168 



GREEN PLANTS AS FOOD MAKERS 



layer of actively dividing 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 outer wood and new bark inside the old bark. 



In a very young dicotyledonous stem before the wood of the 

 bundles has formed an annual ring, these individual fibro-vascular 

 bundles are quife separate, arranged in a circle around the cen- 

 tral pith. Each bundle consists of three parts : the outer part, 

 pJdeom, made up of the bast fibers and sieve tubes, through 

 which liquids pass downwards: the middle part, cambium, or 



growth portion which soon 

 develops also between the 

 bundles and thus forms the 

 cambium layer: and an 

 inner part, xylem, made up 

 of woody fibers and ducts 

 with woody walls through 

 which liquids pass upward 

 through the stem. 



Use of the outer bark. 

 The outer bark of a tree is 

 protective. The cells are 

 dead, but the heavy woody 

 skeletons prevent the 

 evaporation of fluids from 

 within. The bark also 

 protects the tree from 

 attacks of plants or animals 

 which might harm it . Most 

 trees are provided with a 

 layer of corklike cells. This 

 layer in the cork oak is 

 thick enough to be of com- 

 mercial importance. There 

 are many lenticels scattered 

 These can be seen easily in a 



In this experiment the willow twig was girdled by 

 taking off the bark. Can food now reach the part be- 

 low the ring ? Why have roots come out aDove the 

 ring? Why has a sprout appeared below the ring? 



through the surface of the bark 



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



