254 . SCIENCE OF AGRICULTURE. I^art IL 



vegetables, are spread over the surface of the globe, by the winds and other causes already 

 mentioned, all of these seeds which fall in places suitable for their vegetation, and are 

 not destroyed by animals, germinate and produce plants; then among these plants, 

 the strongest, and largest, and those to which the soil is best suited, develope themselves 

 in number and magnitude so as to choke the others. Such is the general progress of 

 nature, and among plants, as among animals, the strong florish at the expense of the 

 weak. These causes have operated for such a length of time, that the greater number 

 of species are now fixed and considered as belonging to certain soils, situations, and 

 climates, beyond which they seldom propagate themselves otherwise than by the hands 

 of man. 



Sect. XI, Evidence and Character of Vegetable Vitality. 



1623. The power of counteracting the laws of chemical affinity is reckoned the best and 

 most satisfactory evidence of the presence and agency of a vital principle as inherent in 

 any subject. This principle, which seems first to have been instituted by Humboldt, is 

 obviously applicable to the case of animals, as is proved by the process of the digestion 

 of the food, and its conversion into chyle and blood ; as well as from the various 

 secretions and excretions effected by the several organs, and effecting the growth and 

 developement of the individual, in direct opposition to the acknowledged laws of 

 chemical aflfinity, which, as soon as the vital principle is extinct, begin immediately to 

 give indication of their action in the incipient symptoms of the putrefaction of the 

 deady body. But the rule is also applicable to the case of vegetables, as is proved by the 

 intro-susception, 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 ? 



1624. 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. 



1625. The stimulating influence of light upon the vital principle of the plant is discoverable, whether 

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

 the color of its 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 sua 

 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 is clear. The leaves of the mallow are said to exhibit but slight indi- 

 cations of this susceptibility, as also sword-shaped leaves ; and also 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 atriplex in a 

 stove heated to 25" of Reaumur. Yet the stems were not inchned 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 inferior side 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 mimosa pudica fold themselves up along the common toot-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 in 

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

 lotus of the Euphrates, as described by Theophrastus, which he represents 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 has been denominated by botanists The Horologium. 

 FlorcE. Flowers requiring but a shght 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 

 cereiw. But it seems somewhat doubtful whether or not light is the sole, agent in the present case; for 



