V] VENATION CHARACTERS. 99 



genera'. Speaking of detached leaves, Bentham says : — " I do 

 not know of a single one which, in outline or venation, is 

 exclusively characteristic of the order, or of any one of its 

 genera." Species of Orevillea, Hakea and a few other genera 

 are more or less familiar in plant houses, but the leaf-forms 

 illustrated by the commoner members of the family convey no 

 idea of the enormous variation which is met with not only in 

 the family as a whole, but in the different species of the same 

 genus. The striking diversity of leaf within the limits of a 

 single genus will be dealt with more fully in volume II. under 

 the head of Fossil Dicotyledons. 



There is a common source of danger in attempting to carry 

 too far the venation characters as tests of affinity. The parallel 

 venation of Monocotyledons is by no means a safe guide to 

 follow in all cases as a distinguishing feature of this class of 

 plants. In addition to such leaves as those of the Gymnosperm 

 Oordaites and detached pinnae of Cycads, there are certain 

 species of Dicotyledons which correspond in the character of 

 their venation to Monocotyledonous leaves. Eryngium m.on- 

 tanum Coult., E. Lassausd Dene., and other species of this 

 genus of Umbelliferse agree closely with such a plant as 

 Pandanus or other Monocotyledons; similarly the long linear 

 leaves of Richea dracophylla, K. Br., one of the EricaccEe, are 

 identical in form with many monocotyledonous leaves. In- 

 stances might also be quoted of monocotyledonous leaves, such 

 as species of Smilax and others which Lindley included in his 

 family of Dictyogens which correspond closely with some types 

 of Dicotyledons^ Venation characters must be used with care 

 even in determining classes or groups, and with still greater 

 reserve if relied on as family or generic tests. 



It is too frequently the case that while we are conversant 

 with the most detailed histological structure of a fossil plant 

 stem, its external form is a matter of conjecture. The conditions 

 which have favoured the petrifaction of plant tissues have as a 

 rule not been favourable for the preservation of good casts or 

 impressions of the external features ; and, on the other hand, 

 in the best impressions of fern fronds or other plants, in which 

 1 See also Bunbury (83) p. 309. " Seward (96) p. 208. 



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