OF VEGETABLE VITALITY. 



127 



life; just in tlie same way as the processes of 

 digestion, assimulation, and circulation are carried 

 on in the sleeping animal. 



Heat, then, is a most important stimulus in 

 the operations of the vegetable functions, accel- 

 erating the motion of the sap, and exciting all 

 the other actions; for the sap flows much more 

 copiously as the warmth of the season increases; 

 or as the artificial temperature of the hot-house 

 is raised. At the same time, it is ascertained 

 that excessive heat impedes the progress of vege- 

 tation as well as extreme cold, both being equally 

 prejudicial. Hence the sap flows more copiously 

 in the spring and autumn, than in either the 

 summer or winter months. This may readily 

 be proved by watching the progress of the growth 

 of the annual shoot, which, after having sprung 

 up rapidly in spring, remains for a while sta- 

 tionary during the great heat of summer, but is 

 again elongated during the more moderate tem- 

 perature of autumn. 



Other stimulants have been found to accelerate 

 the growth of vegetables when dissolved in water, 

 and applied to the roots or branches. Thus the 

 germination of peas is accelerated by moistening 

 them in water impregnated with oxymuriatic 

 acid gas, a fact first ascertained by Humboldt; 

 and the vegetation of the bulbs of the hyacinth 

 and narcissus is accelerated by. the application 

 of nitre in solution. Dr Barton of Philadel- 

 phia found that a decaying branch of Urioden- 

 dron tulipiferum, and a faded flower of the yel- 

 low iris, recovered, and continued long fresh, 

 when put into water impregnated with camphor, 

 though a flower and branches, in all respects 

 similar, did not recover when put into common 

 water. 



Plants have also the power of generating 

 heat, and in this respect show a wonderful coin- 

 cidence with the more perfect vital powers of 

 animals. The heat of plants is evinced by the 

 more speedy melting of snow when in contact 

 with their leaves or stems, compared with what 

 is lodged upon inorganic bodies, provided the 

 preceding frost has been sufficiently permanent 

 to cool those substances thoroughly. Mr Hunter 

 detected this heat by the rise of the thermometer, 

 applied in frosty weather, to the internal parts 

 of vegetables newly opened. And Lamarck 

 mentions an extraordinary degree of heat evolved 

 by the arum maculatum, or wake robin, about 

 the period when the sheath is about to open. 

 Most plants, at the period of inflorescence, ex- 

 hibit this internal heat in a greater degree than 

 at other times. 



Electricity. There can be no doubt but that 

 electricity is one of the stimuli of vegetable life, 

 although its mode of action is not yet ascertained. 

 Vegetation is seen to increase prodigiously dur- 

 ing electric changes of the atmosphere, and es- 

 pecially in that condition of it when the air is 



positively charged with the electric fluid. It 

 is probable, too, as we have explained when 

 treating of the theory of the ascent of the saji, 

 that electricity is concerned in this phenomenon, 

 and perhaps in the elaboration and secretion 

 of the various vegetable juices and products. 

 The influence of light on vegetables has al- 

 ready been explained; and when we call to mind 

 the recent discoveries by which it is shown' that, 

 in the sun's rays, there are some which exercise 

 a powerful chemical action on bodies, it may 

 not be without probability conjectured, that 

 these chemical rays have a considerable influ- 

 ence in imparting colour, and promoting the 

 various .chemical changes which take place in 

 vegetable bodies. 



Irritability. This property of plants corre- 

 sponds somewhat to the muscular imtability of 

 animals. Plants are not only susceptible to the 

 action of light and heat, but also to the contact 

 of external matter. It is true that this suscep- 

 tibility is not very apparent in the generality 

 of plants; but in a few, especially the mimosa, or 

 sensitive plant, a decided shrinking and folding 

 up of the leaves takes place when they are 

 touched by the finger, or any other body. The 

 twisting of tendrils round any other body ; the 

 bending of stems and branches ; and, above all, 

 the extension of roots, as influenced by moisture 

 or particular soils, are all, however, evident 

 proof of this irritability. 



Du Hamel made the following experiments, 

 with a view to ascertain the susceptibility of the 

 sensitive plant. At eight o'clock in the morn- 

 ing of a day in September, a leaf-stalk of a sen- 

 sitive plant, formed with the lower part of the 

 stem an angle of 135°, which, upon being touched, 

 fell to an angle of 80°: an hour afterwards it rose 

 again to 135°; and npon being touched a second 

 time, it fell again also to 80°. An hour and a 

 half afterwards it rose to 145°, and upon being 

 touched fell to 136°, where it remained till five 

 o'clock in the evening, when upon being touched 

 it fell to 110°. Hence it follows, that the sus- 

 ceptibility is greatest in the morning, or during 

 the heat of the day ; but the leaf recovers itself 

 sooner or later, according to the vigour of the 

 plant, the season of the year, and temperature 

 of the atmosphere, as well as the hour of the 

 day at which the experiment is made, though 

 it does not always recover itself in the same way ; 

 for sometimes the common foot-stalk recovers 

 first, sometimes the lateral foot-stalk, and some- 

 times the leaflets themselves. 



The leaves of dicmcea muscipula, or Venus 

 fly-trap, are also extremely susceptible to the 

 action of accidental stimuli. They are all 

 radical, and approaching to battledore-shaped, 

 with a sort of circulai- process at the apex, 

 which is bisected by a tendril, and ciliated with 

 fine hairs like an eyelash. This circular process is 



