638 ECOLOGY, 



as a class dicotyls have a much greater capacity for propagation by leaves than have 

 monocotyls. In a great many species whose leaves appear unable to give rise to 

 new plants, roots originate somewhat readily from the leaves. These plants rarely 

 exhibit leaf propagation in nature, chiefly because living leaves rarely fall to the 

 ground. Hence in seed plants the capacity for leaf propagation is not especially 

 advantageous. 



Conductive tissues in leaves. — General features. — Veins are the mechanical 

 framework of leaves and also the paths of conduction, and diflferences in venation 

 form most conspicuous leaf features. Most dicotyl leaves have a prominent mid- 

 rib, whose branches fork and anastomose repeatedly, thus forming a reticulated net- 



FiG. 935. — The skeletonized edge of a leaf of a Ficus, showing the anastomosing of 

 the finer veins. — From Land. 



work of small veins (fig. 93S). The midrib and the larger veins are the main trunk 

 lines along which water and foods pass, and the smaller veins connect these 

 with the chlorenchyma. In some dicotyls (as in the maples, fig. 779) there 

 are several primary veins, while most monocotyls have several to many equal and 

 more or less parallel primary veins, connected by rather obscure transverse veins. 

 The anastomosing of veins is highly advantageous, since materials may pass to 

 or from any point by more than one route ; in case of injury to a large vein, this 

 insures continued activity in all parts of the leaf. In most conifers and in many 

 narrow-leaved angiosperms there is a midrib with few or no branches. Veins 

 often are inconspicuous in succulent xerophytes because they are deeply buried, and 

 in submersed hydrophytes, because the are poorly developed. 



Structural features. — Leaf veins, like conductive tracts generally (p. 682), 

 are composed of water-conducting elements {hadrome), food-conducting elements 



