84 STRUCTURE OF LEAVES. 



spersed through the parenchyma external to the curvature. There are 

 also other veins of less extent (costal veins) given off by the midrib, 

 and these give origin to small veinlets. In some oases, as Sycamore 

 and Cinnamon, in place of there being only a single central rib, there 

 are several which diverge from the part where the blade joins the 

 petiole or stem. Thus, the primary veins give off secondary veins, 

 and these in their turn give off tertiary veins, and so on, until a com- 

 plete network of vessels is produced. To such a distribution of veins 

 the name of Reticulated or Netted venation has been applied. 



In the leaves of some plants there exists a central rib or midrib, 

 with veins running nearly parallel to it from the base to the apex of 

 the leaf, as in grasses (fig. 210) ; or with veins diverging in more or 

 less parallel lines, as in Fan Palms ; or with veins coming off from it 

 throughout its whole course, and running parallel to each other in a 

 straight or curved direction towards the margin of the leaf, as in Plan- 

 tain and Banana (fig. 150). In these cases the veins are often united 

 by cross veinlets, which do not, however, form, an angular network. 

 These are called Parallel-veined. 



Leaves may thus be divided into two great classes, according to 

 their venation — Reticulated or netted-veined leaves, in which there is an 

 angular network of vessels, as seen generally in dicotyledonous plants ; 

 and Parallel-veined leaves, in which the vessels run in a straight or 

 curved manner from base to apex, or from the midrib to the margin of 

 the leaf, and in which, if there is a union, it is effected by transverse 

 veins which do not form an angular network. This kind of leaf 

 occurs commonly in monocotyledonous plants. In many acotyledonous 

 plants there is no true vascular venation, but when it is present, there 

 is frequently a tendency in the veins to divide in a forked (furcate) 

 manner. This is seen in many Ferns, which have hence been called 

 Fork-veined. Condensed cellular tissue forming false venation is seen 

 in mosses and in seaweeds. 



Tabulae Aebanqbment of Venation. 



A. — Reticulated Venation. 



I. Unicostate {unus, one). A single rib or costa in the middle (midrib). 



1. Primary veins coming off at different points of the midrib. 



«.. Veins ending in curvatures within the margin (fig. 148), and forming 

 ■what have been called true netted leaves (Lilac'). 



i. Veins going directly to the margin (fig. 149), tmAiormmgfeather-veinM 

 leaves (Oak and Chestnut). 



2. Primary veins coming off along with the midrib (fig. 158) from the base 



of the leaf. 



II. Multicostate {mullus, many). More than one rib. In such cases there are 



frequently three (tricostate), as in fig. 177, or five (quinquecostate), 

 as in fig. 173. Authors usually give to these leaves the general 

 name of costaie or ribbed. 

 1. Convergent. Ribs converging, running from base to apex in a curved 



