Book I. EVIDENCE OF VEGETABLE VITALITY. 255 



introsusception, digestion, and assimilation of the food necessary to their developement ; 

 all indicating the agency of a principle capable of counteracting the laws of chemical 

 affinity, which, at the period of what is usually called the death of the plant, begin also 

 immediately to act, and to give evidence of their action in the incipient symptoms of the 

 putrefaction of the vegetable. Vegetables are therefore obviously endowed with a species 

 of vitality. But, admitting the presence and agency of a vital principle inherent in 

 the vegetable subject, what are the peculiar properties by which this principle is cha- 

 racterised ? 



1657. Excitability . One of the most distinguishable properties of the vital principle of vegetables is 

 that of its excitability, or capacity of being acted upon by the application of natural stimuli, impelling it 

 to the exertion of its vegetative powers ; the natural stimuli thus impelling it being light and heat. 



1658. The stimulating influence of light upon the vital principle of the plant is discoverable, whether 

 in the stem, leaf, or flowei-. The direction of the stem is influenced by the action of light, as well as 

 the colour of the leaves. Distance from direct rays of light or weak light produces etiolation, and its 

 absence blanching. The luxuriance of branches depends on the presence and action of light, as is par- 

 ticularly observable in the case of hot-house plants, the branches of which are not so conspicuously 

 directed, either to the flue in quest of heat, or to the door or open sash in quest of air, as to the sun 

 in quest of light. Hence also the branches of plants are often more luxuriant on the south, than on 

 the north, side; or at least on the side that is best exposed to light. The position of the leaf is also 

 strongly affected by the action of light, to which it uniformly turns its upper surface. This may be readily 

 perceived in the case of trees trained to a wall, from which the upper surface of the leaf is by con- 

 sequence always turned ; being on a south wall turned to the south, and on a north wall turned to the 

 north : and if the upper surface of the leaf is forcibly turned towards the wall, and confined in that 

 position for a length of time, it will soon resume its primitive position upon regaining its liberty, but 

 particularly if the atmosphere be clear. The leaves of the mallow are said to exhibit but slight indi- 

 cations of this susceptibility, as also sword-shaped leaves; and those of the mistletoe are equally 

 susceptible on both sides. It had been conjectured that these effects are partly attributable to the 

 agency of heat ; and to try the value of the conjecture. Bonnet placed some plants of the J'triplex in a 

 stove heated to 25 of Reaumur. Yet the stems were not inclined to the side from which the greatest 

 degree of heat came ; but to a small opening in the stoves. Heat, then, does not seem to exert any 

 perceptible influence in the production of the above effects. Does moisture ? Bonnet found that the 

 leaves of the vine exhibited the same phenomenon when immersed in water, as when left in the open 

 air. Whence it seems probable that light is the sole agent in the production of the effects in question. 

 But as light produces such effects upon the leaves, so darkness or the absence of light produces an effect 

 quite the contrary ; for it is known that the leaves of many plants assume a very different position in the 

 night from what they have in the day. This is particularly the case with winged leaves, which, though 

 fully expanded during the day, begin to droop and bend down about sunset and during the fall of the 

 evening dew, till they meet together on the inferiorside of the leaf-stalk ; the terminal lobe, if the leaf 

 is furnished with one, folding itself back till it reaches the first pair; or the two side lobes, if the leaf is 

 trifoliate, as in the case of common clover. So, also, the leaflets of the false acacia and liquorice hang 

 down during the night, and those of Mimbsa pud'ica fold themselves up along the common foot-stalk 

 so as to overlap one another. Linnaeus has designated the above phenomenon by the appellation of 

 The Sleep of Plants. The expansion of the flower is also effected by the action of light. Many plants 

 do not fully expand their petals except when the sun shines : and hence alternately open them during 

 the day and shut them up during the night. This may be exemplified in the case of papilionaceous 

 flowers in general, which spread out their wings in fine weather to admit the rays of the sun, and again 

 fold them up as the night approaches. It may be exemplified also in the case of compound flowers, as 

 the dandelion and hawkweed. But the most singular case of this kind is perhaps that of the lotus of the 

