Blakeslee: A Unifoliate A'lutation 



155 



seems improbable that such a strikin^^ 

 variation as the one photographed 

 could have escaped notice. The same 

 is true of 5,938 plants grown in the 

 greenhouse. A total, then, of over 

 122,000 plants were grown under close 

 observation without showing another 

 mutant of this kind. 



Since the simple-leaved form is sterile 

 and moreover retains its leaves in the 

 fall after other plants have become 

 defoliated, it is a conspicuous object at 

 time of harvest. If such a plant had 

 been present in the 228,000 and more 

 individuals grown in multiplication ped- 

 igrees in 1918, it would have stood good 

 chance of being discovered at har^'est, 

 although these multiplication pedigrees 

 were .not so often nor so closely in- 

 spected during the growing season. It 

 is not entirely improbable, therefore, that 

 the mutant occurred only once among 

 over 450,000 plants which were kept 

 under more or less strict observation. 



Numerous varieties of the Adzuki 

 Bean have been introduced and grown 

 in trial plats by the department of 

 Forage Crop Investigation of the U. S. 

 Bureau of Plant Industry during the 

 last twenty years. Dr. C. V. Piper and 

 Mr. W. J. Morse, who have had charge 

 of the tests in this department, report 

 that they have never seen a plant of the 

 Aszuki Bean with unifoliolate leaves. 



Another representative of the genus 

 Phaseolus {P. Vulgaris) has been studied 

 by Dr. J. Arthur Harris in reference to 

 the occurrence of abnormalities in the 

 seedlings. He has kindly informed the 

 writer that only once has he found plants 

 with unifoliolate leaves. Three indi- 

 viduals in a pedigree, the offspring from 

 a single parent, had undivided leaves and 

 were otherwise abnormal. Outside of 

 this single pedigree, none of the several 

 million plants of the pea bean individ- 



ually examined for abnormalities was 

 found with simple leaves. 



The Legumenosae are predominately 

 characterized by compound leaves. 

 In a few genera, however, the leaves are 

 simple. As familiar examples of the 

 latter condition may be mentioned the 

 Redbud (Cercis), the Rattlebox (Cro- 

 tolaria) and the Dyer's Greenweed 

 Genista). In Rhynchosia, typically a 

 trifoliolate genus, one species {R. sim- 

 plex) has only unifoliolate leaves. These 

 facts may suggest that in the evolution 

 of the Leguminosae the present pre- 

 dominately compound foliage has been 

 derived from a simple-leaved condition. 

 Such a supposition might be supported 

 by the genus Bauhinia, which appar- 

 ently shows various stages in the process 

 of conversion of a simple to a compound 

 leaf by progressive furrowing between 

 lobes. The unifoliolate representatives 

 of the family might, therefore, be con- 

 sidered arrested stages in evolution or 

 reversions to an ancestral type. Under 

 this interpretation, the mutation under 

 discussion would be in the nature of a 

 reversion. It must not be denied, 

 however, that some may prefer to read 

 the evolutionary trend in the reversed 

 direction and to consider the mutation 

 a progressive one. 



All the facts presented show that the 

 unifoliolate type here reported for the 

 Adzuki Bean is an extremely rare var- 

 iant. Despite the fact that its inherit- 

 ance could not be established by breed- 

 ing experiments, it has been called a 

 mutation. Its failure to appear more 

 than once in so large a number of indi- 

 viduals indicates that it is a variation 

 genotypic in nature, since it could 

 scarcely- be attributed to the reappear- 

 ance of a character through normal 

 segregation nor be considered a mere 

 modification induced by environmental 

 factors. 



