OF VEGETABLE VITALITY. 



123 



gjanised vegetable structure, is its susceptibility 

 of being acted on by external stimuli, whereby 

 all the phenomena of vegetable life are called 

 into existence. These stimuli are light, heat, 

 and electricity; and we shall consider their in- 

 fluence separately. 



Light. The action of light is very percepti- 

 ble in plants, as affecting their leaves, stems, 

 ilowers, and even roots. It is to this influence 

 of light that the bending of the stem and leaves 

 of plants is attributable, producing in them, as 

 it were, a direct motion towards the luminous 

 quarter. Bonnet planted three beans for the 

 purpose of comparative experiment; one in the 

 open air, another in a tube of glass covered at 

 the top; and a third in a tube of wood covered 

 at the top also. The first plant was strong and 

 luxuriant, the second was also strong, and in- 

 clined towards the sun; but the third, though it 

 grew tall, was pale and sickly. Hence it is, upon 

 the principle of the exclusion of light, that 

 plants are blanched, as in the case of the blanch- 

 ing of celery. The direction and luxuriance of 

 the branches depend also upon the influence of 

 light, as may be remarked in all plants growing 

 in hot-houses, the branches of which are not so 

 conspicuously directed either to the heated place 

 in quest of heat, or to the door or open sash, in 

 quest of ait, as to the sun by a mysterious attrac- 

 tion for his light. It may be observed also, that 

 the branches of vegetables turned towards the 

 south, are always more luxuriant than those on 

 the north, or at least, on that side which is most 

 fully exposed to the influence of 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 observed 

 in trees trained to a wall, from which the upper 

 surface of the leaf is necessarily alwaj^s averted, 

 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 assume its 

 original position upon regaining its liberty, but 

 particularly if the atmosphere is clear. Bonnet 

 tried to retain a leaf in its inverted position, by 

 means of twisting the leaf stalk; but it was 

 always found to untwist itself again in the course 

 of a short time, and again to present its upper 

 surface to the sun or light. This it was some- 

 times found to do, even in the night ; but always 

 tlie most expeditiously in young subjects. If 

 the experiment is often repeated, the leaf resumes 

 its original position with more difficulty, and 

 exhibits evident marks of being injured by the 

 exertion, in the appearance of several black 

 spots about the veins of the under surface, and 

 in the scaling off of the cuticle. All leaves, 

 however, ai-e not equally susceptible to the action 

 of the stimulus of light. Thus the leaves of 



the common mallow show few signs of this sus- 

 ceptibility; and it is the same with all those 

 leaves of a narrow shape. The leaves of the 

 misletoe have never been known to resume a 

 former position in consequence of any change in 

 the position of the branch, because, as Sxnith 

 observes, they are perhaps equally susceptible 

 on both sides. Succulent leaves, notwithstand- 

 ing their thick and firm texture, are said to be 

 particularly susceptible ; and if the leaf of the 

 vine is even sepai-ated from a branch, and sus- 

 pended by a fine thi'ead, so as that the upper 

 surface shall be turned from the light, it will 

 yet gradually alter its position till it comes round 

 again to it. This experiment requires to be 

 made with great care and delicacy, lest the leaf 

 should be made to turn by means of the effect 

 of the atmosphere upon the thread. 



In speculating on these effects, it may be asked 

 if light is here the sole agent; or, whether some- 

 thing may not also be due to the influence of 

 heat and moisture. To set this question at rest. 

 Bonnet placed several plants in a heated stove, 

 and he found that the stems were not turned to 

 the side from whence the greatest heat proceeded, 

 but to a small opening in the stove, admitting 

 the light. He also found that the leaves of the 

 vine turned towards the light exactly in the same 

 manner when placed in water, as when left in 

 the open air; whence it maybe naturally infeiTcd, 

 that to the influence of light alone is due the 

 motions of plants already described. 



But as light produces such effects upon the 

 leaves, so darkness, or the absence of light, pro- 

 duces an effect quite the contrary ; for it is well 

 known that the leaves of many plants assume a 

 very different position during the night from what 

 they have in the day. This is particularly the 

 case with winged leaves, which, though fully ex- 

 panded during the day, begin to droop and bend 

 down about sun-set, and during the fall of the 

 evening dew, tiU they meet together on the in- 

 ferior 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, which fact seems to have been first 

 observed by the daughter of Linneeus. So also 

 the leaflets of the false acacia and liquorice hang 

 down during the night, on each side of the mid- 

 rib, but do not meet beneath it. The leaves of 

 the sensitive plant {mimosa pudica) fold them- 

 selves up along the common foot-stalk, so as to 

 overlap one another. But, perhaps, this effect 

 is produced partly by the agency of moisture, as 

 it is accelerated by dews and rains, and may even 

 be occasioned by artificial watering. On referr- 

 ing to our account of the theories of Dutrochet 

 of endosmose and exosmose, already given, some 

 farther light on this subject may bo obtained. 



During the absence of light, too, it is liiglily 



