Part 23 (1/2)
The leaves arise from an almost woody axis; they [page 333] are linear, much attenuated towards their tips, and several inches in length. The upper surface is concave, the lower convex, with a narrow channel down the middle. Both surfaces, with the exception of the channel, are covered with glands, supported on pedicels and arranged in irregular longitudinal rows. These organs I shall call tentacles, from their close resemblance to those of Drosera, though they have no power of movement. Those on the same leaf differ much in length. The glands also differ in size, and are of a bright pink or of a purple colour; their upper surfaces are convex, and the lower flat or even concave, so that they resemble miniature mushrooms in appearance. They are formed of two (as I believe) layers of delicate angular cells, enclosing eight or ten larger cells with thicker, zigzag walls. Within these larger cells there are others marked by spiral lines, and apparently connected with the spiral vessels which run up the green multi-cellular pedicels. The glands secrete large drops of viscid secretion. Other glands, having the same general appearance, are found on the flower-peduncles and calyx.
FIG. 14. (Drosophyllum lusitanic.u.m.) Part of leaf, enlarged seven times, showing lower surface.
Besides the glands which are borne on longer or shorter pedicels, there are numerous ones, both on the upper and lower surfaces of the leaves, so small as to be scarcely visible to the naked eye. They are colourless and almost sessile, either circular or oval in outline; the latter occurring chiefly on the backs of the leaves (fig. 14).
Internally they have exactly the same structure as the larger glands which are supported on pedicels; [page 334] and indeed the two sets almost graduate into one another. But the sessile glands differ in one important respect, for they never secrete spontaneously, as far as I have seen, though I have examined them under a high power on a hot day, whilst the glands on pedicels were secreting copiously. Nevertheless, if little bits of damp alb.u.men or fibrin are placed on these sessile glands, they begin after a time to secrete, in the same manner as do the glands of Dionaea when similarly treated. When they were merely rubbed with a bit of raw meat, I believe that they likewise secreted.
Both the sessile glands and the taller ones on pedicels have the power of rapidly absorbing nitrogenous matter.
The secretion from the taller glands differs in a remarkable manner from that of Drosera, in being acid before the glands have been in any way excited; and judging from the changed colour of litmus paper, more strongly acid than that of Drosera. This fact was observed repeatedly; on one occasion I chose a young leaf, which was not secreting freely, and had never caught an insect, yet the secretion on all the glands coloured litmus paper of a bright red. From the quickness with which the glands are able to obtain animal matter from such substances as well-washed fibrin and cartilage, I suspect that a small quant.i.ty of the proper ferment must be present in the secretion before the glands are excited, so that a little animal matter is quickly dissolved.
Owing to the nature of the secretion or to the shape of the glands, the drops are removed from them with singular facility. It is even somewhat difficult, by the aid of a finely pointed polished needle, slightly damped with water, to place a minute particle of any kind on one of the drops; for on withdrawing the [page 335] needle, the drop is generally withdrawn; whereas with Drosera there is no such difficulty, though the drops are occasionally withdrawn. From this peculiarity, when a small insect alights on a leaf of Drosophyllum, the drops adhere to its wings, feet, or body, and are drawn from the gland; the insect then crawls onward and other drops adhere to it; so that at last, bathed by the viscid secretion, it sinks down and dies, resting on the small sessile glands with which the surface of the leaf is thickly covered.
In the case of Drosera, an insect sticking to one or more of the exterior glands is carried by their movement to the centre of the leaf; with Drosophyllum, this is effected by the crawling of the insect, as from its wings being clogged by the secretion it cannot fly away.
There is another difference in function between the glands of these two plants: we know that the glands of Drosera secrete more copiously when properly excited. But when minute particles of carbonate of ammonia, drops of a solution of this salt or of the nitrate of ammonia, saliva, small insects, bits of raw or roast meat, alb.u.men, fibrin or cartilage, as well as inorganic particles, were placed on the glands of Drosophyllum, the amount of secretion never appeared to be in the least increased. As insects do not commonly adhere to the taller glands, but withdraw the secretion, we can see that there would be little use in their having acquired the habit of secreting copiously when stimulated; whereas with Drosera this is of use, and the habit has been acquired.
Nevertheless, the glands of Drosophyllum, without being stimulated, continually secrete, so as to replace the loss by evaporation. Thus when a plant was placed under a small bell-gla.s.s with its inner surface and support thoroughly wetted, there was no loss by evaporation, and so much [page 336] secretion was acc.u.mulated in the course of a day that it ran down the tentacles and covered large s.p.a.ces of the leaves.
The glands to which the above named nitrogenous substances and liquids were given did not, as just stated, secrete more copiously; on the contrary, they absorbed their own drops of secretion with surprising quickness. Bits of damp fibrin were placed on five glands, and when they were looked at after an interval of 1 hr. 12 m., the fibrin was almost dry, the secretion having been all absorbed. So it was with three cubes of alb.u.men after 1 hr. 19 m., and with four other cubes, though these latter were not looked at until 2 hrs. 15 m. had elapsed.
