That this action, which is comparable to digestion, is not wholly due to the secretion, as at first deposited, seems probable, since very little change took place in any of the substances when placed in the fluid drawn from the pitchers, and put in glass tubes, nor even in substances immersed in the pitchers, when the plants have been removed into a room the temperature of which was far below that of the normal temperature in which the plant flourishes. In the latter case, as soon as the plant was taken back into a higher, and more normal temperature, the immersed substances were immediately acted upon.
Comparing the action of meat, and other substances, placed in tubes containing fluid drawn from the pitchers, with similar substances placed in tubes containing distilled water, it was observed that disintegration was three times as rapid in the fluid as in the water, but this disintegration was wholly different in its character from that which followed the immersion of like substances in the fluid still contained in the pitchers of growing plants.
The conclusions arrived at from the above experiments were thus summarised. "From these observations it would appear probable that a substance acting as pepsine is given off from the inner wall of the pitcher, but chiefly after placing animal matter in the acid fluid; but whether this active agent flows from the glands, or from the cellular tissue in which they are imbedded, I have no evidence to show."
Reverting to the lids of the pitchers, on which honey-secreting glands are stated to have been found in all species but one, it must be recorded that Dr. Lawson Tait disputes the fact. He says: "On the under surface of the lids of most pitchers are to be seen glands identical in structure with those occurring on the inner surface of the pitcher. Dr. Hooker believes these to be honey-glands, but I differ from that eminent authority, for the following reasons:—That they are identical in structure with the true digestive glands, and that they are better marked in the pitchers where the lids cover the mouth completely, than in those which do not; that in many (such as in N. distillatoria) they are hooded in exactly the same way as the glands of the pitcher; that when the gland is excited by food, I have been able to detect acid secretion collected in the hoods of the lid-glands of N. distillatoria; that nectaries are usually very inconspicuous, and only a small spot of tissue which, without being transformed, produces the nectar."
The above appears to be very circumstantial, but perhaps it did not occur to the writer to inquire for what purpose acid-secreting, or digestive, glands occur on the lid of the pitcher, or whether the acid secretion found collected in the hoods might not have been accidental, or whether he might not have overlooked the "inconspicuous" honey-secreting glands, and whether himself, and the authority with whom he joins issue, may not, after all, have seen and examined two different classes of glands. Had they both really seen the same glands, it is difficult to conceive how they could have arrived at such opposite conclusions. From analogy, as well as probability, the presumption is certainly most strongly in favour of glands at the orifice secreting a sweet, attractive, rather than a digestive fluid.
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Fig. 12.—Pitcher of Nepenthes Chelsoni.
The form of the glands in the pitchers has been detailed by Mr. Gilburt, as seen in one species, and this applies pretty generally to all. "All the glands we have hitherto treated of," he writes, "are imbedded; these project entirely above the surface. They are also over-arched by a canopy crescentic in form, and have each of them direct communication with a twig of the vascular system. The arrangement of the position of these glands is quite irregular and unsymmetrical. The canopies, or hoods, however, of those nearest the mouth of the pitcher cover the glands more completely than do those lower down, while at the base the glands are wholly exposed. The figure shows the relation of hood to gland about midway between the two extremes.
"The glands vary very much in size, and are composed of five or six layers of thin-walled cells, without intercellular spaces. The cells of the superficial layer have their longest diameter at right angles to the surface, those of the next layer are about equal in all directions, while those beneath are flattened in all horizontally, and have walls of extreme thinness. At the base of the gland are generally to be seen one or two spirally-marked cells, in a group of which the vascular twig always terminates."
Dr. Lawson Tait constructed an elaborate table, in which are shown the variations in size of the glands found in the pitchers of one species. Progressing downwards, from the opening to the bottom of the pitcher, the glands were measured at every five millimetres, commencing in size at .045, and gradually increasing until they attained a diameter of .2 mm. Through the whole course downwards an increase was maintained, which was gradual for the first third of the distance, then suddenly the diameter is almost doubled. During the second third of the course the diameter is increased about one-third, whilst in the last third the increase is very slight. The estimated number of glands in a square millimetre are 73 in the upper zone, 25 in the middle zone, and 36 in the lowest. So that, where the diameter of the glands only ranges between .045 and .07 mm., the number is 73 in a given space (one square millimetre); but when the glands are .135 to .19 mm. in diameter, or twice to nearly three times as large, there are only one-third of the number, or 25, in the same area.
<!-- image -->Fig. 13.—Section of gland.
In the upper zone (of Nepenthes Rafflesiana), where the glands are small, though numerous, they are wholly covered by the hood. "The complete covering of the glands in this zone," writes Dr. Lawson Tait, "may be of advantage in protecting them and their secretion from accident and the depredation of insects, for the glands here are much more likely to be uncovered by water than those further down. I think it is also very likely that these hoods store up the digestive principles of the pitcher until they are required, or until it is washed out by the contact of water, it being retained in their cavities by capillary attraction."
