Miscellaneous. 419 



regard to the quantity of nervures. It is consequently necessary to 

 indicate and fix this number m every drawing or description of a 

 fossil leaf ; without it new species cannot be determined. 



When the secondary nervures of a simple leaf go from the central 

 nervure to the margin or even a little beyond the parenchyma, these 

 are nervures running towards the margin (Randliitifer) . 



They are simple when the first pair of secondary nervures above 

 the petiole is without tertiary nervures, as m the beeches, in Alnus 

 glutinosa and Castanea vesca. If tertiary nervures arise from the 

 lower side of the first secondary nervures, these are winged nervures. 



The nervures however do not always attain the margin of the leaf. 

 Very frequently they stop at a certain distance from the margin with 

 so much constancy and regularity that they form a new and very ex- 

 tensive division which is capable of many subdivisions. This consti- 

 tutes the system of arched nervures. Two neighbouring nervures are 

 bent towards one another and united in an elegant arch so exactly, 

 that it is only possible by close observation to ascertain where one 

 stops and the other commences. Nevertheless, at the point of union 

 there is always a small elevation, from which, usually close to the su- 

 perior nervure, a common nervure arises, which goes to the margin 

 of the leaf and terminates in a point or tooth of the margin. The 

 superior nervure sends a branch downwards ; but the essential branch 

 curves upwards to join the secondary nervure next above it in a 

 similar arch ; this continues to the apex of the leaf. A series of con- 

 tinuous arches is formed, sometimes ten or more in succession. The 

 lines of the folds of the leaf divide these arches in the middle, but do 

 not attain the margin. This pretty fonn of neuration is one of the 

 most common ui our plants. It is exhibited in the Hieracia, the 

 Bipsacece, and very distinctly in the Epilobium angustifoliuni ; it is 

 also met with in many shrubs and trees, such as the walnut, orange 

 and lemon trees, and the holly. In tropical plants with projecting 

 ribs it is always the case : drawings allow one to trace the course of 

 the nervures, except that they do not indicate their continuation to 

 the margin. 



The nervures running towards the apex {Spitzlaufer) are not less 

 striking. In these the lateral nervures run in elegant curves from 

 the base between the margin and the central nervure, uniting again 

 with this nervure at or near the apex. In the latter case some more 

 secondary nervures separate from the central rib, the last pair of which 

 reunite with it at the point. The first are complete, as in nearly all 

 the CaryophijUeee, in many species of Laurus and Zizyphus. The 

 second are incomplete, as in Cornus, Philadelphus and Ceanothus. 



Another neuration is especially exhibited by tropical plants ; in 

 this the nervure follows the margin from base to apex, completely 

 surrounduig the leaf and terminating exactly at the apex. Secondary 

 nervures can scarcely ever attaui the margin. They are generally 

 very near one another and very fine ; they divide and lose themselves 

 in the nervures of the circumference. These are marginal nervures 

 {Saumlaufer) . This form belongs to most of the Myrtacece and the 

 Banksice ; it is also probably that of Buxus. 



