In making these experiments it was proved that in order to exhibit fairly their nocturnal movements, the soil of the plants must not be kept too dry. Also that the temperature must not be kept too low; but this would naturally vary in different plants, those which are natives of hot countries requiring a higher temperature to exhibit their natural activity, than those which are denizens of a more temperate clime. A plant out of doors, although in good health, did not exhibit any nocturnal phenomena, whilst a plant of the same species in a warm greenhouse had its leaflets all drooping at night. In the case of many plants it was also found indispensable that the leaves should be well illuminated during the day in order to their sleeping at night. One plant which had its leaves violently agitated by the wind during the day was thereby prevented sleeping by night.
In some instances, as has been detailed, the leaves, or leaflets, are elevated at night, and in others they are depressed. Of the genera examined experimentally there are thirty-seven in which the leaves or leaflets rise and thirty-two in which they sink at night. In a species of Bauhinia from Brazil the nocturnal movement was different from any others to which we have alluded. In this plant the leaves are large and broad, with a deep notch at the ends. At night the two halves rise, and close together, with the upper faces closely applied to each other, like closing a book. In young plants the petioles rise also at the same time. Owing to the closing up of the leaves in this manner, the plants have a much more compact appearance at night than during the day.
Without attempting to go over all the argument, either as to the causes which operate in producing these nocturnal movements, or their utility in the economy of the plant, to the latter of which we have already alluded, we may refer to one point which has not been evident from our narrative, but which is insisted upon by Mr. Darwin in his summary. This is in reference to the continuous movement, to a greater or a less extent, during night as well as day, except where the close pressure or imbrication of the leaves prevents motion.
"Any one who had never observed continuously a sleeping plant," he says, "would naturally suppose that the leaves moved only in the evening when going to sleep, and in the morning when awaking; but he would be quite mistaken, for we have found no exception to the rule that leaves which sleep continue to move during the whole twenty-four hours; they move, however, more quickly when going to sleep, and when awaking, than at other times. That they are not stationary during the day has been demonstrated. It is troublesome to observe the movements of leaves in the middle of the night, but this was done in a few cases; and tracings were made during the early part of the night of the movements in the case of several plants (in Oxalis, Amphicarpaea, two species of Erythrina, Cassia, Passiflora, Euphorbia, and Marsilea) and the leaves after they had gone to sleep were found to be in constant movement. When, however, opposite leaflets come into close contact with one another, or with the stem at night, they are, as we believe, mechanically prevented from moving, but this point was not sufficiently investigated."
It is very certain that the more, and the closer, the growth of plants is investigated, the more evident does it become that there is a continual motion of some kind going on, and that a state of life is a state of motion. We have only of late years began to appreciate the fact that plant organs are capable of spontaneous movement, and although the earlier chapters in the history of such phenomena have been written, these are, doubtless, only preliminary to fuller and more elaborate details which the future will reveal.
The number of genera in which nocturnal movements have been observed is not more than about eighty-six. All the species in a genus will, as a rule, exhibit the same kind of motion. Here and there an erratic species may be found expressing its movements in the language of another genus, but usually they are tolerably uniform. The large natural order of Leguminosae or pod-bearers, to which the pea and bean belongs, contains the largest number of genera possessed of distinct nocturnal movements. It is in this order that the "sensitive plants" are located. There are in fact more genera belonging to this order in which "sleeping partners" have been observed than in all the other families put together.
It is difficult to establish a definite boundary between plants which exhibit nocturnal movements and those which exhibit similar movements, but in a less pronounced degree, as for instance in those which elevate or depress their leaves but slightly, or those which only exhibit the same kind of rotation in the evening and morning as at mid-day. This feature adds force to the suggestion made by Darwin, that the movements which are termed "nocturnal" or "sleep" movements, are caused in the same manner, and only differ in degree from the ordinary rotation of leaves and branches. In fact "nyctitropism" or the sleep of leaves is "merely a modification of their ordinary circumnutating movement, regulated in its period and amplitude, by the alterations of light and darkness."
Viewed in any aspect, and under any name, these phenomena are most interesting and curious. Whether we choose to call them "sleep," or only "periodic," it matters not, the facts themselves cannot but enlarge our ideas of plant existence. If we have been in the habit of considering that in its manifestations of activity the vegetable world is much the inferior of the animal world, investigations of this kind will compel us to modify our somewhat rash generalisations, and look with increased respect on the organisms we have unduly depreciated.
Floral apostles, that, in dewy splendour, Weep without woe, and blush without a crime, Oh, may I deeply learn, and ne'er surrender Your lore sublime.
CHAPTER XII
METEORIC FLOWERS.