The same result followed in between 1 hr. 15 m. and 1 hr. 30 m. when particles both of cartilage and meat were placed on several glands.
Lastly, a minute drop (about 1/20 of a minim) of a solution of one part of nitrate of ammonia to 146 of water was distributed between the secretion surrounding three glands, so that the amount of fluid surrounding each was slightly increased; yet when looked at after 2 hrs., all three were dry. On the other hand, seven particles of gla.s.s and three of coal-cinders, of nearly the same size as those of the above named organic substances, were placed on ten glands; some of them being observed for 18 hrs., and others for two or three days; but there was not the least sign of the secretion being absorbed. Hence, in the former cases, the absorption of the secretion must have been due to the presence of some nitrogenous matter, which was either already soluble or was rendered so by the secretion. As the fibrin was pure, and had been well washed in distilled water after being kept in glycerine, and as the cartilage had been soaked in water, I suspect that these substances must [page 337] have been slightly acted on and rendered soluble within the above stated short periods.
The glands have not only the power of rapid absorption, but likewise of secreting again quickly; and this latter habit has perhaps been gained, inasmuch as insects, if they touch the glands, generally withdraw the drops of secretion, which have to be restored. The exact period of re-secretion was recorded in only a few cases. The glands on which bits of meat were placed, and which were nearly dry after about 1 hr. 30 m., when looked at after 22 additional hours, were found secreting; so it was after 24 hrs. with one gland on which a bit of alb.u.men had been placed. The three glands to which a minute drop of a solution of nitrate of ammonia was distributed, and which became dry after 2 hrs., were beginning to re-secrete after only 12 additional hours.
Tentacles Incapable of Movement.--Many of the tall tentacles, with insects adhering to them, were carefully observed; and fragments of insects, bits of raw meat, alb.u.men, &c., drops of a solution of two salts of ammonia and of saliva, were placed on the glands of many tentacles; but not a trace of movement could ever be detected. I also repeatedly irritated the glands with a needle, and scratched and p.r.i.c.ked the blades, but neither the blade nor the tentacles became at all inflected. We may therefore conclude that they are incapable of movement.
On the Power of Absorption possessed by the Glands.--It has already been indirectly shown that the glands on pedicels absorb animal matter; and this is further shown by their changed colour, and by the aggregation of their contents, after they have been left in contact with nitrogenous substances or liquids. The following observations apply both to the glands supported on [page 338] pedicels and to the minute sessile ones. Before a gland has been in any way stimulated, the exterior cells commonly contain only limpid purple fluid; the more central ones including mulberry-like ma.s.ses of purple granular matter.
A leaf was placed in a little solution of one part of carbonate of ammonia to 146 of water (3 grs. to 1 oz.), and the glands were instantly darkened and very soon became black; this change being due to the strongly marked aggregation of their contents, more especially of the inner cells. Another leaf was placed in a solution of the same strength of nitrate of ammonia, and the glands were slightly darkened in 25 m., more so in 50 m., and after 1 hr. 30 m. were of so dark a red as to appear almost black. Other leaves were placed in a weak infusion of raw meat and in human saliva, and the glands were much darkened in 25 m., and after 40 m. were so dark as almost to deserve to be called black. Even immersion for a whole day in distilled water occasionally induces some aggregation within the glands, so that they become of a darker tint. In all these cases the glands are affected in exactly the same manner as those of Drosera. Milk, however, which acts so energetically on Drosera, seems rather less effective on Drosophyllum, for the glands were only slightly darkened by an immersion of 1 hr. 20 m., but became decidedly darker after 3 hrs. Leaves which had been left for 7 hrs. in an infusion of raw meat or in saliva were placed in the solution of carbonate of ammonia, and the glands now became greenish; whereas, if they had been first placed in the carbonate, they would have become black. In this latter case, the ammonia probably combines with the acid of the secretion, and therefore does not act on the colouring matter; but when the glands are first subjected to an organic [page 339] fluid, either the acid is consumed in the work of digestion or the cell-walls are rendered more permeable, so that the undecomposed carbonate enters and acts on the colouring matter. If a particle of the dry carbonate is placed on a gland, the purple colour is quickly discharged, owing probably to an excess of the salt. The gland, moreover, is killed.
Turning now to the action of organic substances, the glands on which bits of raw meat were placed became dark-coloured; and in 18 hrs. their contents were conspicuously aggregated. Several glands with bits of alb.u.men and fibrin were darkened in between 2 hrs. and 3 hrs.; but in one case the purple colour was completely discharged. Some glands which had caught flies were compared with others close by; and though they did not differ much in colour, there was a marked difference in their state of aggregation. In some few instances, however, there was no such difference, and this appeared to be due to the insects having been caught long ago, so that the glands had recovered their pristine state.
