STRUCTURE.] VARIETIES OF VENATION. 



These distinctions may to some appear over-refined; but I 

 am convinced that no one can very precisely describe a leaf 

 without the use either of them, or of equivalent terms yet 

 to be invented. With respect to their venation only, leaves 

 may be conveniently arranged under the following heads : 



1. Veinless (avenium), when no veins at all are formed, except 



a slight approach to a midrib, as in Mosses, Fuci, &c. 

 Leaves of this description exist only in the lowest tribes 

 of foliaceous plants, and must not be confounded with 

 the fleshy or thickened leaves common among the higher 

 orders of vegetation, in which the veins are by no means 

 absent, but only concealed within the substance of the 

 parenchyma. (See No. 10.) Of this De Candolle has 

 two forms, first, his folia nullinervia, in which there is 

 not even a trace of a midrib, as in Ulva ; and second, his 

 folia falsinervia, in which a trace of a midrib is percep- 

 tible. These terms appear to me unnecessary ; but, if 

 they be employed, the termination nervia must be changed 

 to venia. 



2. Equal-veined (aequalivenium), when the midrib is perfectly 



formed, and the veins are all of equal size, as in Ferns. 

 This kind of leaf has not been before distinguished : it 

 may be considered intermediate between those without 

 veins and those in which primary veins are first apparent. 

 The veins are equal in power to the proper veinlets of 

 leaves of a higher class. 



3. Straight-veined (rectivenium) . In this the veins are en- 



tirely primary, generally very much attenuated, and 

 arising from towards the base of the midrib, with which 

 they lie nearly parallel : they are connected by proper 

 veinlets; but there are no common veinlets. The leaves 

 of Grasses and of Palms and Orchidaceous plants are of 

 this nature. This form has been called by Link 

 parallels- and convergenti-nervosum, according to the 

 degree of parallelism of the primary veins ; and to these 

 two he has added what he calls venuloso-nervosum, when 

 the primary veins are connected by proper veinlets ; but 

 as this is always so, although it is not in all cases equally 

 apparent, the term is superfluous. Ach. Richard calls 



