We may anticipate one or two objections which may possibly be urged against this little volume. One of these may be that we have made very free use of the many researches of Dr. Charles Darwin, in certain phenomena of plant life, without adding to them, in number or in illustration. To this we plead guilty, with the excuse that by so doing we should contribute something towards the diffusion of a knowledge, and, as we hope, of a more general appreciation of the important additions he has made to our knowledge of vegetable life. Some there are who have been content to associate his name only with a theory which they may not comprehend, but do not fail to condemn. With that theory we are not now concerned; but there is another aspect in which we desire that this accurate and indefatigable observer should be known and remembered, outside an exclusively scientific circle; and that is, as a collector of facts, the results of patient observations, illustrative of the life history of plants and animals. The volumes which he has written are unequalled as a cyclopedia of facts; and his bitterest foe has never accused him of distorting, or misrepresenting facts, for the benefit of any theory whatever. As a biological historian, therefore, we commend him to our readers, and, if we have added so little to the subjects which he has investigated, it is because he has done this so completely that further amplification was unnecessary.
The second objection which we may anticipate is the miscellaneous character of the subjects which we have brought together within the two ends of this one book. If the object with which this was undertaken be kept in view, we would fain think that such an objection is also untenable. We profess to be writing a popular volume, on a somewhat unpopular subject. We confess to a design of endeavouring to interest those who are not botanists, and do not pretend to any but a most superficial knowledge of plant life. For such we have collected together, under the headings of a certain number of chapters, a quantity of what we consider curious and interesting phenomena and facts, in the hope that by such means we might stimulate in them an interest in trees, plants, and flowers, which they never felt before. If we succeed in doing this, and, at the same time, in enlarging their views of the power and beneficence of the great Author of all these marvels, our work will have been accomplished.
Then wherefore, wherefore were they made, All dyed with rainbow light, All fashioned with supremest grace, Upspringing day and night;
Springing in valleys green and low, And on the mountains high, And in the silent wilderness Where no man passes by?
Our outward life requires them not— Then wherefore had they birth? To minister delight to man, To beautify the earth;
To comfort man—to whisper hope, Whene'er his faith is dim; For Who so careth for the flowers Will much more care for him.
CHAPTER II
CARNIVOROUS PLANTS—THE SUNDEWS.
IT is very many years since we wandered about the low swampy parts of Hampstead Heath, in search of the little sundew. It had in those days an interest from its comparative rarity, since it inhabits such localities as are not to be found in every district; but it had also other interest, in the beautiful sparkling glands of the leaves, and its mysterious association with dead insects. This little plant is so inconspicuous that it must be hunted for, amongst the bog moss, in the swampy places in which it delights to grow. The little leaves are nearly as round as a shirt button, and seldom so much as half an inch in diameter, attached at the lower edge to long slender stalks. These stalks radiate from a central point, a short root-stock, and the leaves lie flat on the ground, like a little rosette. In the centre rises the flowering stem, sometimes from four to six inches high, with a few minute white flowers towards the top (fig. 1). The leaves and the ends of the leaf-stalks are covered with curious hairs or tentacles, with clubbed ends, which sparkle in the sun, as if they bore on their extremity a minute dew-drop. These leaves, and their curious appendages, are the objects to which our attention must be confined, if we would comprehend why the little sundew has been called a "carnivorous plant."
The leaves, of which the plant seldom bears more than half a dozen, and often less, are covered on the upper surface with glandular hairs, to which the name "tentacles" has been applied. Of these, from 130 to 250 have been counted on single leaves. Those in the centre are shortest and erect, becoming longer and more oblique towards the margin. Each tentacle has a hair-like stem, and bears an expanded oblong gland at the apex. This is surrounded by a viscid secretion, which imparts the glistening dewy appearance that originated the name. If we remove one of these glands, and cut it down the centre, we shall see that it has an external layer of many-sided cells, which are small and filled with purple granular contents (fig. 2). Beneath this is another layer of different-shaped cells, with similar contents. In the centre is a group of elongated cylindrical cells, each with a spiral fibre winding round within it, and containing a limpid fluid. From these spiral cells a spiral vessel runs down through the centre of the stalk or pedicel of the gland. Other and more minute rudimentary hairs are found mixed with the tentacles, or covering those parts from which the tentacles are absent.
Fig. 1.—Round-leaved Sundew, Drosera rotundifolia.
<!-- image --> <!-- image -->Fig. 2.—Section of gland of Drosera rotundifolia magnified.
