Similar movements may be observed in the rosebay willow herb (Epilobium angustifolium). The pistil in the centre of the flower has a four-lobed stigma, supported on an erect style, which is rather longer than the filaments of the stamens. When the flower first opens, the lobes are closely applied to each other, and the style and stamens hang down (a). As the anthers become mature the style becomes erect, and the stamens begin to elevate themselves (b). By the time the anthers are fully matured the lobes of the stigma divide and curl outwards and downwards in a circinate manner, so that they may be reached by the anthers; the filaments become erect, and the pollen is discharged upon the lobes of the stigma (c). After discharging the contents of their anthers the stamens droop and become pendulous again, whilst the style remains erect (d).
<!-- image -->Peculiar movements have been observed in other parts of flowers, and in some orchids, as in Megaclinium falcatum, the lip, or labellum, is said to exhibit spontaneous movement. Alluding to these remarkable plants, Dr. Lindley says, "Among many other remarkable peculiarities the irritability of the labellum must not be passed over in silence. This is extremely striking in some species." In Caleana nigrita the column is a boat-shaped box, resembling a lower lip; the labellum forms a lip that exactly fits it, and is hinged on a claw which reaches the middle of the column. When the flower opens, the labellum turns round within the column and falls back, so that, the flower being inverted, it stands fairly over the latter. The moment a small insect touches its point, the labellum makes a sudden revolution, brings the point to the bottom of the column, passing the anther in its way, and thus makes prisoner any insect which the box will hold. When it catches an insect it remains shut while its prey continues to move about, but if no capture is made the lid soon recovers its position. Another plant (Drakaea elastica) has a single flower placed at the end of a slender, smooth, erect scape, from twelve to eighteen inches high; and its labellum, which is hammer-shaped, and placed on a long arm, with a movable elbow-joint in the middle, is stated to resemble an insect suspended in the air, and moving with every breeze. Another plant of this description is Spiculaea ciliata, whose rusty flowers when spread open may be compared to long-legged spiders; the lip, with a long solid lamina, looking like their body, while an appendage at its apex, which is apparently movable, is not unlike the head of such a creature.
One of the species with an irritable labellum is called the Toad Orchis (Megaclinium bufo), and was figured and thus described, many years ago, in the "Botanical Register." "Let the reader imagine a green snake to be pressed flat, like a dried flower, and then to have a row of toads, or some such speckled reptiles, drawn up along the middle in single file, their backs set up, their fore-legs sprawling right and left, and their mouths wide open, with a large purple tongue wagging about convulsively, and a pretty considerable approach will be gained to an idea of this plant, which, if Pythagoras had but known of it, would have rendered all arguments about the transmigration of souls superfluous." This orchid is a native of Sierra Leone.
Mr. Darwin, writing in general terms, says, "Of the many singular properties of orchids the irritability of the labellum, in several distantly-allied forms, is highly remarkable. When touched it is described as quickly moving. This is the case with some of the species of Bulbophyllum." We have not, however, considered it well to multiply examples, since to understand and appreciate them some special knowledge of the structure and anatomy of orchid flowers is essential. As our remarks on the spontaneous movements of plants are drawing to a close, we may allude to the subject of a memoir by Professor Caspary, on the effect of extreme cold in producing movements in the branches of trees in frosty weather. The amount of motion seems to be directly proportionate to the intensity of the cold, but how it is produced has not yet been explained.
M. Lecoq has also described certain rhythmical tremors in the leaves of Colocasia esculenta, to which Dr. Masters has directed attention. These are stated to occur at intervals, the plant in the meantime being perfectly at rest; so violent are the vibrations that on one occasion the pot in which the plant was growing shook so violently that it could with difficulty be steadied. This statement has also been confirmed by another observer. The emission of water from a pore near the apex of the leaf has been occasionally observed in this plant, and it has been suggested that the tremors may have been occasioned by the efforts of the plant to rid itself of the water. But, as Dr. Masters remarks, it is certain that in many cases no such aperture is visible in the plant in question, and that the emission of water is not by any means a common phenomenon.
CHAPTER XI
SLEEP OF PLANTS.
As long ago as the time of Pliny, nearly two thousand years, certain appearances in plants were observed, which seemed to indicate a condition of repose, as exhibited in the phenomena since designated as the "sleep of plants." The celebrated Linnaeus devoted an essay to this subject, and it has since been made the theme of various authors. By the "sleep of plants," is generally included such movements of the leaves as take place periodically towards the close of the day, and which consist in moving upwards or downwards into such a position that the blade of the leaf shall be vertical or nearly so. There is no real analogy between the sleep of animals and the sleep of plants, and the latter term must therefore be accepted rather as a poetic simile, than as a record of fact. The term "nyctitropism" has been proposed as a less objectionable synonym, but its use will scarcely be necessary here, since the more popular term is not liable to be misapplied.
"The fact that the leaves of many plants place themselves at night in widely-different positions from what they hold during the day, but with the one point in common, that their upper surfaces avoid facing the zenith, often with the additional fact that they come into close contact with opposite leaves or leaflets, clearly indicates, as it seems to us, that the object gained is the protection of the upper surfaces from being chilled at night by radiation. There is nothing improbable in the upper surface needing protection more than the lower, as the two differ in function and structure."
