Carnivorous plant digestion and protein secretion

A report quoted in Darwin's book measured digestive power against a standard. It set the power of a liquid containing pepsin with the usual proportion of hydrochloric acid at 100. Three other acids under investigation then scored 14.0, 35.4 and 30.2 on the same scale¹. A second experiment used the same procedure, except that all the tubes shared one water-bath and the residues were dried at 115° C.¹. The quoted report names them only as the three acids under investigation.

Darwin concluded at the end of the chapter. He found a remarkable accordance in digestive power between animal gastric juice, with its pepsin and hydrochloric acid, and the secretion of Drosera, with its ferment and an acid of the acetic series². He wrote that one could hardly doubt the ferment was closely, if not identically, the same in both². He called it a new and wonderful fact that a plant and an animal should pour forth nearly the same complex secretion for the same purpose³. He said he would return to the subject in his fifteenth chapter².

Bayliss (1915) summarised how the animal side was understood in his time. Pavlov's observations showed that meat placed in the stomach caused secretion in an isolated small stomach, even with nervous influences excluded⁴. Edkins (1907) showed that extracts of the pyloric mucous membrane increased the formation of an acid gastric juice⁵. Maydell (1913) confirmed this by injecting such extracts under the skin of a dog with a chronic gastric fistula, which increased its secretion⁶. Bayliss's account concerns control of secretion in the animal stomach.