DISTRIBUTION OF WATER THROUGHOUT LEAF 115 



with the mid-rid, but some run an independent course from 

 base to apex, as seen in grass leaves. These leaves are called 

 parallel-veined (Fig. 58, B}. In the netted- veined sort the 

 veins divide and subdivide until the meshes are extremely small- 

 in some leaves approximately 0.2 mm. in diameter, and the 

 ultimate branches end free in the mesophyll. In the parallel- 

 veined type the main veins running from base to apex are united 

 by frequent cross branches (Fig. 58, D). Where the ultimate 

 branches are 0.2 mm. apart water from them needs to flow 



FIG. 59. Semi-diagrammatic cross-section of a leaf showing by arrows how the water 

 passes from the tracheal elements of a vein into the border parenchyma cells, and thence into 

 the palisade and spongy parenchyma, from which it evaporates into the intercellular spaces 

 and passes from the leaf through the stomata. a, upper epidermis; b, lower epidermis; c, 

 palisade parenchyma; g, spongy parenchyma; d, border parenchyma; e, tracheal elements; 

 and the stippled cells below e, the phloem cells. 



laterally only o. i mm. to supply the mesophyll cells at the center 

 of the mesh. Illustrations of this kind make clear the general 

 fact that the provision for the distribution of water throughout 

 the leaf is very efficiently wrought out. 



In the successively smaller branches of the bundles in leaves 

 we find the tracheal elements becoming fewer and smaller until 

 the smallest ramifications may have but a single line of tracheids. 

 The phloem elements also are successively reduced in the smaller 

 and smaller branches; the sieve tubes give way to undivided 



