It has already been remarked that plants growing in a state of nature have the margins of their leaves much more strongly incurved than those grown in pots and prevented from catching many insects.  We have seen that insects washed down by the rain from all parts of the leaf often lodge within the margins, which are thus excited to curl farther inwards; and we may suspect that this action, many times repeated during the life of the plant, leads to their permanent and well-marked incurvation.  I regret that this view did not occur to me in time to test its truth.

It may here be added, though not immediately [page 381] bearing on our subject, that when a plant is pulled up, the leaves immediately curl downwards so as almost to conceal the roots,—­a fact which has been noticed by many persons.  I suppose that this is due to the same tendency which causes the outer and older leaves to lie flat on the ground.  It further appears that the flower-stalks are to a certain extent irritable, for Dr. Johnson states that they “bend backwards if rudely handled."*

Secretion, Absorption, and Digestion.—­I will first give my observations and experiments, and then a summary of the results.

[The Effects of Objects containing Soluble Nitrogenous Matter.

(1) Flies were placed on many leaves, and excited the glands to secrete copiously; the secretion always becoming acid, though not so before.  After a time these insects were rendered so tender that their limbs and bodies could be separated by a mere touch, owing no doubt to the digestion and disintegration of their muscles.  The glands in contact with a small fly continued to secrete for four days, and then became almost dry.  A narrow strip of this leaf was cut off, and the glands of the longer and shorter hairs, which had lain in contact for the four days with the fly, and those which had not touched it, were compared under the microscope and presented a wonderful contrast.  Those which had been in contact were filled with brownish granular matter, the others with homogeneous fluid.  There could therefore be no doubt that the former had absorbed matter from the fly.

(2) Small bits of roast meat, placed on a leaf, always caused much acid secretion in the course of a few hours—­in one case within 40 m.  When thin fibres of meat were laid along the margin of a leaf which stood almost upright, the secretion ran down to the ground.  Angular bits of meat, placed in little pools of the secretion near the margin, were in the course of

* ‘English Botany,’ by Sir J.E.  Smith; with coloured figures by J. Sowerby; edit. of 1832, tab. 24, 25, 26. [page 382]