THE LEAVES. 



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

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



FiG. 164. — A few meslies of the finest \'eins of a leaf of A nthyllis. ju, main vein ; /'. ^, 

 branches : a. ij, a. a closed mesh : r. ends of the finest veins ^-ithin the mesh. The 

 drawing sliows only the xylem bundles ; the phloem btmdles accompanving them and 

 the mesophyll cells tilliiig the meshes are not sho\\n. Mod£ratel_\- magtiitied. — After 

 Sachs. 



sion of a single apical cell, and the further di\'ision 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 bv growth throughout the whole extent of the leat". 

 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 monocotvledons, a zone 

 of grou-th occupies the entire base of the blade. By the 

 di\'ision of these cells, chiefly at right angles to the length 



