72 STRUCTURE OP LEAVES. 



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 complete net-work of vessels is produced. To such a dis- 

 tribution of veins, the name of Reticulated or Netted venation has 

 been applied. 



142. In the leaves of some plants there exists a central rib or mid- 

 rib, with veins running nearly parallel to it from the base to the apex 

 of the leaf, as in grasses (fig. 194) and Fan palms; or with veins com- 

 ing 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 Plantain and Banana (fig. 135). In these cases the veins 

 are often united by cross veinlets, which do not, however, form an 

 angular net-work. These are called Parallel-veined. 



143. Leaves may thus be divided into two great classes, according 

 to their venation Reticulated or netted leaves, in which there is an 

 angular net-work of vessels, as occurs generally in the leaves of exo- 

 genous or dicotyledonous plants; and Parallel-veined, in which the 

 veins run in a straight or curved manner from base to apex, or from 

 the midrib to the margin of leaf, and in which, if there is a union, 

 it is effected by transverse veins which do not form an angular net- 

 work. This kind of leaf occurs commonly in endogenous or monocotyle- 

 donous plants. 



144 TABULAR ARRANGEMENT OP VENATION. 



A. Reticulated Venation. 

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



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



a. Veins ending in curvatures within the margin (fig. 133), and forming 

 what have been called true netted leaves (Lilac). 



6. Veins going directly to the margin (fig. 134), and forming feather- 

 veined leaves (Oak and Chestnut). 



2. Primary veins coming off along with the midrib (fig. 143) from the base 



of the leaf. 



II. Midticostate (multus, many). More than one rib. In such cases there 

 are frequently three (tricostate), as in fig. 162 ; or five (quinquecos- 

 tate), as in fig. 158. Authors usually give to these leaves the 

 general name of costate, or ribbed. 



1. Concostate (con, together, costa, a rib). Ribs converging, running from 



base to apex in a curved manner, as in Cinnamon, Laurus Cinna- 

 momum(fig. 158). There is occasionally an obscure rib running close 

 to the edge of the leaf, and called intramarginal, as in the Myrtle. 



2. Discostate (dis, separate). Ribs diverging or proceeding in a radiating 



manner; this is called radiating venation, and is seen in Sycamore, 

 Vine, Geranium (figs. 144, 146). 



B. Parallel Venation. The term parallel is not strictly applicable, for the veins 

 often proceed in aradiating manner, but it is difficult to find a compre- 

 hensive term. This venation may be characterised as not reticulated. 

 L Veins proceeding from midrib to margin, usually with convexity towards 

 the midrib, as in Musa and Canna (fig. 135). 



