(15) Seeds.—A considerable number of seeds or fruits selected by hazard, some fresh and some a year old, some soaked for a short time in water and some not soaked, were tried. The ten following kinds, namely cabbage, radish, Anemone nemorosa, Rumex acetosa, Carex sylvatica, mustard, turnip, cress, Ranunculus acris, and Avena pubescens, all excited much secretion, which was in several cases tested and found always acid. The five first-named seeds excited the glands more than the others. The secretion was seldom copious until about 24 hrs. had elapsed, no doubt owing to the coats of the seeds not being easily permeable. Nevertheless, cabbage seeds excited some secretion in 4 hrs. 30 m.; and this increased so much in 18 hrs. as to run down the leaves. The seeds or properly the fruits of Carex are much oftener found adhering to leaves in a state of nature than those of any other genus; and the fruits of Carex sylvatica excited so much secretion that in 15 hrs. it ran into the incurved edges; but the glands ceased to secrete after 40 hrs. On the other hand, the glands on which the seeds of the Rumex and Avena rested continued to secrete for nine days.
The nine following kinds of seeds excited only a slight amount of secretion, namely, celery, parsnip, caraway, Linum grandiflorum, Cassia, Trifolium pannonicum, Plantago, onion, [page 386] and Bromus. Most of these seeds did not excite any secretion until 48 hrs. had elapsed, and in the case of the Trifolium only one seed acted, and this not until the third day. Although the seeds of the Plantago excited very little secretion, the glands continued to secrete for six days. Lastly, the five following kinds excited no secretion, though left on the leaves for two or three days, namely lettuce, Erica tetralix, Atriplex hortensis, Phalaris canariensis, and wheat. Nevertheless, when the seeds of the lettuce, wheat, and Atriplex were split open and applied to leaves, secretion was excited in considerable quantity in 10 hrs., and I believe that some was excited in six hours. In the case of the Atriplex the secretion ran down to the margin, and after 24 hrs. I speak of it in my notes “as immense in quantity and acid.” The split seeds also of the Trifolium and celery acted powerfully and quickly, though the whole seeds caused, as we have seen, very little secretion, and only after a long interval of time. A slice of the common pea, which however was not tried whole, caused secretion in 2 hrs. From these facts we may conclude that the great difference in the degree and rate at which various kinds of seeds excite secretion, is chiefly or wholly due to the different permeability of their coats.
Some thin slices of the common pea, which had been previously soaked for 1 hr. in water, were placed on a leaf, and quickly excited much acid secretion. After 24 hrs. these slices were compared under a high power with others left in water for the same time; the latter contained so many fine granules of legumin that the slide was rendered muddy; whereas the slices which had been subjected to the secretion were much cleaner and more transparent, the granules of legumin apparently having been dissolved. A cabbage seed which had lain for two days on a leaf and had excited much acid secretion, was cut into slices, and these were compared with those of a seed which had been left for the same time in water. Those subjected to the secretion were of a paler colour; their coats presenting the greatest differences, for they were of a pale dirty tint instead of chestnut-brown. The glands on which the cabbage seeds had rested, as well as those bathed by the surrounding secretion, differed greatly in appearance from the other glands on the same leaf, for they all contained brownish granular matter, proving that they had absorbed matter from the seeds.
That the secretion acts on the seeds was also shown by some of them being killed, or by the seedlings being injured. Fourteen cabbage seeds were left for three days on leaves and excited [page 387] much secretion; they were then placed on damp sand under conditions known to be favourable for germination. Three never germinated, and this was a far larger proportion of deaths than occurred with seeds of the same lot, which had not been subjected to the secretion, but were otherwise treated in the same manner. Of the eleven seedlings raised, three had the edges of their cotyledons slightly browned, as if scorched; and the cotyledons of one grew into a curious indented shape. Two mustard seeds germinated; but their cotyledons were marked with brown patches and their radicles deformed. Of two radish seeds, neither germinated; whereas of many seeds of the same lot not subjected to the secretion, all, excepting one, germinated. Of the two Rumex seeds, one died and the other germinated; but its radicle was brown and soon withered. Both seeds of the Avena germinated, one grew well, the other had its radicle brown and withered. Of six seeds of the Erica none germinated, and when cut open after having been left for five months on damp sand, one alone seemed alive. Twenty-two seeds of various kinds were found adhering to the leaves of plants growing in a state of nature; and of these, though kept for five months on damp sand, none germinated, some being then evidently dead.
