Seven cabbage seeds of the previous year were placed on the same number of leaves. Some of these leaves were moderately, but the greater number only slightly inflected, and most of them re-expanded on the third day. One, however, remained clasped till the fourth, and another till the fifth day. These leaves therefore were excited somewhat more by the seeds than by inorganic objects of the same size. After they re-expanded, the seeds were placed under favourable conditions on damp sand; other seeds of the same lot being tried at the same time in the same manner, and found to germinate well. Of the seven seeds which had been exposed to the secretion, only three germinated; and one of the three seedlings soon perished, the tip of its radicle being from the first decayed, and the edges of its cotyledons of a dark brown colour; so that altogether five out of the seven seeds ultimately perished.
Radish seeds (Raphanus sativus) of the previous year were placed on three leaves, which became moderately inflected, and re-expanded on the third or fourth day. Two of these seeds were transferred to damp sand; only one germinated, and that very slowly. This seedling had an extremely short, crooked, diseased, radicle, with no absorbent hairs; and the cotyledons were oddly mottled with purple, with the edges blackened and partly withered.
Cress seeds (Lepidum sativum) of the previous year were placed on four leaves; two of these next morning were moderately and two strongly inflected, and remained so for four, five, and even six days. Soon after these seeds were placed on the leaves and had become damp, they secreted in the usual manner a layer of tenacious mucus; and to ascertain whether it was the absorption of this substance by the glands which caused so much inflection, two seeds were put into water, and as much of the mucus as possible scraped off. They were then placed on leaves, which became very strongly inflected in the course of 3 hrs., and were still closely inflected on the third day; so that it evidently was not the mucus which excited so [page 128] much inflection; on the contrary, this served to a certain extent as a protection to the seeds. Two of the six seeds germinated whilst still lying on the leaves, but the seedlings, when transferred to damp sand, soon died; of the other four seeds, only one germinated.
Two seeds of mustard (Sinapis nigra), two of celery (Apium graveolens)—both of the previous year, two seeds well soaked of caraway (Carum carui), and two of wheat, did not excite the leaves more than inorganic objects often do. Five seeds, hardly ripe, of a buttercup (Ranunculus), and two fresh seeds of Anemone nemorosa, induced only a little more effect. On the other hand, four seeds, perhaps not quite ripe, of Carex sylvatica caused the leaves on which they were placed to be very strongly inflected; and these only began to re-expand on the third day, one remaining inflected for seven days.
It follows from these few facts that different kinds of seeds excite the leaves in very different degrees; whether this is solely due to the nature of their coats is not clear. In the case of the cress seeds, the partial removal of the layer of mucus hastened the inflection of the tentacles. Whenever the leaves remain inflected during several days over seeds, it is clear that they absorb some matter from them. That the secretion penetrates their coats is also evident from the large proportion of cabbage, raddish, and cress seeds which were killed, and from several of the seedlings being greatly injured. This injury to the seeds and seedlings may, however, be due solely to the acid of the secretion, and not to any process of digestion; for Mr. Traherne Moggridge has shown that very weak acids of the acetic series are highly injurious to seeds. It never occurred to me to observe whether seeds are often blown on to the viscid leaves of plants growing in a state of nature; but this can hardly fail sometimes to occur, as we shall hereafter see in the case of Pinguicula. If so, Drosera will profit to a slight degree by absorbing matter from such seeds.]
A Summary and Concluding Remarks on the Digestive Power of Drosera.
When the glands on the disc are excited either by the absorption of nitrogenous matter or by mechanical irritation, their secretion increases in quantity and becomes acid. They likewise transmit [page 129] some influence to the glands of the exterior tentacles, causing them to secrete more copiously; and their secretion likewise becomes acid. With animals, according to Schiff,* mechanical irritation excites the glands of the stomach to secrete an acid, but not pepsin. Now, I have every reason to believe (though the fact is not fully established), that although the glands of Drosera are continually secreting viscid fluid to replace that lost by evaporation, yet they do not secrete the ferment proper for digestion when mechanically irritated, but only after absorbing certain matter, probably of a nitrogenous nature. I infer that this is the case, as the secretion from a large number of leaves which had been irritated by particles of glass placed on their discs did not digest albumen; and more especially from the analogy of Dionaea and Nepenthes. In like manner, the glands of the stomach of animals secrete pepsin, as Schiff asserts, only after they have absorbed certain soluble substances, which he designates as peptogenes. There is, therefore, a remarkable parallelism between the glands of Drosera and those of the stomach in the secretion of their proper acid and ferment.
The secretion, as we have seen, completely dissolves albumen, muscle, fibrin, areolar tissue, cartilage, the fibrous basis of bone, gelatine, chondrin, casein in the state in which it exists in milk, and gluten which has been subjected to weak hydrochloric acid. Syntonin and legumin excite the leaves so powerfully and quickly that there can hardly be a doubt that both would be dissolved by the secretion. The secretion
* ‘Phys. de la Digestion,’ 1867, tom. ii. pp. 188, 245. [page 130]
failed to digest fresh gluten, apparently from its injuring the glands, though some was absorbed. Raw meat, unless in very small bits, and large pieces of albumen, &c., likewise injure the leaves, which seem to suffer, like animals, from a surfeit. I know not whether the analogy is a real one, but it is worth notice that a decoction of cabbage leaves is far more exciting and probably nutritious to Drosera than an infusion made with tepid water; and boiled cabbages are far more nutritious, at least to man, than the uncooked leaves. The most striking of all the cases, though not really more remarkable than many others, is the digestion of so hard and tough a substance as cartilage. The dissolution of pure phosphate of lime, of bone, dentine, and especially enamel, seems wonderful; but it depends merely on the long-continued secretion of an acid; and this is secreted for a longer time under these circumstances than under any others. It was interesting to observe that as long as the acid was consumed in dissolving the phosphate of lime, no true digestion occurred; but that as soon as the bone was completely decalcified, the fibrous basis was attacked and liquefied with the greatest ease. The twelve substances above enumerated, which are completely dissolved by the secretion, are likewise dissolved by the gastric juice of the higher animals; and they are acted on in the same manner, as shown by the rounding of the angles of albumen, and more especially by the manner in which the transverse striae of the fibres of muscle disappear.