in their Structure and Functions. "^^ 1 19 



gous : the pith gives off its medullary rays, which are distri- 

 buted over every part of the plant, exactly in the same way 

 as the spinal cord gives off the nerves, and diffuses them 

 through the animal body. In the second place, phytologists 

 have discovered that the pith contains within its cells a num- 

 ber of globular bodies, resembling nervous ganglia. The 

 number of these nervous globules bears a close proportion 

 to the quantity of pith. Their size and number differ very 

 much in different plants, and in the same plants at different 

 stages of their growth. In the third place, the pith is most 

 abundant, and the globular corpuscles found in it are the 

 most numerous, in plants which are young and growing ; the 

 period when vegetable life is in a state of its greatest vigour. 

 So, in infancy, when the increase of the body is most rapid, 

 the nervous system is proportionally larger than in adult age. 

 The brain, at birth, forms the sixth part of the whole body ; 

 but in full-grown man it forms only the thirty-fifth part. 

 Some have supposed that the pith is essential to the produc- 

 tion of the fruit ; some, that it promotes the circulation of the 

 sap ; and others, that it supplies the leaves with moisture for 

 exhalation. These opinions, however, have not been esta- 

 blished by facts. Indeed, from the circumstances which 

 I have mentioned, there can be little doubt that the pith 

 performs functions very similar to, if not identical with, 

 those of the nervous system. It appears that the one, as 

 well as the other, is the source of vital action : for we find 

 them both presenting the same form, the same arrangement, 

 and the same distribution; and we observe them both ex- 

 hibiting the greatest magnitude when the functions of growth 

 and nutrition are the most actively exercised, and when the 

 vital power, upon which these functions depend, is required 

 to act with the greatest energy. The other proofs of the 

 existence of a nervous system in plants I have drawn from 

 the effects produced upon them by certain poisonous agents, 

 and also, from their capability of preserving a certain degree 

 of temperature under a great variety of circumstances. Most 

 persons are acquainted with the deleterious effects which 

 prussic acid, belladonna, nux vomica, and similar substances 

 are capable of exerting upon the animal frame. If a large 

 dose of prussic acid be administered to an animal, it produces 

 death in the course of a few minutes. If a less quantity be 

 given, it occasions loss of sensibility, and other alarming 

 symptoms. Results similar to these are observed to take 

 place in plants exposed to the influence of prussic acid. For 

 instance, if concentrated prussic acid be dropped upon a plant, 

 it speedily destroys its life ; but if the diluted acid be em-^ 



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