The Effects of Objects not containing Soluble Nitrogenous Matter.
(16) It has already been shown that bits of glass, placed on leaves, excite little or no secretion. The small amount which lay beneath the fragments was tested and found not acid. A bit of wood excited no secretion; nor did the several kinds of seeds of which the coats are not permeable to the secretion, and which, therefore, acted like inorganic bodies. Cubes of fat, left for two days on a leaf, produced no effect.
(17) A particle of white sugar, placed on a leaf, formed in 1 hr. 10 m. a large drop of fluid, which in the course of 2 additional hours ran down into the naturally inflected margin. This fluid was not in the least acid, and began to dry up, or more probably was absorbed, in 5 hrs. 30 m. The experiment was repeated; particles being placed on a leaf, and others of the same size on a slip of glass in a moistened state; both being covered by a bell-glass. This was done to see whether the increased amount of fluid on the leaves could be due to mere deliquescence; but this was proved not to be the case. The particle on the leaf caused so much secretion that in the course of 4 hrs. it ran down across two-thirds of the leaf. After 8 hrs. the leaf, which was concave, was actually filled with very viscid [page 388] fluid; and it particularly deserves notice that this, as on the former occasion, was not in the least acid. This great amount of secretion may be attributed to exosmose. The glands which had been covered for 24 hrs. by this fluid did not differ, when examined under the microscope, from others on the same leaf, which had not come into contact with it. This is an interesting fact in contrast with the invariably aggregated condition of glands which have been bathed by the secretion, when holding animal matter in solution.
(18) Two particles of gum arabic were placed on a leaf, and they certainly caused in 1 hr. 20 m. a slight increase of secretion. This continued to increase for the next 5 hrs., that is for as long a time as the leaf was observed.
(19) Six small particles of dry starch of commerce were placed on a leaf, and one of these caused some secretion in 1 hr. 15 m., and the others in from 8 hrs. to 9 hrs. The glands which had thus been excited to secrete soon became dry, and did not begin to secrete again until the sixth day. A larger bit of starch was then placed on a leaf, and no secretion was excited in 5 hrs. 30 m.; but after 8 hrs. there was a considerable supply, which increased so much in 24 hrs. as to run down the leaf to the distance of 3/4 of an inch. This secretion, though so abundant, was not in the least acid. As it was so copiously excited, and as seeds not rarely adhere to the leaves of naturally growing plants, it occurred to me that the glands might perhaps have the power of secreting a ferment, like ptyaline, capable of dissolving starch; so I carefully observed the above six small particles during several days, but they did not seem in the least reduced in bulk. A particle was also left for two days in a little pool of secretion, which had run down from a piece of spinach leaf; but although the particle was so minute no diminution was perceptible. We may therefore conclude that the secretion cannot dissolve starch. The increase caused by this substance may, I presume, be attributed to exosmose. But I am surprised that starch acted so quickly and powerfully as it did, though in a less degree than sugar. Colloids are known to possess some slight power of dialysis; and on placing the leaves of a Primula in water, and others in syrup and diffused starch, those in the starch became flaccid, but to a less degree and at a much slower rate than the leaves in the syrup; those in water remaining all the time crisp.]
From the foregoing experiments and observations we [page 389] see that objects not containing soluble matter have little or no power of exciting the glands to secrete. Non-nitrogenous fluids, if dense, cause the glands to pour forth a large supply of viscid fluid, but this is not in the least acid. On the other hand, the secretion from glands excited by contact with nitrogenous solids or liquids is invariably acid, and is so copious that it often runs down the leaves and collects within the naturally incurved margins. The secretion in this state has the power of quickly dissolving, that is of digesting, the muscles of insects, meat, cartilage, albumen, fibrin, gelatine, and casein as it exists in the curds of milk. The glands are strongly excited by chemically prepared casein and gluten; but these substances (the latter not having been soaked in weak hydrochloric acid) are only partially dissolved, as was likewise the case with Drosera. The secretion, when containing animal matter in solution, whether derived from solids or from liquids, such as an infusion of raw meat, milk, or a weak solution of carbonate of ammonia, is quickly absorbed; and the glands, which were before limpid and of a greenish colour, become brownish and contain masses of aggregated granular matter. This matter, from its spontaneous movements, no doubt consists of protoplasm. No such effect is produced by the action of non-nitrogenous fluids. After the glands have been excited to secrete freely, they cease for a time to secrete, but begin again in the course of a few days.