STEMS AND THEIR FUNCTIONS 



159 



among which are scattered strands of cambium, most numerous toward the out- 

 side. These cambium strands produce phloem without and xylem within, as in 

 the dicotyledonous stem; but the whole of the cambium is eventually transformed 

 into phloem and xylem. In the mature stem the completed bundles are without 

 cambium and are incapable of further growth; they are therefore called closed 

 bundles. The xylem and phloem produced by the original cambium must function 

 throughout the life of the plant, and the total quantity of vascular tissue cannot 

 be increased. Growth in diameter of the stem is usually slight, and is caused almost 



Fig. 11.6. Cross section of outer part of stem of corn (maize), a monocot, showing scattered 

 vascular bundles. (Courtesy General Biological Supply House, Inc.) 



entirely by increase in the breadth of the individual cells left behind by the 

 advancing meristem of the growing point. 



THE FUNCTIONING OF THE STEM 



The functions of the stem are primarily (1) support of the leaf spread, 

 and (2) transport of inorganic materials, food, and metabolic products from 

 one part of the plant to another. Support is accomplished largely by the 

 various mechanical tissues, aided by the turgor pressure within the cells. 

 In young dicot stems the outer cylinder of collenchyma adds stiffness to 

 the structure, but the mechanical strength of older stems resides almost 

 wholly in the xylem, or wood. Transport is accomplished by the xylem 

 and phloem, which together make up the vascular system. 



The xylem, or wood. Mature xylem consists chiefly of mechanical 

 and conductive tissues but contains a certain amount of parenchyma 

 that serves as a storage tissue. 



The mechanical tissue is made up of fibers — elongated cells with very 

 thick walls strengthened by deposits of an impermeable substance called 

 lignin. The walls of these cells become so thick and impenetrable that 

 the enclosed cell finally dies, and the space that it occupied is nearly 



