called annual rings. 

 The distance between one ring 

 and the next (see Figure) usu- 

 ally represents the amount of 

 growth in one year. Growth 

 takes place from an actively dividing layer of cells, known as the 

 cambium layer. This layer forms wood cells from its inner surface 

 and bark from its outer surface. Thus new wood is formed as a 

 distinct ring around the old wood. 



Use of the Outer Bark. The outer bark of a tree is protective. 

 The cells are dead, the heavy woody skeletons serving to keep out 

 cold and dryness, as well as prevent the evaporation of fluids from 

 within. The bark also protects the tree from attack of other 

 plants or animals which might harm it. Most trees are provided 

 with a layer of corky cells. This layer in the cork oak is thick 

 enough to be of commercial importance. The function of the 

 corky layer in preventing evaporation is well seen in the case of 

 the potato, which is a true stem, though found underground. If 



Section of a twig of box elder three years 

 old, showing three annual growth rings. 

 The radiating lines (ra) which cross the 

 wood (w) represent the pith rays, th3 

 principal ones extending from the pith 

 in the center to the cortex or bark. 

 (From Coulter's Plant Relations.) 



