136 General Botany 



are manufactured primarily in the leaves, there must be food- 

 conducting tissues that are adequate to carry them to all parts 

 of the stem and roots. The food-conducting tissues also transfer 

 food from the leaves to the seeds and growing parts, and when 

 food has accumulated in the stem or roots it may pass up through 

 the conductive tissues of the stem to other parts of the plant. 



The mechanical tissue is made up of cyHndrical or spindle- 

 shaped cells with very heavy walls. Indeed, the walls at ma- 

 turity may be so thick as to render the cells almost solid. Ordi- 

 nary cellulose is not very hard, but the walls of the mechanical 

 tissue are hardened and thickened by a deposit of lignin, a sub- 

 stance composed of cellulose and certain aromatic compounds. 

 The difference between hard and soft woods is for the most part 

 due to the thickening of the walls of the mechanical cells ; sec- 

 ondarily it is due to chemical changes in the walls themselves 

 (lignification) . 



Mechanical tissue is found on both the water-conducting and 

 food-conducting sides of the bundles. On the food-conducting 

 side it lies outside the food-conducting tissue, and is made up of 

 long, exceedingly slender, nearly solid, spindle-shaped cells. 

 These cells are called hast fibers, and the tissue that is made up 

 of them is called the hast. 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 manufacture of thread and cordage. 



The cells of the mechanical tissue on the water-conducting 

 side of the bundle are somewhat shorter and thicker than the 

 bast fibers. They are known as wood fibers, and make up what 

 is properly called the wood. In most dicots the wood fibers 

 (Fig. 91) are mixed with the water-conducting vessels and Hving 

 thin-walled cells called wood parenchyma, and the whole inner part 

 of the bundles is known as wood. In woody dicots this mechanical 

 tissue is present in abundance and forms the bulk of the stem. 

 The lumber that is obtained from dicotyledonous trees is derived 