When any small object is placed on the glands, it causes a movement in the tentacles. The impulse is transmitted from those which are touched to others which surround them, and, one by one, the tentacles bend over towards the centre of the leaf, in order to enclose the irritating object. If the latter is a living object it is more speedily and effectively clasped than a dead one. The time required to cause all the tentacles to close over an object depends upon circumstances. The inflection is more rapid over a thin-skinned insect than a tough-coated one, and the period varies from one to four or five hours for all the tentacles to be closed down upon the captive. If the glands are only touched by a hair or thread, and nothing is left upon them, the tentacles at the margin will curve inwards. This movement may be caused by touching a gland three or four times, and in ten seconds from being touched the movement has been seen to commence. 1
When an insect is caught by this process, a much more remarkable phenomenon takes place, which was thoroughly examined by Mr. Darwin and declared in the following terms:—"When an object, such as a bit of meat or an insect, is placed on the disc of a leaf, as soon as the surrounding tentacles become considerably inflected, their glands pour forth an increased amount of secretion. I ascertained this by selecting leaves with equal-sized drops on the two sides, and by placing bits of meat on one side of the disc; and as soon as the tentacles on this side became much inflected, but before the glands touched the meat, the drops of secretion became larger. This was repeatedly observed, but a record was kept of only thirteen cases, in nine of which increased secretion was plainly observed; the four failures being due either to the leaves being rather torpid, or the bits of meat too small to cause much inflection." This is an important fact, as it shows conclusively some relationship between the action of inflection in the hairs and the amount of viscid secretion exuded.
There is, however, another important fact which must be taken into account in connexion with that just recorded. It is, that not only is the secretion increased in quantity, but it also undergoes a change in its nature, becoming more acid. This acidulation takes place before the glands have touched the object on the leaf, and so long as the tentacles remain bent downwards does the secretion continue to exude, and continues also its acid properties. It might be shown here, as the result of experiment, that fragments of meat, and other substances, placed on the leaves and submitted to the action of this secretion, remained clean and free from putrefaction, whilst other fragments of equal size, placed at the same time on damp moss, became mouldy, or disintegrated, and swarming with infusoria. This fact indicates some preservative power in the acidulated secretion.
It has been demonstrated that most insects are killed within a quarter of an hour from the time of their being caught. The respiration of insects is accomplished by means of breathing pores, or tracheæ, on the surface of their bodies. The viscid secretion from the glands tends to close and choke up these tracheæ, so that the insect is killed by suffocation. Every additional gland, as it closes over the captured insect, contributes of its viscid secretion, which soon bathes and involves the little insect, so that respiration is impossible. The struggles of an insect when first caught only serve to touch and stimulate other tentacles, and increase the number of those which close over it, and pour forth their viscid secretion, and thus hasten its death.
We may well assume, as experiments justify the assumption, that the acidulated secretion, which is discharged over the insect from the inflected glands, aids in the digestion by the plant of this animal food. It is abundantly certain that all these phenomena, the sensibility, or irritability of the tentacles when touched, their power of closing over the object on the leaf, the increase of its viscid secretion, and the acquisition of acid properties, are not performed without a purpose, and that purpose appears to be the capture of animal food, its digestion, and ultimate absorption by the plant.
There can be no doubt that the glands of the leaf do really possess the power of absorption, which may be tested by placing upon them small quantities of such substances as carbonate of ammonia, the absorption of which causes a change of colour consequent upon the aggregation of their contents. It may be assumed also from the fact that the tentacles remain closed longer over an object which contains soluble nitrogenous matter than over one which does not. The sundew has very delicate roots, which are scarcely more than suckers for obtaining moisture which the plant requires in great abundance. As Mr. Darwin observes, "a plant of sundew with the edges of its leaves curled inwards, so as to form a temporary stomach, with the glands of the closely inflected tentacles pouring forth their acid secretion, which dissolves animal matter afterwards to be absorbed, may be said to feed like an animal. But, differently from an animal, it drinks by means of its roots; and it must drink largely so as to retain many drops of viscid fluid round the glands, sometimes as many as 260, exposed during the whole day to a glaring sun."
<!-- image -->Fig. 3.—Leaf with tentacles of Drosera rotundifolia, inflected.
Thus we have taken a cursory glance at the little sundew, and some of the phenomena which it exhibits, in order to comprehend still better the more explicit details of some of the individual features in its history, to which we shall have to return. We have described the leaves, which are in fact the traps by means of which living insects are caught, and, not only this, but the stomach also in which the animal food is digested. To prove that these are not fanciful notions, but have plenty of evidence in support, the important features will have to be examined in detail.