IT was a happy idea of Linnaeus to construct a "floral clock," with the hours representing the opening or closing of certain flowers. It was also the same botanist who applied the name of "meteoric flowers" to such as closed and expanded periodically, at, or near the same period of time, or such as appeared to be influenced especially by definite atmospheric changes in opening or closing. Pretty and poetical as such a theory may be, it is doubtful if it extends beyond this. So many circumstances may modify the periodicity to such an extent as to upset any horological arrangement. A dull day and a bright sunny one, a dry morning or a moist one, will certainly not produce the same results. The opening and closing depending so much on light and temperature will be related more to the bright, clear sky, and the warm genial atmosphere, than to the particular hour of the day. Admitting all these influences and conditions, it is doubtless true that under a normal condition there are many flowers which open or close nearly at the same time, or within an hour. It might be said that certain flowers have a manifest tendency to open or close at, or about a certain time, unless this tendency is disturbed or thwarted by special interference. Probably this was all that Linnaeus ever intended, and that his design was to indicate that some flowers expanded with the first break of day, others not until noon, and others again in the evening, or during the night. Thus much is undoubtedly true and in this aspect the subject has its interest, sufficient to demand a brief notice at our hands, although practically of but little scientific value.
The catalogue, as constructed by Linnaeus would be too long for insertion here, and we shall be content with enumerating the abbreviated list as corroborated by De Candolle, the hours of waking being first stated.
2 a.m. Purple Convolvulus (Ipomoea purpurea) 3-4 a.m. Fiore di Notte (Ipomoea nil) s Great Bindweed (Calystegia sepium) 4-5 a.m. Goatsbeard (Tragopogon pratense) and other Cichoraceae 5 a.m. Yellow Arctic Poppy (Papaver nudicaule)
Although light and temperature appear to have an influence on the opening and closing of flowers, yet in some species at least, which open or close near a definite time, there is a decided effort made to open or close when the appointed hour arrives, even when the external conditions are unfavourable. De Candolle made some experiments with flowers excluded from the daylight, and found that in some cases a plant which was accustomed to flower in daylight at a certain time, still followed the clock hour in their opening and closing.
Fritsch made some extensive observations at Prague on the opening and closing of flowers. He was of opinion that there is no hour of the day when the flowers of some plant do not open, and in the majority of instances they are closed at sunset. With the exception of a few hours near midnight, there is no hour at which blossoms do not begin to close—the maximum about six o'clock in the evening. In a few plants the complete expansion lasts only an hour, sometimes not so much. In those blossoms which are fully expanded in the morning the duration of expansion is short. In those blossoms which expand in the afternoon, the condition of waking is limited by the length of time the sun is above the horizon. In those blossoms which are fully expanded in the night the duration of sleep is shortest. Flowers which are fully expanded in the morning open more rapidly than they close. Those which open in the afternoon close more rapidly than they open. For other facts relative to light, temperature, and other influences on the movements of flowers, we must refer to the memoir from which we have quoted.
Image
Fig. 42.—Scarlet Pimpernel (Anagallis arvensis).
The pretty Arenaria rubra, says Edwin Lees, that opens its purple petals wide before the mid-day sun closes them instantly as soon as plucked, or folds them close should a storm obscure the welkin with dark clouds. The daisy "goes to bed," as it is said, before the sun goes down, but the bright yellow wort (Chlora perfoliata) closes its flowers before five in the afternoon, and the yellow goat's beard (Tragopogon pratensis), so common now in upland meadows, even before noon—hence its colloquial name "Go-to-bed-at-noon." The little pimpernel (Anagallis arvensis) sullenly keeps its scarlet petals closely shut on a cloudy or a rainy day, and this so constantly and certainly, that it has been called the "Shepherd's weather-glass," for whatever the barometer may indicate, if the red pimpernel has its flowers expanded fully in the morning, there will, to a certainty, be no rain of any consequence on that day, and the umbrella and the macintosh may be dispensed with.
Come, tell me, thou coy little flower, Converging thy petals again, Who gave thee the magical power Of shutting thy cup on the rain, While many a beautiful bow'r Is drenched in nectareous dew, Seal'd up is your scarlet-tinged flower, And the rain peals in vain upon you?
The cowslip and primrose can sip The pure mountain dew as it flows, But you, ere it touches your lip, Coyly raise your red petals and close; The rose and the sweet briar drink With pleasure the stores of the sky, And why should your modesty shrink From a drop in that little pink eye?
Many of the night-blooming flowers are fragrant; it would seem that because they expand at a period when, on account of the darkness, beauty of appearance would be futile to attract, they compensate for this by diffusing delicious odours. The great water-lily (Victoria regia) is really nocturnal, as are some others of the water lilies, and the magnificent night-blooming cereus (Cereus grandiflorus) is one of the finest, as well as one of the most fragrant of flowers.