Insectivorous Plants

by Charles Darwin

Although in the case of uninjured flower-stems and leaves the carbonate seems to be absorbed only by the glands, yet it enters a cut surface much more quickly than a gland. Strips of the rind of a flower-stem were torn off, and the cells of the pedicels were seen to contain only colourless transparent fluid; those of the glands including as usual some granular matter. These strips were then immersed in the same solution as before (one part of the carbonate to 109 of water), and in a few minutes granular matter appeared in the lowercells of all the pedicels. The action invariably commenced (for I tried the experiment repeatedly) in the lowest cells, and therefore close to the torn surface, and then gradually travelled up the hairs until it reached the glands, in a reversed direction to what occurs in uninjured specimens. The glands then became discoloured, and the previously contained granular matter was aggregated into larger masses. Two short bits of a flower-stem were also left for 2 hrs. 40 m. in a weaker solution of one part of the carbonate to 218 of water; and in both specimens the pedicels of the hairs near the cut ends now contained much granular matter; and the glands were completely discoloured.

Lastly, bits of meat were placed on some glands; these were examined after 23 hrs., as were others, which had apparently not long before caught minute flies; but they did not present any [page 348] difference from the glands of other hairs. Perhaps there may not have been time enough for absorption. I think so as some glands, on which dead flies had evidently long lain, were of a pale dirty purple colour or even almost colourless, and the granular matter within them presented an unusual and somewhat peculiar appearance. That these glands had absorbed animal matter from the flies, probably by exosmose into the viscid secretion, we may infer, not only from their changed colour, but because, when placed in a solution of carbonate of ammonia, some of the cells in their pedicels become filled with granular matter; whereas the cells of other hairs, which had not caught flies, after being treated with the same solution for the same length of time, contained only a small quantity of granular matter. But more evidence is necessary before we fully admit that the glands of this saxifrage can absorb, even with ample time allowed, animal matter from the minute insects which they occasionally and accidentally capture.

Saxifraga rotundifolia (?).—The hairs on the flower-stems of this species are longer than those just described, and bear pale brown glands. Many were examined, and the cells of the pedicels were quite transparent. A bent stem was immersed for 30 m. in a solution of one part of carbonate of ammonia to 109 of water, and two or three of the uppermost cells in the pedicels now contained granular or aggregated matter; the glands having become of a bright yellowish-green. The glands of this species therefore absorb the carbonate much more quickly than do those of Saxifraga umbrosa, and the upper cells of the pedicels are likewise affected much more quickly. Pieces of the stem were cut off and immersed in the same solution; and now the process of aggregation travelled up the hairs in a reversed direction; the cells close to the cut surfaces being first affected.

Primula sinensis.—The flower-stems, the upper and lower surfaces of the leaves and their footstalks, are all clothed with a multitude of longer and shorter hairs. The pedicels of the longer hairs are divided by transverse partitions into eight or nine cells. The enlarged terminal cell is globular, forming a gland which secretes a variable amount of thick, slightly viscid, not acid, brownish-yellow matter.

A piece of a young flower-stem was first immersed in distilled water for 2 hrs. 30 m., and the glandular hairs were not at all affected. Another piece, bearing twenty-five short and nine long hairs, was carefully examined. The glands of the latter contained no solid or semi-solid matter; and those of only two [page 349] of the twenty-five short hairs contained some globules. This piece was then immersed for 2 hrs. in a solution of one part of carbonate of ammonia to 109 of water, and now the glands of the twenty-five shorter hairs, with two or three exceptions, contained either one large or from two to five smaller spherical masses of semi-solid matter. Three of the glands of the nine long hairs likewise included similar masses. In a few hairs there were also globules in the cells immediately beneath the glands. Looking to all thirty-four hairs, there could be no doubt that the glands had absorbed some of the carbonate. Another piece was left for only 1 hr. in the same solution, and aggregated matter appeared in all the glands. My son Francis examined some glands of the longer hairs, which contained little masses of matter, before they were immersed in any solution; and these masses slowly changed their forms, so that no doubt they consisted of protoplasm. He then irrigated these hairs for 1 hr. 15 m., whilst under the microscope, with a solution of one part of the carbonate to 218 of water; the glands were not perceptibly affected, nor could this have been expected, as their contents were already aggregated. But in the cells of the pedicels numerous, almost colourless, spheres of matter appeared, which changed their forms and slowly coalesced; the appearance of the cells being thus totally changed at successive intervals of time.

