106 



STRUCTURE AND FUNCTIONS OF LEAVES 



those of the stem and consist merely of portions of the latter 

 which pass outward and upward from the stem into the leaf 

 under the name of leaf traces. 



The manner in which fibre-vascular bundles pass from the 

 stem through the petiole into the leaf and are there distributed 

 can readily be gathered from an examination of Figs. 116118. 

 Their wood cells and vessels serve to 

 carry water into the leaf, while their 

 sieve cells carry plant food from its 

 place of manufacture in the blade of 

 the leaf down into the stem. 



FIG. 118. Part of the fibro- 

 vascular skeleton of a but- 

 tercup leaf 



Much magnified. After Bon- 

 nier and Sablon 



FIG. 119. Termination 

 of a vein in a leaf 



v, spirally thickened 

 cells of the vein; p, 

 parenchyma cells of 

 the spongy interior 

 of the leaf, with chlo- 

 rophyll bodies ; n, nu- 

 cleated cells. x about 345 diameters 



126. Nutrition. The series of processes by which the plant 

 (1) takes up the raw materials to form its food, (2) unites these 

 into foods, and finally (3) constructs tissue from these foods, or 

 (4) stores them, constitutes nutrition. 



A good deal of that portion of nutrition included under 

 (1) is carried on by the roots. But all kinds of nutritive work 

 are carried on in green leaves, and the portion numbered (2) is 

 a specialty of green plant cells, particularly of those in leaves. 