"In the second zone the glands gradually alter from a round shape to an oval one, increasing at the same time in size as they are viewed from above downwards, and they become less covered by the hoods. The relative gland area is also greatly increased. The greatest amount of work would necessarily fall on the third zone, so we have here the glands at their maximum, and almost uncovered by the hoods, which still remain in existence however. The glands are so large and so close that the bulk of the surface is occupied by them."
If we were to summarise these details we should arrive at something like the following conclusions as to the pitchers of Nepenthes. That they are adapted for the capture and retention of insects; that at the orifice are certain attractions, such as the production of a sweet fluid, which would be likely to allure insects into the traps; that the mouth is protected by an overhanging lid, which would prevent the falling in of small and useless objects, but insufficient to obstruct the entrance of living prey; that this lid may also prevent the admission of an excess of external moisture; that the internal structure is extensively glandular, the glands being elevated, but at the same time protected by a hood; that the glands are largest, cover the largest surface, and are least protected at the bottom and the lower third of the inner surface; that these glands secrete a digestive fluid, with an acid reaction; that insects are commonly found at the bottom of the pitchers where they become disintegrated; that they are probably digested by the excreted fluid, and their soluble nitrogenous matter absorbed and assimilated for the advantage of the plant. Hitherto we have failed to discover, in the records of the investigations, any satisfactory evidence of the aggregation of protoplasm, which in other plants has afforded so strong an argument in favour of the absorption of animal matter. From all this, if the summary is a fair one, we are naturally led to conclude that the pitchers of the pitcher-plants are traps to catch animals, as well as stomachs to digest them, and that there are sufficient grounds for including the species of Nepenthes amongst insectivorous or carnivorous plants. Further investigation may probably discover something analogous to aggregation.
The Australian pitcher-plant (Cephalotus follicularis, fig. 15) is a smaller and much more unpretending plant than the true pitcher plants of the tropics. The leaves are produced in a rosette close to the ground, and consist of true leaves and pitchers. The latter are attached to their footstalk in an opposite direction to the pitchers of Nepenthes, the mouth being directed towards the axis. The minute structure and anatomy of these pitchers has been pretty exhaustively examined. The thimble-shaped pitchers are surmounted by a lid, which is elevated, so that the entrance to the pitchers is open and free. We are not concerned with the external structure, which is fully detailed in the memoir by Professor Dickson.
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Fig. 14.—Pitcher of Cephalotus.
Fig. 15.—Section of pitcher of Cephalotus.
The mouth of the pitcher is furnished with a corrugated rim which ends abruptly on the inner margin in a row of inflexed teeth, extending along the front of the orifice to the base of the lid (see section, fig. 15). Below the rim is a ledge extending round the inside of the pitcher, with its acute edge projecting downwards into the cavity, forming a kind of contracted neck. This is called the conducting-shelf. Below this, again, the upper two-thirds of the walls are smooth and glandular. At the lower margin of this smooth surface an oblique curved elevation extends on each side, and below all is the bottom of the pitcher, which is smooth and without glands. The surface of the conducting-shelf is furnished with hairs projecting downwards.
Looking at this arrangement as that of a fly-catching apparatus, it seems to be admirably adapted to the purpose, the projecting ledge, the downward directed hairs with which it is furnished, and the incurved teeth of the rim, all present obstacles to the exit of any insect which may enter.
<!-- image -->Fig. 16.—Glands of Cephalotus, in section (Gilburt).
Small glands with six cells, two of which are central and four peripheral, are scattered over the under-surface of the lid and upon the rim. The upper walls of the interior immediately beneath the ledge contain large glands of from thirty to forty cells. The oblique curved elevations below the glandular walls are studded with similar glands, but nearly twice as large. Mixed with these are smaller hyaline glands, which consist of a central oval cell surrounded by two to four others; the appearance is somewhat that of stomata, but the central cell, as Mr. Gilburt believes, is glandular, and Dr. Dickson surmises that its secretion may serve to dilute the other secreted matter (fig. 16, c).
Of the oblique curved elevations on which these latter glands are situated, Mr. Gilburt writes: "On the lateral walls of the pitchers near the base there are two crescent-shaped raised patches inclined forward and downward towards the front of the pitcher at a very acute angle. These are without doubt the most active secreting organs of the plant. In mature pitchers these patches are coloured a deep crimson; whether they serve in any sense as a bait to lure the insects into the pitcher I do not know, but such an idea is not improbable." And Dr. Dickson says that "the red coloured cell-contents very soon after injury of the cells, or treatment by reagents, change to a bright blue."