Read Jan. 26, 1758.
ACosta and Prosper Alpinus, who both wrote near the conclusion of the XVIth century, are, I believe, the first, who recorded that nocturnal change in the leaves of plants, which has since been called somnus. It is an observation indeed as old as Pliny’s time, that the leaves of trefoil assume an erect situation[15] upon the coming of storms. The same is observable of our wood-sorrel; and Linnæus adds, of almost all plants with declinated stamina[16]. In the Trifolium pratense album C. B. or common white-flowered meadow trefoil, it is so obvious, that the common people in Sweden remark, and prognosticate the coming of tempests and rain from it.
The examples of sleeping plants instanced by Alpinus are but few. That author says, it was common to several Egyptian species[17]; but specifies only the Acaciæ, Abrus, Absus, Sesban, and the Tamarindtree. Cornutus some time afterwards remarked this property in the Pseudo-acacia Americana. From that time it has remained almost unnoticed, till Linnæus, ever attentive to nature’s works, discovered that the same affair was transacted in many other plants; and his observations have furnished us with numerous and obvious examples thereof. Mr. Miller mentions it in the Medicago arborea Lin. Sp. Pl. 778. and we may add to the list two other common plants not mentioned by Linnæus: these are the Phaseolus vulgaris, common kidney-bean; and the Trifolium pratense purpureum majus, or clover-grass: in both which this nocturnal change is remarkably displayed. Doubtless the same property exists in numberless other species; and future observation will very probably confirm Dr. Hill’s sentiment, that no “plant or tree is wholly unaffected by it.”
It is now more than twenty years since Linnæus first attended to this quality in plants. In his Flora Lapponica, when speaking of the Trifolium pratense album, as above-mentioned, he remarks, that the leaves of the Mimosa, Cassia, Bauhinia, Parkinsonia, Guilandina, and others in affinity with them, were subject to this change in the night time: and he had then carried his observations so far, as to find, that heat and cold were not the cause of this quality; since they were alike influenced by it when placed in stoves, where the temperature of the air was always the same.
The merit of reviving this subject is therefore due to the illustrious Swede; and the naturalist is greatly indebted to him for so far extending his observations thereon.
The subject of the somnus plantarum cannot but be highly entertaining to the lovers of natural knowlege: and such, I apprehend, cannot be less entertained with that faculty, which Linnæus calls vigiliæ florum; of which we shall give a brief account.
Previous to our explanation of this affair it is proper to observe, that the flowers of most plants, after they are once opened, continue so night and day, until they drop off, or die away. Several others, which shut in the night-time, open in the morning either sooner or later, according to their respective situation in the sun or shade, or as they are influenced by the manifest changes of the atmosphere. There are however another class of flowers, which make the subject of these observations, which observe a more constant and uniform law in this particular. These open and shut duly and constantly at certain and determinate hours, exclusive of any manifest changes in the atmosphere; and this with so little variation in point of time, as to render the phænomenon well worth the observation of all, whose taste leads them this way.
This faculty in the flowers of plants is not altogether a new discovery; but we are indebted to the same hand for additional observations upon this head likewise. It is so manifest in one of our common English plants, the Tragopogon luteum, that our country people long since called it John-go-to-bed-at-noon. Linnæus’s observations have extended to near fifty species, which are subject to this law. What we find principally upon this subject is in the Philosophia Botanica, p. 273. We will enumerate these plants, and mention the time when the flowers open and shut, that those, who have opportunity and inclination, may gratify themselves, and probably at the same time extend this branch of botanic knowlege still farther.
It is proper to observe, that as these observations were made by Linnæus in the academical garden at Upsal, whoever repeats them in this country will very probably find, that the difference of climate will occasion a variation in point of time: at least this will obtain in some species, as our own observations have taught us; in others the time has corresponded very exactly with the account he has given us.
Whether this faculty hath any connexion with the great article of fecundation in the oeconomy of flowers, I cannot determine: in the mean time it is not improbable. Future and repeated observations, and well-adapted experiments, will tend to illustrate this matter, and it may be lead the way to a full explanation of the cause.