124 



Science of Plant Life 



bamboo stem photographed through a 

 microscope. The large openings in each 

 bundle are the water-conducting tubes. 



that is made up of them is 

 called the bast. Bast may be 

 seen in the stringy fibers on a 

 grapevine or in the bark of 

 trees. It is the bast fibers 

 from flax, hemp, jute, and 

 other dicotyledonous plants 

 that are used in the manufac- 

 ture of thread and cordage. 



The cells of the mechanical 



FIG. 72. Cross section of a portion of tissue on the water-conducting 



side of the bundle are some- 

 what shorter and thicker than 

 the bast fibers. They are 



known as wood fibers and make up what is properly called 

 the wood, although in most dicots the wood fibers are mixed 

 with the water-conducting vessels, and the whole inner part 

 of the bundle is known as wood. In woody plants this me- 

 chanical tissue is present in abundance and forms the bulk 

 of the stem. The lumber that is obtained from dicotyledon- 

 ous trees is derived from the inner parts of the bundles and 

 is made up of wood fibers and water-conducting tissues. 



The cambium is a layer of soft tissue between the two sides 

 of the bundle. It is the principal growing tissue of the dicot 

 stem. Growth takes place in it by the longitudinal division 

 of the cells. On its inner face the cells of the cambium layer 

 change into water-conducting cells or wood fibers ; on its 

 outer face they change into food-conducting cells or bast 

 fibers. In this way the bundles of perennial dicots enlarge 

 from year to year, and this causes the stem to increase in 

 thickness. In a tree the cambium cells form a continuous 