We are prepared to accept this as the most feasible reason for the night movement of leaves, strengthened as it is by experiments which were made in this direction, and which demonstrated that leaves which had been fixed in a horizontal position during nights when the temperature was below freezing-point, suffered much more from injury by frost than other leaves on the same plant, which were permitted to assume their usual vertical position.
As it is an important feature to determine what is the probable reason for this "sleep movement" in leaves, we give verbatim the results of one experiment. "We exposed on two occasions during the summer, to a clear sky, several pinned-open leaflets of Trifolium pratense which naturally rise at night, and of Oxalis purpurea which naturally sink at night (the plants growing out of doors) and looked at them early on several successive mornings after they had assumed their diurnal positions. The difference in the amount of dew on the pinned-open leaflets, and on those which had gone to sleep was generally conspicuous; the latter being sometimes absolutely dry, whilst the leaflets which had been horizontal were coated with large beads of dew. This shows how much cooler the leaflets fully exposed must have become, than those which stood almost vertically, either upwards or downwards, during the night."
When a seed germinates in the ground, the first pair of leaf-like organs which appear above the surface are usually different in form and texture from the true leaves which are afterwards developed. These primary organs are the cotyledons, and between them the delicate little bud, or plumule, of the succeeding leaves nestles. This remark applies generally to the large number of plants which have a pair of cotyledons. Up to the period when Dr. Darwin commenced his experiments, from the record of which we have already quoted, sleep movements had only been observed in the cotyledons of two plants. This observer has determined, however, that these movements take place in a large number of plants. In some instances the cotyledons sleep, whilst the leaves do not. In others the cotyledons do not move, whilst the leaves are remarkable for their sleep movements. In some plants the cotyledons rise vertically upwards, whilst the leaves move downwards, and in others the reverse takes place. Reflecting on the reason why this movement should take place in the cotyledons, this author is of opinion that the movement, by means of which the blade is made to rise or fall almost vertically at night, has been acquired for some special purpose, and he does not doubt that purpose to be the protection of the upper surface of the blade, and perhaps also of the little central bud, or plumule, from radiation at night.
No sane person, reading over the details of experiments like these, would hesitate in the conclusion that, whatever the reason may be, there must be some adequate and sufficient cause for the movements which the cotyledons and other plant organs exhibit. That such movements are merely matters of chance is a position wholly untenable, but their orderly and systematic recurrence leads at once to the conclusion that, however much we may differ in our speculations as to the probable reason why such movements take place, there will be no doubt in our minds that they are the agents in some work which is beneficial to the plant which exhibits them. The theory suggested seems to be a sufficient one, and not opposed to known facts, and it appears to be one which is justified by the evidence. Under these circumstances we see no difficulty in accepting it as an interpretation.
We return now to the sleep of leaves. This is a phenomenon not unknown to those who have been in the habit of observing plants, as even children have noticed it. The different appearance which certain leaves present in the evening to that of mid-day could not escape recognition. Such, for instance, as the drooping leaflets of the common acacia-tree, the Robinia of gardeners, and the reflexed leaflets of the wood-sorrel, and white clover. These are the most common and readily observed. Investigation has shown that movements of leaves, upwards or downwards, in the evening, are far more common than has been supposed; to some of the details of which we shall have to refer. Movements of this kind must not be confounded with the closing of flowers on dull days, or at eventide, nor with their turning towards, or in opposition to the sun. For the present we concern ourselves entirely with the periodic leaf movements which are observed night and morning, and which bear an intimate relationship to the elevation and depression of the cotyledons, to which we have already directed attention. The movements called by Darwin the "nyctitropism of leaves."
The pretty little wood-sorrel (Oxalis acetosella) is almost universally known and admired. The leaves are of a beautiful pale green, each leaf consisting of three lobes or leaflets, which are heart-shaped, attached at the base at the top of a slender erect footstalk. During the day these leaflets are spread out nearly flat, radiating equally from the top of the footstalk. In the evening each leaflet gradually falls, until the under side nearly touches the footstalk, and, in so doing, the leaflets which are broadest upwards, are strongly bent inwards, so that each side is deeply concave. In this condition the leaves remain throughout the night (fig. 38). Leaves of this plant were carefully watched for their periodic movements. After half-past five in the evening the leaflets sank rapidly, and at seven o'clock depended vertically, and remained nearly the same until the morning, when by a quarter to seven they had commenced to rise, and continued rising for an hour. Between eleven o'clock in the morning and half-past five in the afternoon they moved four times up and down before the last great fall for the night commenced. The rising and falling during the day was slight as compared with the nocturnal fall. The highest point was reached at noon. This is one of the best plants to obtain for persons who reside in town, and desire to watch the movements of the leaves. It will grow freely and readily in a small flower-pot, by the window, in a sitting room, covering the top with its delicate green leaves, so as to be a pretty object, as well as a most interesting one. Several other species are commonly cultivated, and many of these exhibit similar phenomena, although some of them do not.