 Euphrates, which is described by Theophrastus as rearing and expanding its blossoms by day, closing 

 and sinking down beneath the surface of the water by night so as to be beyond the grasp of the hand, 

 and again rising up in the morning to present its expanded blossom to the sun. The same phenomenon 

 is related also by Pliny. But although many plants open their flowers in the morning and shut them 

 again in the evening, yet all flowers do not open and shut at the same time. Plants of the same species 

 are tolerably regular as to time, other circumstances being the same; and hence the daily opening and 

 shutting of the flower botanists have denominated The Horologium Flurce. Flowers requiring but a 

 slight application of stimulus open early in the morning, while others, requiring more, open somewhat 

 later. Some do not open till noon, and some, whose extreme delicacy cannot bear the action of light at 

 all, open only at night ; such as the Cactus grandiflora, or night-blowing cereus. But it seems somewhat 

 doubtful whether or not light is the sole agent in the present case ; for it has been observed that equatorial 

 flowers open always at the same hour, and that tropical flowers change their hour of opening according 

 to the length of the day. It has been observed, also, that the flowers of plants which are removed from a 

 warmer to a colder climate expand at a later hour in the latter. A flower that opens at six o'clock in the 

 morning in Senegal, will not open in France or England till eight or nine, nor in Sweden till ten ; a 

 flower that opens at ten o'clock in Senegal, will not open in France or England till noon or later, and in 

 Sweden it will not open at all ; and a flower that does not open till noon or later in Senegal, will not open 

 at all in France or England. This seems as if heat or its absence were also an agent in the opening or 

 shutting of flowers ; though the opening of such as blow only in the night cannot be attributed either to 

 light or heat. But the opening or shutting of some flowers depends not so much on the action of the 

 stimulus of light as on the existing state of the atmosphere, and hence their opening or shutting betokens 

 change. If the Siberian sow-thistle shuts at night, the ensuing day will befine; and if it opens, it will be 

 cloudy and rainy. If the African marigold continues shut alter seven o'clock in the morning, rain is 

 near at hand; and if the Convolvulus arv^nsis. Calendula pluvialis, or ^nagallis arv^nsis, is even 

 already open, it will shut upon the approach of rain, the last of which, from its peculiar susceptibility, 

 has obtained the name of the poor man's weatherglass. But some flowers, besides expanding during the 

 light of day, incline also towards the sun, and follow his course, looking towards the east in the morning, 

 towards the south at noon, and towards the west in the evening ; and again returning in the night to 

 their former position in the morning. Such flowers are designated by the appellation of Heliotropes, on 

 account of their following the course of the sun; and the movement they thus exhibit is denominated 

 their nutation. This phenomenon had been observed by the ancients long before they made any con- 

 siderable progress in botany, and had even been interwoven into their mythology, having originated, 

 according to the records of fabulous history, in one of the metamorphoses of early times. Clytie, inconsol- 

 able for the loss of the affections of Sol, by whom she had been formerly beloved, and of whom she was 

 still enamoured, is represented as brooding over her griefs in silence and solitude; where, refusing all 

 sustenance, and seated upon the cold ground, with her eyes invariably fixed on the sun during the day, 

 and watching for his return during the night, she is at length transformed into a flower, retaining, as 

 much as a flower can retain it, the same unaltered attachment to the sun. This is the flower which is 

 denominated //eliotropium by the ancients, and described by Ovid as Flos qui ad solern vertitur. But it 

 is to be observed, that the flower alluded to by Ovid cannot be the //eliotrbpium of the moderns, 

 because Ovid describes it as resembling the violet : much less can it be the sun-flower, which is a 

 native of America, and could not consequently have been known to Ovid ; so that the true Helioixl,- 



