Cap. VI. DIGESTION. 131 
prepared casein, which is said to consist of two sub- 
stances; and although Schiff asserts that casein in 
this state is not attacked by gastric juice, he might 
easily have overlooked a minute quantity of some 
albuminous matter, which Drosera would detect and 
absorb. Again, fibro-cartilage, though not properly 
dissolved, is acted on in the same manner, both by 
the secretion of Drosera and gastric juice. But this 
substance, as well as the so-called hematin used by 
me, ought perhaps to have been classed with indi- 
gestible substances. 
That gastric juice acts by means of its ferment, 
pepsin, solely in the presence of an acid, is well 
established ; and we have excellent evidence that a 
ferment is present in the secretion of Drosera, which 
likewise acts only in the presence of an acid; for we 
have seen that when the secretion is neutralised by 
minute drops of the solution of an alkali, the diges- 
tion of albumen is completely stopped, and that on 
the addition of a minute dose of hydrochloric acid it 
immediately recommences. 
The nine following substances, or classes of sub- 
stances, namely, epidermic productions, fibro-elastic 
tissue, mucin, pepsin, urea, chitine, cellulose, gun 
cotton, chlorophyll, starch, fat and oil, are not acted 
on by the secretion of Drosera; nor are they, as far as 
is known, by the gastric juice of animals. Some 
soluble matter, however, was extracted from the mucin, 
pepsin, and chlorophyll, used by me, both by the 
secretion and by artificial gastric juice. 
The several substances, which are completely dis- 
solved by the secretion, and which are afterwards 
absorbed by the glands, affect the leaves rather dif- 
ferently. They induce inflection at very different 
