MORPHOLOGY OF HIGHER PLANTS. 353 



such a system of venation is known as a closed system of venation 

 (Fig. 202, A). The leaves of Veratrum and Zea Mays furnish 

 other examples of parallel-nerved leaves. 



In palms the venation is somewhat different. The veins, 

 instead of converging toward the apex as they do in the more or 

 less lanceolate leaf of lily-of-the-valley, radiate from the base 

 to the margin of the more or less round leaf, and a leaf of this 

 type is said to be palmi-nerved. 



There is still a third type of venation in Monocotyledons. In 

 this instance one principal vein runs from the base to the apex 

 of the leaf, and from this branches run parallel to the margin. 

 The banana furnishes an example of this type, and is said to be 



PINNI-NERVED. 



Venation in Dicotyledons. — Here the veins are character- 

 ized by their habit of repeatedly branching and anastomosing, 

 whatever the general type of venation may be, and thus form a 

 net-work or reticulum, hence the leaves are said to be reticulate 

 or netted-veined. The principal types are as follows : A chest- 

 nut or chinquapin leaf (Fig. 202) furnishes a good illustration 

 of a pinnately-reticulate leaf. The principal vein which runs from 

 the base to the apex is called the midrib, while the secondary 

 veins which arise from it and run more or less parallel to the 

 margin are sometimes spoken of as ribs and may be likened to the 

 plumes on the shaft of a feather. 



In other cases several large veins arise at the base and diverge 

 toward the margin, giving rise to palmately-veined leaves, as 

 in the leaf of maple. There are still other types, as in cinnamon, 

 which is said to be rib-netted, etc. 



Surface of Leaves. — In addition to the markings of leaves 

 due to veining there are certain other characters which serve 

 to distinguish them. Hairs are of frequent occurrence on leaves, 

 being generally most abundant on the dorsal surface, especially the 

 veins, and various terms having reference to the kinds of hairs 

 have been applied to leaves. 



Plant Hairs. — When the surface of the plant (either of stems 



or leaves) is covered with short, fine hairs, which are not very 



dense and not matted, the surface is described as pubescent.- 



when the hairs are relatively long but scattered the surface is said 



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