Trifolium pratense quinquefolium and others.
"Darwin and Modern Science" by A.C. Seward and Others
H. Muller, "Fertilisation of Flowers," page 186, describes hive-bees visiting Trifolium pratense for the sake of the pollen.
"More Letters of Charles Darwin Volume II" by Charles Darwin
Trifolium pratense, 73, 94.
"On the Origin of Species by Means of Natural Selection" by Charles Darwin
Trifolium pratense L. Pulse family.
"Texas Honey Plants" by C. E. Sanborn