THE LEAVES. 



137 



169. Growth.- -The growth of the leaf is at first apical. 

 In fern-worts its tissues are produced by the continued divi- 



FIG. 164. A few meshes of the finest veins of a leaf of A nthyllis. tii, main vein ; b, 6, 

 branches ; a, a, a, a closed mesh ; c, ends of the finest veins within the mesh. The 

 drawing shows only the xylem bundles ; the phloem bundles accompanying them and 

 the mesophyll cells filling the meshes are not shown. Moderately magnified. After 

 Sachs. 



sion of a single apical cell, and the further division of each 

 of the segments so produced (/, fig. 76). The branches of 

 such leaves, therefore, arise in acropetal succession. In most 

 seed plants, instead of a single apical cell, there is a cluster 

 of such cells. Growth at the apex often ceases early, and is 

 replaced by growth throughout the whole extent of the leaf. 

 This intercalary growth is sometimes localized between the 

 fundament of leaf base and blade, producing the petiole 

 when one is formed. In elongated leaves without distinct 

 petiole, as in grasses and many other monocotyledons, a zone 

 of growth occupies the entire base of the blade. By the 

 division of these cells, chiefly at right angles to the length 



