PITCHER-PLANTS.
THE important discoveries of Mr. Darwin have led to the attention of many investigators being drawn to plants which are provided with apparatus for the purposive trapping of insects. Foremost amongst these stands the distinguished Director of the Royal Gardens at Kew, who, in his address at Belfast, brought prominently into notice a large class of plants which had previously been regarded as little more than botanical freaks. We are finding, however, on all sides, that such a thing as a mere freak exists nowhere in Nature, and that the more carefully we study the details of any object, however apparently eccentric, the more do we see how powerful an agent usefulness becomes in the modification of structure.
The question may well be asked, as it often has been, how can an organ in an early stage of development be useful ; as in the case of the wonderful trap of the Diono3a, if the trap has been evolved by usefulness, what was its usefulness before it was a trap complete and perfect in the form as we now see it ? In this particular instance, we are bound to confess that our imper- fect knowledge can as yet give no answer, but this, of course, is not conclusive that a solution of the question is for ever beyond our reach.
In the case of the plants now more immediately in question, the history of the evolution of their special organs can be almost com- pletely sketched, and nothing can be more convincing of the actual process of evolution by increased usefulness than a careful study of Pitcher-plants. Of these, there are five principal families known, and these are all nearly allied, and have very interesting geographical distributions. The first three are found in America, the first, or Sarracenia family, being confined to North America ; the second, the Darlingtonia californica, to a very limited district in California ; and the third, the Heliamphora, to an equally limited area in Guiana. A fourth family, represented by the Cephalotus follicularis, is limited to a marshy district near St. George's Sound, on the north-east coast of Australia, and is chiefly remarkable for its closeness of relation to the fifth family, the Nepenthes, of which there are about twenty varieties, all of them confined to the islands on the south coast of Asia and Madagascar. The pitchers of these plants consist of some modi- fication of the leaves, as in the Sarracenias, where the whole leaf is changed ; or in the Cephalotus, where only some leaves are modified ; or in the Nepenthes, where it is only a glandular struc- ture at the end of the midrib, which has been altered for a special function.
If we examine the leaves of plants generally with special refer- ence to their function, we find that they all have, to a greater or less degree, the power of absorbing waters and aqueous solution of certain substances, especially of ammonia. We do not know that the question has yet been specially investigated, but there is strong reason to suspect that it is rather the rule than the ex- ception for plants to absorb a large proportion of their food by their leaves ; and enough is already known about a special function of digestion in certain leaves, to warrant the belief that it will be found in many plants besides those described by Darwin, Hooker, and others. The absorption of water and ammonia by all leaves we may regard as the first step towards the absorption of more complex substances in special instances ; and the cupping seen in many of the most ordinary leaves, useful in retaining moisture, may be considered as the first step in the evolution of a special modification of the leaves as reservoirs. In the event of plants with cupped leaves being subjected to the influences of a climate where a hot sunshine was occasionally in- terrupted by short, heavy showers, the plants with the most cupped leaves would have the best chance for survival, so that the evolution of plants of which some of the Sarracenhe may be taken as examples, would be easy. They have leaves modified only in shape, so that they retain water for the advantage of the plant under the influence of the hot American sun. Under such conditions, however, it would be scarcely possible for the capture of insects to be avoided. Eager as they are in search of water, they would frequently visit these reservoirs of water, and some of them would get drowned, as they constantly do in our tumblers and milk-jugs. A watery solution of their bodies would be of advantage to the plant, being absorbed, so that it would necessarily follow that the best traps would be encouraged, till the complete and perfect form, as seen in the Sarracenia purpurea, would be developed. Another circum- stance in the surroundings of the plant has stepped in at this point to its further advantage, in that certain maggots of a carnivorous fly are deposited in the mass of putrifyingin sect- bodies collected in the pitcher. These contribute to the more rapid and complete utilisation of the food, by eating and excret- ing it, and afterwards they bore a hole at the bottom of the pitcher, through which the refuse is poured on to the roots of the plants, and there utilised as manure.
In the various kinds of Sarracenim the pitchers are found to be of different forms, modified doubtless by the various kinds of food which they find in their neighbourhood. Thus the S. purpurea has a wide, open pitcher, which can take any kind of food with im- punity ; whilst the S. flava has long, slender pitchers, adapted only for small and easily decomposed flies. If a large English blue- bottle fly creeps into a pitcher of this plant kept in a greenhouse in this country, the leaf is killed, or at least withers all round the fly. This is due to over-feeding, and is extremely suggestive of the accidents which happen to the human digestive organs after over-feeding ; but if they are supplied with the food to which they are habituated, the plants thrive well. They have no special secreting glands by which they provide a digestive fluid, as will be found in the more complex pitchers of the Nepenthes, and in the leaves of the Droseraceae, where the mechanism of deglutition seems to be superadded to a perfect digestive organ.
