4 BULLETIN 75, U. S. DEPARTMENT OF AGEICULTURE. 



Henslow ^ in discussing self -fertility in Medicago sativa wrote as 

 follows : 



This plant, when protected, yielded seeds, as compared with unprotected, in the 

 ratio of 101:77. Hence it is highly self -fertile, though specially modified, in having 

 "irritable" stamens, for cross-fertilization. 



This note of Henslow has been cited by later writers, but it is 

 really an erroneous abstract from Darwin's discussion of Medicago 

 lupulina. Darwin ^ writes as follows : 



Medicago lupulina (Leguminosse). On account of the danger of losing the seeds, 

 I was forced to gather the pods before they were quite ripe; 150 flower-heads on 

 plants visited by bees yielded pods weighing 101 grains; while 150 heads on pro- 

 tected plants yielded pods weighing 77 grains. The inequality would probably have 

 been greater if the matiure seeds could have been all safely collected and compared. 



As Henslow's paper is primarily a review of Darwin's book, it is 

 clear from the two quotations that Henslow erroneously wrote 

 '^sativa" in place of ''lupulina." This is rendered the more certain 

 as Henslow in his earher paper on Medicago sativa had referred to 

 Darwin's work in a footnote, where the data are properly stated to 

 apply to Medicago lupulina. 



In 1895 appeared a paper by Burkill,^ who reviews the principal 

 contributions to this subject by previous writers and adds important 

 new observations and "experiments. He verifies the conclusions of 

 earher investigators that the explosive action of the flower depends 

 on the uppermost stamens of the stamineal tube. Burkill obtained 

 no pods in a considerable number of flowers covered with nets to 

 prevent insect visits, for which phenomenon he presents an interesting 

 explanation : 



Pollen is ehed in the bud and lies round the stamens and stigma in a little lena- 

 ehaped space made by the carina. . . .No seeds are set in the unexploded 

 flower in spite of the pollen in contact with the stigma. This is explained by the 

 fact that the stigma does not become receptive until rubbed or until its cells are 

 injiured in some manner. My proof is, I think, conclusive. Firstly, the stigma 

 appears not to be moist, but when rubbed on glass leaves a sticky mark. Secondly, 

 I have caused flowers to set seed though unexploded, (1) by pinching the stigma 

 through the keel, (2) by perforating the keel and rubbing the stigma with a stiff paint 

 brush, and (3) by cutting off the tip of the keel and rubbing the stigma with a stiff 

 paint brush. An insect visitor exploding the flower will injure the stigmatic papillae 

 and bring about fertilization. 



Burkill gives a list of 31 insects which he observed visiting alfalfa 

 flowers in and near Cambridge, England. In no case did he see a 

 butterfly causing the flower to trip, but on one hot afternoon he 



' Ilenslow, George. On the self-fertilization of plants. Transactions, Linnsean Society, London, Botany, 

 8. 2,v. l,pt. G,p. 3C1, 1S79. 



2 Darwin, Charles. The Effects of Cross and Self Fertilization in the Vegetable Kingdom. New York, 

 ^877, p. 3CS. 



'Burkill, I. H. On the fertilization of some species of Medicago L. In England Proceedings, Cambridge 

 Philosophical Society, v. 8, pt. 3, p. 142-147, 1894. 



