88 CIRCULATION AND USES OF PLANT FOOD 



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 called 

 med'ullary rays, or pith rays, radiating outward from the pith 



toward the bark. These are 

 Cambium layer yy Annudl rings thin plates of pith which sepa- 

 ^hrdys rate the wood into a number 

 of wedge-shaped masses . The 

 masses of wood contain 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 continuation of 

 the bundles of tubes which 

 are found in the root. In sec- 

 tions 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) usually represents the amount of growth 

 in one year. Growth takes place from a layer of actively dividing 

 cells, known as the cam'hium 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, but the heavy woody skeletons prevent the evap- 

 oration of fluids from within. The bark also protects the tree from 

 attack of 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. 



There are small breathing holes known as len'ticels scattered 

 through the surface of the bark. These can be seen easily in a 

 young stem of apple, beech, or horse-chestnut. 



Diagram of a twig of box elder three years 

 old, showing three annual growth rings. 

 The radiating lines which cross the wood 

 represent the pith rays or medullary rays. 



