GROWTH AND NUTRITION 23 



the further food supply, as previously stated, must be 

 constructed from the water and mineral salts of the soil, 

 coming through the roots, and the carbon dioxide of the 

 air, obtained through the leaves. These are combined 

 into more complex carbon compounds and nitrogenous 

 foods in the green portions of the plant, chiefly in the 

 leaves, of which there are now several, located at some 

 distance from the roots and from each other, and the 

 plant is of considerable size. There is need, therefore, of 

 some means of passage or translocation of materials to 

 the places where needed. Such means is soon provided 

 in the form of the fibrovascular bundles, which pass the 

 entire length of the stem and roots, and, by branching, 

 into the leaves (mid-ribs, nerves, veins). These are 

 readily seen in a crude form, as threads through the pith, 

 when a dry stalk of maize is broken through an internode. 



24. Fibrovascular bundles and leaf structure. There 

 are two distinct portions of a bundle in cereal plants, the 

 phloem or outer portion, and the xylem or inner portion. 

 In each of these portions tubes, running lengthwise of the 

 bundle, and therefore of the stem or leaf, are formed 

 by rows of elongated cells placed end to end. In the 

 xylem these tubes or vessels, called tracheids, are very 

 large and have lost their adjacent end walls and proto- 

 plasm and carry chiefly water. In the phloem they are 

 called sieve tubes and carry soluble or finely divided 

 (emulsified) foods (Pfeffer I, p. 577). These vessels are 

 differently formed, having their adjacent cell walls devel- 

 oped into perforated plates, through which their retained 

 protoplasm is continuous. 



A cereal leaf in cross-section is shown to be made up of 

 three main portions, the upper epidermis, a middle por- 

 tion or parenchyma called mesophyll, and the lower epi- 



