190 SCIENCE OF GARDENING. PART II. 



855. Irritability. Plants are not only susceptible of the action of the natural stimuli 

 of light and heat, exciting them gradually to the exercise of the functions of their dif- 

 ferent organs in the regular progress of vegetation ; they are susceptible also of the action 

 of a variety of accidental or artificial stimuli, from the application of which they are 

 found to give indications of being endowed also with a property similar to what we 

 call irritability in the animal system. This property is well exempb'fied in the genus 

 Mimosa ; but particularly in that species known by the name of the Sensitive Plant ; 

 and the dionsea muscipula and drosera. But sometimes the irritability resides in 

 the flower, and has its seat either in the stamens or style. The former case is ex- 

 emplified in the flower of the berberry and cactus tuna, and the latter in stylidium 

 glandulosum. 



856. Sensation. From the facts adduced in the preceding sections, it is evident that 

 plants are endowed with a capacity of being acted upon by the application of stimuli, 

 whether natural or artificial, indicating the existence of a vital principle, and forming 

 one of the most prominent features of its character. But besides this obvious and ac- 

 knowledged property, it has been thought by some phytologists that plants are endowed 

 also with a species of sensation. Sir J. E. Smith seems rather to hope that the doctrine 

 may be true, than to think it so. 



857. Instinct. There is also a variety of phenomena exhibited throughout the extent 

 of the vegetable kingdom, some of which are common to plants in general, and some 

 peculiar to certain species, that have been thought by several botanical writers to exhibit 

 indications, not merely of sensation, but of instinct. The tendency of plants to incline 

 their stem and to turn the upper surface of the leaves to the light, the direction which-, 

 the extreme fibres of the root will often take to reach the best nourishment, the folding 

 up of the flower on the approach of rain, the rising and falling of the water-lily, and 

 the peculiar and invariable direction assumed by the twining stem in ascending its prop, 

 are among the phenomena that have been attributed to instinct. Keith has endeavoured 

 (Lin. Trans, xi. p. 11.) to establish the doctrine of the existence and agency of an in- 

 stinctive principle in the plant, upon the ground of the direction invariably assumed by 

 the radicle and plumelet respectively, in the germination of the seed. 



858. Definition of the plant. But if vegetables are living beings endowed with 

 sensation and instinct, or any thing approaching to it, so as to give them a resemblance 

 to animals, how are we certainly to distinguish the plant from the animal ? At the ex- 

 tremes of the two kingdoms the distinction is easy ; the more perfect animals can never 

 be mistaken for plants, nor the more perfect plants for animals, but at the mean, where 

 the two kingdoms may be supposed to unite, the shades of discrimination are so very faint 

 or evanescent that of some individual productions it is almost impossible to say to which 

 of the kingdoms they belong. Hence it is that substances which have at one time been 

 classed among plants, have at another time been classed among animals ; and there are 

 substances to be met with whose place has not yet been satisfactorily determined. Of 

 these I may exemplify the genus Corallina (Jig. 66.), which Linnaeus placed among 



66 



animals, but which Gaertner places among plants. Linnaeus, Bonnet, Hedwig, and 

 Mirbel, have each given particular definitions. According to Keith, a vegetable is an 

 organised and living substance springing from a seed or gem, which it again produces ; 

 and effecting the developement of its parts by means of the intro-susception and assimila- 

 tion of unorganised substances, which it derives from the atmosphere or the soil in which 

 it grows. The definition of the animal is the counterpart : an animal is an organised 

 and living being proceeding from an egg or embryo, which it again produces ; and ef- 

 fecting the developement of its parts by means of the intro-susception of organised sub- 

 stances or their products. For all practical purposes, perhaps plants may be distinguished 



