4 BULLETIN 75, U. S. DEPARTMENT OF AGRICULTURE. 
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 (Leguminose). 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 mature 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 earlier 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 
earlier 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 shed in the bud and lies round the stamens and stigma in a little lens- 
shaped 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 
injured 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 papille 
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 
1 Henslow, George. Ontheself-fertilization of plants. Transactions, ter Society, London, Botany, 
s.2, Vv. 1, pt. 6, p. 361, 1879. 
- sae Charles. The Effects of Cross and Self Fertilization in the yun Kingdom. New York, 
3 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. 