The glands on a young flower-stem, after having been left for 2 hrs. 45 m. in a strong solution of one part of the carbonate to 109 of water, contained an abundance of aggregated masses, but whether generated by the action of the salt, I do not know. This piece was again placed in the solution, so that it was immersed altogether for 6 hrs. 15 m., and now there was a great change; for almost all the spherical masses within the gland-cells had disappeared, being replaced by granular matter of a darker brown. The experiment was thrice repeated with nearly the same result. On one occasion the piece was left immersed for 8 hrs. 30 m., and though almost all the spherical masses were changed into the brown granular matter, a few still remained. If the spherical masses of aggregated matter had been originally produced merely by some chemical or physical action, it seems strange that a somewhat longer immersion in the same solution should so completely alter their character. But as the masses which slowly and spontaneously changed their forms must have consisted of living protoplasm, there is nothing surprising in its being injured or killed, and its appearance wholly changed by long immersion in so strong a solution of the carbonate as that [page 350] employed. A solution of this strength paralyses all movement in Drosera, but does not kill the protoplasm; a still stronger solution prevents the protoplasm from aggregating into the ordinary full-sized globular masses, and these, though they do not disintegrate, become granular and opaque. In nearly the same manner, too hot water and certain solutions (for instance, of the salts of soda and potash) cause at first an imperfect kind of aggregation in the cells of Drosera; the little masses afterwards breaking up into granular or pulpy brown matter. All the foregoing experiments were made on flower-stems, but a piece of a leaf was immersed for 30 m. in a strong solution of the carbonate (one part to 109 of water), and little globular masses of matter appeared in all the glands, which before contained only limpid fluid.

I made also several experiments on the action of the vapour of the carbonate on the glands; but will give only a few cases. The cut end of the footstalk of a young leaf was protected with sealing-wax, and was then placed under a small bell-glass, with a large pinch of the carbonate. After 10 m. the glands showed a considerable degree of aggregation, and the protoplasm lining the cells of the pedicels was a little separated from the walls. Another leaf was left for 50 m. with the same result, excepting that the hairs became throughout their whole length of a brownish colour. In a third leaf, which was exposed for 1 hr. 50 m., there was much aggregated matter in the glands; and some of the masses showed signs of breaking up into brown granular matter. This leaf was again placed in the vapour, so that it was exposed altogether for 5 hrs. 30 m.; and now, though I examined a large number of glands, aggregated masses were found in only two or three; in all the others, the masses, which before had been globular, were converted into brown, opaque, granular matter. We thus see that exposure to the vapour for a considerable time produces the same effects as long immersion in a strong solution. In both cases there could hardly be a doubt that the salt had been absorbed chiefly or exclusively by the glands.

On another occasion bits of damp fibrin, drops of a weak infusion of raw meat and of water, were left for 24 hrs. on some leaves; the hairs were then examined, but to my surprise differed in no respect from others which had not been touched by these fluids. Most of the cells, however, included hyaline, motionless little spheres, which did not seem to consist of protoplasm, but, I suppose, of some balsam or essential oil.

Pelargonium zonale (var. edged with white).—The leaves [page 351] are clothed with numerous multicellular hairs; some simply pointed; others bearing glandular heads, and differing much in length. The glands on a piece of leaf were examined and found to contain only limpid fluid; most of the water was removed from beneath the covering glass, and a minute drop of one part of carbonate of ammonia to 146 of water was added; so that an extremely small dose was given. After an interval of only 3 m. there were signs of aggregation within the glands of the shorter hairs; and after 5 m. many small globules of a pale brown tint appeared in all of them; similar globules, but larger, being found in the large glands of the longer hairs. After the specimen had been left for 1 hr. in the solution, many of the smaller globules had changed their positions; and two or three vacuoles or small spheres (for I know not which they were) of a rather darker tint appeared within some of the larger globules. Little globules could now be seen in some of the uppermost cells of the pedicels, and the protoplasmic lining was slightly separated from the walls of the lower cells. After 2 hrs. 30 m. from the time of first immersion, the large globules within the glands of the longer hairs were converted into masses of darker brown granular matter. Hence from what we have seen with Primula sinensis, there can be little doubt that these masses originally consisted of living protoplasm.

A drop of a weak infusion of raw meat was placed on a leaf, and after 2 hrs. 30 m. many spheres could be seen within the glands. These spheres, when looked at again after 30 m., had slightly changed their positions and forms, and one had separated into two; but the changes were not quite like those which the protoplasm of Drosera undergoes. These hairs, moreover, had not been examined before immersion, and there were similar spheres in some glands which had not been touched by the infusion.