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T. G. TUTIN 



leaflets. In Lathyrus these are basipetal with the result that they commonly have three 

 prominent vascular bundles, the lateral ones arising near the base and running almost 

 the whole length of the leaflet, while in Vicia the venation is of the normal pinnate type. 

 This is a character of developmental as well as morphological significance and one which 

 in my opinion should be taken into account in defining the genera. An additional vegetative 

 character, possibly associated with the tendency to reduction in the leaf surface, is the 

 development of wings of tissue from the angles of the stem in Lathyrus. Lathyrus niger 

 is an example of one of the few species in which floral and vegetative characters do not 

 agree. Here the flowers are those of a Lathyrus but the leaflets and unwinged stems 

 suggest a Vicia. Perhaps the happiest solution is to put it, as has been done in the past, 

 in a separate genus, Orohus. 



The satisfactory recognition of genera by the traditional method of using macroscopic 

 morphological characters of the reproductive parts becomes difficult or impossible in 

 families where there is extreme reduction or great uniformity of pattern in these parts. 



A good indication of this difficulty is obtained by comparing the number of genera 

 recognised in certain families in the 18th century and at the present time. To simplify 

 the comparison and to make it more exact it will be confined to British genera. In the 

 Ranunculaceae, where generic differences are large and obvious, the numbers are the 

 same, if we exclude Nigella, as even now scarcely naturalised, and Eranthis, a recent 

 introduction. 



In the Papilionaceae, where the floral pattern is more uniform but the fruit still 

 varied, if we make the same allowances the number of genera is also the same. 



In the Umbelliferae with small and very uniform flowers and somewhat inconspicuous 

 fruit characters there is a considerable difference. In the 18th century 32 genera were 

 recognised as British, while excluding, once more, recent introductions, 40 genera are 

 now listed. The change has in fact been greater than these figures suggest as the British 

 representatives of Anethum and Athamanta are now separated from those genera and 

 Imperatoria and Phellandrium are no longer recognised as distinct. This change, while 

 in part due to more careful study along traditional lines, no doubt owes something to 

 the improvement in lenses and the increased use of the dissecting microscope. 



Taking the Gramineae as an example of a family with small and very reduced flowers, 

 fruits showing little variety superficially, and vegetative parts which appear to conform 

 to a common pattern, we find, as would be expected, an even greater difference. 



Withering (1796) gives 24 genera which include most of the species (and in some 

 genera several more) and nearly all the genera which are today regarded as forming the 

 56 genera recognised as British. The change has once again been due to more detailed 

 examination, and to some extent to the use of the compound microscope, combined with 

 the recognition of the fact that resemblances in the inflorescence and even in the gross 

 morphology of the spikelet are not necessarily criteria for making a satisfactory classifica- 

 tion of plants with inflorescences as reduced as these. 



An excellent example of this is the genus Parapholis. The British species were 

 originally put in Rottboellia, together with tropical species now included in the tribe 

 Andropogoneae. This genus was defined by characters of the inflorescence and was in 

 reality a highly heterogeneous one. Later the British species were transferred to Lepturus, 

 which after many vicissitudes was eventually shown by Hubbard (1946) to be only slightly 

 less heterogeneous than the original Rottboellia. Using all the available characters, including 

 microscopic ones such as starch-grains, silica cells and chromosome number and sizes, he 

 showed that the species included in Lepturus and the comparatively recently but unsatis- 

 factorily distinguished genera Pholiurus and Monerma should in fact be placed in five 

 genera and no fewer than three tribes. 



