Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/149

INSECTIVOROUS PLANTS 137 of Dioiifca are completely indifferent to wind and rain. The surface of the blade is very slightly sensitive ; it may be roughly handled or scratched without causing movement, but closes when its surface or midrib is deeply pricked or cut. Irritation of the triangular area on each lobe enclosed by the sensitive filaments causes closure. The footstalk is quite insensitive. Inorganic or non-nitrogenous bodies, placed on the leaves without touching the sensitive fila ments, do not excite movement, but nitrogenous bodies, if in the least degree damp, cause after several hours the lobes to close slowly, So too the leaf which has closed over a digestible body applies a gradual pressure, which serves to bring the glands on both sides into contact with the body, and may also, as Balfour suggests, aid in absorp tion. Thus we see that there are two kinds of movement, adapted for different purposes, one rapid, excited mechani cally, the other slow, excited chemically. Leaves made to close over insoluble bodies reopen in less than twenty-four hours, and are ready, even before being fully expanded, to shut again. But if they have closed over nitrogen-yielding bodies, they remain closely shut for many days, and after re-expanding are torpid, and never act again, or only after a considerable time. Even in a state of nature, the most vigorous leaves are very rarely able to digest more than twice, or at most thrice, during their life. The secretion is a true gastric juice containing formic acid, and like gastric juice has remarkable antiseptic powers. Lindsay fed leaves with such quantities of meat as to kill them with indigestion, yet showed that the meat inside the leaf remained perfectly fresh while portions hanging outside putrefied. While evidence is thus afforded of the absorption of the products of digestion by the complete disappearance of fibrin, albumen, &c., placed upon the leaf of Dionsea, Fraustadt was able, by feeding leaves with albumen dyed with aniline-red, to colour the contents and nuclei of the gland-cells. The motor impulse, as in Drosera, is transmitted through the cellular tissue. Burdon Sanderson has demonstrated the existence of a normal electric current in the leaf of Dionsea, and the negative variation under gone by that current at the mo ment of closure of the leaf due to the conversion of electromotive force into mechanical work. This discovery, which is of the highest importance as showing the pro found resemblance between the closure of the leaf of Dioneea and the contraction of a muscle, has been followed up and extend ed by Munk. C. de Candolle a- scribes the clos ure of the valves to variations in the turgescence of the paren chyma of their upper surface. Aldrovanda j FIG. 8. Aldrovanda veticulosa A, whorl of leaves; B, leaf VeSlCUiOSCl. pressed open and enlarged, showing glands, sensitive This " minute nlamcnts > an(1 quadrifid hairs. (After Darwin.) aquatic Dionsea" floats freely, and is destituts of roots. Its whorled leaves have two lobes, with slightly inflected margins, which open only about as much as the valves of a living mussel-shell, and thus capture the more easily the small crustaceans and mollusks which may get between them. Part of the upper surface of each lobe next the midrib bears colourless glands (like those of Diono&a, but stalked), together with numerous long sensitive filaments which have both median and basal articulations ; the outer thinner portion bears small quadrifid hairs. Darwin holds that the glands secrete and digest, while the quadrifids are destined to the absorption of decaying animal matter, the two regions of the leaf thus serving for very different purposes. Drosophyllumlusitanicum. This plant catches such vast numbers of flies in a state of nature that the Portuguese cottagers call it the fly-catcher, and hang up branches of it in their houses for this purpose. Its linear leaves are thickly covered with stalked glands which resemble in the main the tentacles of Drosera, save in that they are incapable of movement, and that their secretion is acid before excitement. The secretion too is less viscid, and freely leaves the gland to wet the insect, which, creeping onward, soon clogs its wings and dies. There are, moreover, many minute colourless sessile glands which only begin to secrete when stimulated by the absorption of nitrogenous matter, with which they seem . i rio.9. Part of leaf to be mainly concerned. of prosophyiium Roridula and ByUis resemble Droso- " x 7. 7 77 t i. it i j t 1 Showing lower phyUum, but their glands are of simpler surface. (After structure than those of the latter, scarcely Danvin -) differing appreciably from the glandular hairs of other plants. Mr Darwin has thrown considerable light upon the question of how far the glands of plants not adapted for capturing insects share the power of absorption exhibited by those of the Drosemcese. Choosing a number of plants at hazard, he found that the glands of two species of Saxifraga, a genus distantly allied to Drosera, of a Primula, and of Pelargonium have the power of rapid absorption, and exhibit movements of aggregation in their protoplasm, whereas those of Erica, Miralilis, and Nicotiana appear to have no such power. Heckei has made similar observations on the floral glands of Parnassia pahistris, and on the leaf-glands of Geranium sparmannia, <fec. The glandular hairs of at least some plants are known to be capable of absorbing ammonia, both in solu tion and in vapour, and probably some obtain animal matter from the insects which are occasionally entangled in the viscid secretion. FIG. 10. A, leaf of Butterwort (Pinguicula vulgaris), with left margin inflected over a row of small flies. (After Darwin.) 13, glands from surface of leaf ( x 300). Pinguicula or Buttemvort. The large thick radical leaves of this genus have a very viscous surface and a pale colour, and bear two sets of glands, the larger borne on usually unicellular pedicels, the smaller almost sessile. When a fly is captured, the viscous secretion becomes strongly acid, the naturally incurved margins of the leaf XIII. 1 8