In one case, a group of the sessile colourless glands, to which a small fly adhered, presented a peculiar appearance; for they had become purple, owing to purple granular matter coating the cell-walls. I may here mention as a caution that, soon after some of my plants arrived in the spring from Portugal, the glands were not plainly acted on by bits of meat, or insects, or a solution of ammonia--a circ.u.mstance for which I cannot account.
Digestion of Solid Animal Matter.--Whilst I was trying to place on two of the taller glands little cubes of alb.u.men, these slipped down, and, besmeared with secretion, were left resting on some of the small sessile glands. After 24 hrs. one of these cubes was found [page 340]
completely liquefied, but with a few white streaks still visible; the other was much rounded, but not quite dissolved. Two other cubes were left on tall glands for 2 hrs. 45 m., by which time all the secretion was absorbed; but they were not perceptibly acted on, though no doubt some slight amount of animal matter had been absorbed from them. They were then placed on the small sessile glands, which being thus stimulated secreted copiously in the course of 7 hrs. One of these cubes was much liquefied within this short time; and both were completely liquefied after 21 hrs. 15 m.; the little liquid ma.s.ses, however, still showing some white streaks. These streaks disappeared after an additional period of 6 hrs. 30 m.; and by next morning (i.e.
48 hrs. from the time when the cubes were first placed on the glands) the liquefied matter was wholly absorbed. A cube of alb.u.men was left on another tall gland, which first absorbed the secretion and after 24 hrs. poured forth a fresh supply. This cube, now surrounded by secretion, was left on the gland for an additional 24 hrs., but was very little, if at all, acted on. We may, therefore, conclude, either that the secretion from the tall glands has little power of digestion, though strongly acid, or that the amount poured forth from a single gland is insufficient to dissolve a particle of alb.u.men which within the same time would have been dissolved by the secretion from several of the small sessile glands. Owing to the death of my last plant, I was unable to ascertain which of these alternatives is the true one.
Four minute shreds of pure fibrin were placed, each resting on one, two, or three of the taller glands. In the course of 2 hrs. 30 m. the secretion was all absorbed, and the shreds were left almost dry. They [page 341] were then pushed on to the sessile glands. One shred, after 2 hrs. 30 m., seemed quite dissolved, but this may have been a mistake.
A second, when examined after 17 hrs. 25 m., was liquefied, but the liquid as seen under the microscope still contained floating granules of fibrin. The other two shreds were completely liquefied after 21 hrs.
30 m.; but in one of the drops a very few granules could still be detected. These, however, were dissolved after an additional interval of 6 hrs. 30 m.; and the surface of the leaf for some distance all round was covered with limpid fluid. It thus appears that Drosophyllum digests alb.u.men and fibrin rather more quickly than Drosera can; and this may perhaps be attributed to the acid, together probably with some small amount of the ferment, being present in the secretion, before the glands have been stimulated; so that digestion begins at once.
Concluding Remarks.--The linear leaves of Drosophyllum differ but slightly from those of certain species of Drosera; the chief differences being, firstly, the presence of minute, almost sessile, glands, which, like those of Dionaea, do not secrete until they are excited by the absorption of nitrogenous matter. But glands of this kind are present on the leaves of Drosera binata, and appear to be represented by the papillae on the leaves of Drosera rotundifolia.
Secondly, the presence of tentacles on the backs of the leaves; but we have seen that a few tentacles, irregularly placed and tending towards abortion, are retained on the backs of the leaves of Drosera binata.
There are greater differences in function between the two genera. The most important one is that the tentacles of Drosophyllum have no power of movement; this loss being partially replaced by the drops of viscid [page 342] secretion being readily withdrawn from the glands; so that, when an insect comes into contact with a drop, it is able to crawl away, but soon touches other drops, and then, smothered by the secretion, sinks down on the sessile glands and dies. Another difference is, that the secretion from the tall glands, before they have been in any way excited, is strongly acid, and perhaps contains a small quant.i.ty of the proper ferment. Again, these glands do not secrete more copiously from being excited by the absorption of nitrogenous matter; on the contrary, they then absorb their own secretion with extraordinary quickness. In a short time they begin to secrete again. All these circ.u.mstances are probably connected with the fact that insects do not commonly adhere to the glands with which they first come into contact, though this does sometimes occur; and that it is chiefly the secretion from the sessile glands which dissolves animal matter out of their bodies.
RORIDULA.
Roridula dentata.--This plant, a native of the western parts of the Cape of Good Hope, was sent to me in a dried state from Kew. It has an almost woody stem and branches, and apparently grows to a height of some feet. The leaves are linear, with their summits much attenuated.
Their upper and lower surfaces are concave, with a ridge in the middle, and both are covered with tentacles, which differ greatly in length; some being very long, especially those on the tips of the leaves, and some very short. The glands also differ much in size and are somewhat elongated. They are supported on multicellular pedicels.
This plant, therefore, agrees in several respects with [page 343]