The Structures and Processes of Stems 121 



General structure of the dicot bundle. The bundles in a 

 plant stem terminate above in the veins of the leaves, and 

 below they connect with the bundles of the roots. . In the 

 dicot stem these bundles contain four tissues : (i) the water- 

 conducting tissue, (2) the food-conducting tissue, (3) the 

 cambium, and (4) the mechanical tissue. The cambium is a 

 layer of thin-walled cells that lies lengthwise in the bundle and 

 separates the inner water-conducting tissue from the outer 

 food-conducting tissue. 



Tissues of the dicot bundle. The water-conducting tissue 

 contains long, tubelike vessels made up of cylindrical cells 

 joined end to end, often for considerable distances without 

 end walls between them. These tubes (tracheae) usually 

 have heavy walls marked by spiral and lattice-form thicken- 

 ings. When mature they are empty of protoplasm. In 

 other words, they are the coverings of dead cells joined to- 

 gether to form tubes usually several inches, more rarely several 

 feet, in length. Mixed with them are smaller and shorter 

 tubes, and cylindrical cells that retain the cell contents. 

 All together these tissues form the passageway for the move- 

 ment of water and mineral salts, and sometimes sugar, to all 

 parts of the plant. The general direction of the water move- 

 ment in this tissue is upward, because the lifting of the water 

 is brought about principally by transpiration from the leaves 

 (page 134). 



The food-conducting tissue differs from the water-conducting 

 tissue in being composed of smaller, thin-walled cells, all of 

 which retain their living protoplasm. The largest of these 

 cells are set end to end, and the end walls have holes in them 

 like the top of a salt shaker. These rows of cells, therefore, 

 form tubes with sievelike cross walls in them, and on this 