On the Darlingtonia two forms of pitcher are found, one peculiar to the young condition of the plant, and the other to its mature state. The change from the one form to the other is quite sudden in the case of the individual plant, no intermediate stage being met with ; and these two forms seem to present a combina- tion of the representatives of various kinds of Sarracenim. This remarkable fact has been pointed out by Dr. Hooker, who well says that, occurring as it does in one outlying species of a small order, it must be regarded as marvellously significant for the law of evolution. It is, in fact, a missing link of the most important kind, for here we have in an isolated plant the actual process occur- ring under our eyes which we assume to have taken place to create out of one species two plants which, to the uneducated eye, seem as different as a beaver and a squirreL In the Cephalotus we have another link in the chain, for in it two kinds of leaves are found, one kind having the character of ordinary leaves, whilst the other shows the modification of the same leaves into pitchers.
It is in the great family of Nepenthes, however, that we find the most interesting series of changes, for these bridge over the chasm which would otherwise exist between the simple structure of a cupped leaf holding water, and the complicated mechanism of the sun-dew and Venus's fly-trap, in which are to be found a complete digestive system and the rudiments of nervous action. It has been shown by Dr. Hooker that the pitchers of the Nepenthes are not formed by a modification of the leaf itself, but from a peculiar glandular substance at the end of the mid- ribs. This may account for the introduction into the structure of the pitchers of a new element, in the shape of special secreting glands, which are found in all the members of this family. Of these there are about twenty varieties, and they are all found in the islands on the south coast of Asia, with the exception of one in Madagascar and one in Australia.
The pitchers are placed at the end of a stalk, which is continuous with the mid-rib of the leaf, and they are developed only after the leaf is well grown, showing that they are additions to the struc- ture of the plant after it had otherwise been almost completely developed. The stalk is inserted at the base of the pitcher, and the different details of the arrangements of the various kinds of Nepenthes show that they have been evolved under very varying circumstances. Thus some of the plants have their pitchers elaborated into most ingenious traps, with fringed guides to direct the straying insects unerringly into the trap, the stalk rest- ing between the guides with a very evident purpose. In some varieties these fringed guides are absent, whilst in others they exist only in the young state of the plant when food is most needed, and disappear in the pitchers produced when the special need has passed away. The pitchers in some are also covered with a lamella, which seems to serve the purpose of an umbrella in one and a sun-shade in another, to prevent extreme dilution or extreme concentration, as the case may be, of the contents of the pitchers.
The edges of the pitchers are provided with a chevaux de frise of incurved hooks, which must serve as a very effectual hindrance to the escape of any insect which once enters. The inner surface of the pitcher is lined with a special glandular apparatus, which has to do with the special secretion of the digestive fluid, and which closely resembles the same structure in the sun-dew and in Venus's fly-trap. In the pitchers is always found a certain quantity of fluid, which is partly the result of water which enters from without, but which is also partly secreted by the inner surface of the pitcher, for it is found in the pitchers before their peculiar valve-like lids have opened. In the virgin or unopened pitcher this fluid contains some animal matter, which consists partly of a substance resembling the pepsin found in the gastric secretion of animals, and some chloride of potash and soda, and it is always perfectly neutral. When removed from the pitcher, the fluid possesses no power of dissolving animal matter unless an acid is added to it, when a series of chemical changes take place which are identical with what takes place in the human or any other stomach. After a pitcher has opened, if a fly or a piece of other nitrogenous food be dropped into it, a change is soon effected in the character of the fluid. It becomes acid, and the quantity of the digestive principle is increased, and the food is slowly digested and absorbed. The details of this wonderful process are not as yet fully known, but they are engaging the attention of many observers, and enough has been already established to make it certain that the processes are identical with those which have been traced in animal digestion.
From the passive traps of the Nepenthes to the slowly moving tentacles of the Droseracem, and thence to the quickly moving jaws of the Dioncea the steps are easy, short, and natural. In animals, the gradation seems to be taken up by the class of edentata, and then the many varieties of toothed animals com- plete the scheme of digestive development by the addition of the process of mastication.
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