GENERIC CRITERIA IN FLOV/ERING PLANTS* 



By T. G. TuTiN 



University College, Leicester > 



The genus, like the species and the family, is in origin a rather ill-defined concept 

 based on morphological features. It is, and, in spite of the efforts of geneticists, cytologists 

 and orthodox taxonomists, seems likely to remain, to a considerable degree subjective. In 

 consequence genera and generic criteria in different families are by no means comparable ; 

 characters which in one family are used for separating genera may appear trifling in 

 comparison with those used for defining sections or even species in another family. In 

 the Ranunculaceae for instance the genera mostly differ from one another in what are 

 usually considered major morphological characters. There are two apparently basically 

 different types of ovary, actinomorphic and zygomorphic flowers occur, and the homologies 

 of the perianth differ in different genera. In the Compositae on the other hand Carduus 

 and Cirsium are distinguished mainly by the nature of the pappus hairs while Hypochoeris 

 and Leontodon, so similar in all other respects, are separated by the occurrence of scales 

 on the receptacle in the former. Doubt is thrown on the validity of the latter character 

 by the fact that Babcock & Cave (1938) found that Rodigia commutata, distinguished from 

 Crepis by the presence of scales on the receptacle, could interchange genes freely with 

 Crepis foetida. The inference is that these two plants, far from being genetically distinct, 

 are in fact more closely related than many of the species traditionally included in the 

 genus Crepis. It would seem therefore that re-investigation of the validity of the pairs 

 of genera Hypochoeris and Leontodon, and Anthemis and Matricaria is desirable. I would 

 venture to suggest that apparently trivial floral characters should be used for distinguishing 

 genera only when the taxa thus circumscribed also differ in several other, perhaps less 

 easily definable, features. 



Indeed I believe that many of the older taxonomists regularly adopted this practice. 

 The best of them spent many years studying the external features of plants with great 

 patience and skill, untroubled by thoughts of cytology, genetics, ecology or physiology. 

 They formed the opinion, probably based more or less subconsciously on a host of vegeta- 

 tive as well as floral characters, that certain species belonged together in one genus. They 

 knew from experience that reproductive parts vary much less than vegetative ones so 

 that, having mentally formed their genus largely on general ' look,' they searched for 

 some reproductive character to define it. 



This procedure worked well on the whole, but from time to time the rigid application 

 of a minor floral feature has led to the inclusion of occasional species in the wrong genus. 

 A case of this kind seems to me to have arisen in Lathyrus, which in its reproductive parts 

 closely resembles Vicia. 



The genera are separated technically by the staminal tube being obliquely truncate 

 and the style either downy all round, glabrous, or bearded below the stigma in Vicia. In 

 Lathyrus on the other hand the staminal tube is transversely truncate and the style bearded 

 on its upper side. Incidentally, in the 3rd edition of Hooker's Students' Flora (1884) 

 these characters are mixed up, the obliquely truncate staminal tube of Vicia being attri- 

 buted to Lathyrus and vice versa, a good indication of how little attention working taxono- 

 mists pay to such characters. 



As Corner (1954) has pointed out, Lathyrus and Vicia differ in the nature of their 



• This paper was read at a meeting of the Systematics Association at Edinburgh. 



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