Experiment 2.--A cube of 1/10 of an inch (i.e. with each side 1/10 of an inch, or 2.54 mm. in length) was placed on a leaf, and after 50 hrs. it was converted into a sphere about 3/40 of an inch (1.905 mm.) in diameter, surrounded by perfectly transparent fluid. After ten days the leaf re-expanded, but there was still left on the disc a minute bit of albumen now rendered transparent. More albumen had been given to this leaf than could be dissolved or digested.

Experiment 3.--Two cubes of albumen of 1/20 of an inch (1.27 mm.) were placed on two leaves. After 46 hrs. every atom of one was dissolved, and most of the liquefied matter was absorbed, the fluid which remained being in this, as in all other cases, very acid and viscid. The other cube was acted on at a rather slower rate.

Experiment 4.--Two cubes of albumen of the same size as the last were placed on two leaves, and were converted in 50 hrs. into two large drops of transparent fluid; but when these were removed from beneath the inflected tentacles, and viewed by reflected light under the microscope, fine streaks of white opaque matter could be seen in the one, and traces of similar streaks in the other. The drops were replaced on the leaves, which re-expanded after 10 days; and now nothing was left except a very little transparent acid fluid.

Experiment 5.--This experiment was slightly varied, so that the albumen might be more quickly exposed to the action of the secretion. Two cubes, each of about 1/40 of an inch (.635 mm.), were placed on the same leaf, and two similar cubes on another

* In all my numerous experiments on the digestion of cubes of albumen, the angles and edges were invariably first rounded. Now, Schiff states ('Leons phys. de la Digestion,' vol. ii. 1867, page 149) that this is characteristic of the digestion of albumen by the gastric juice of animals. On the other hand, he remarks "les dissolutions, en chimie, ont lieu sur toute la surface des corps en contact avec l'agent dissolvant." [page 94]

leaf. These were examined after 21 hrs. 30 m., and all four were found rounded. After 46 hrs. the two cubes on the one leaf were completely liquefied, the fluid being perfectly transparent; on the other leaf some opaque white streaks could still be seen in the midst of the fluid. After 72 hrs. these streaks disappeared, but there was still a little viscid fluid left on the disc; whereas it was almost all absorbed on the first leaf. Both leaves were now beginning to re-expand.]

The best and almost sole test of the presence of some ferment analogous to pepsin in the secretion appeared to be to neutralise the acid of the secretion with an alkali, and to observe whether the process of digestion ceased; and then to add a little acid and observe whether the process recommenced. This was done, and, as we shall see, with success, but it was necessary first to try two control experiments; namely, whether the addition of minute drops of water of the same size as those of the dissolved alkalies to be used would stop the process of digestion; and, secondly, whether minute drops of weak hydrochloric acid, of the same strength and size as those to be used, would injure the leaves. The two following experiments were therefore tried:--

Experiment 6.--Small cubes of albumen were put on three leaves, and minute drops of distilled water on the head of a pin were added two or three times daily. These did not in the least delay the process; for, after 48 hrs., the cubes were completely dissolved on all three leaves. On the third day the leaves began to re-expand, and on the fourth day all the fluid was absorbed.

Experiment 7.--Small cubes of albumen were put on two leaves, and minute drops of hydrochloric acid, of the strength of one part to 437 of water, were added two or three times. This did not in the least delay, but seemed rather to hasten, the process of digestion; for every trace of the albumen disappeared in 24 hrs. 30 m. After three days the leaves partially re-expanded, and by this time almost all the viscid fluid on their discs was absorbed.

Charles Darwin

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