FLUIDS AND CIRCULATION. 17 



The. fluids or juices moving in the vessels of plants contain the 

 nourishment and constitute the essential means by which food is 

 assimilated with their solid substances. A correspondence is thus 

 observable between their functions and the circulation of the blood 

 and other physiological phenomena of animals. They possess powers 

 of motion, irritability, and of reproduction ; they breathe, sleep, digest, 

 and perspire. Their peculiar individual character is preserved by 

 their vital functions which constitute their life ; and when they cease, 

 their bodies are exposed to the chemical processes which act alike on 

 all inorganic substances, and they die. 



" See dying vegetables life sustain ; 

 See life dissolving, vegetate again." 



The circulation, or motion of the juices of plants, is thought to be 

 mechanical, the result of their irritability, the vessels acting as capil- 

 liary tubes, etc. This irritability is destroyed by shocks of electricity, 

 as with animals. Heat and light greatly increase this circulation, as 

 in the spring of the year, while cold as readily checks or suspends it, 

 as in autumn and winter. Long continued heat and rapidity of circu- 

 lation, as in summer, exhausts their powers or irritability, till in au- 

 tumn it is slow and their fluids are thick, as in animal life, both in 

 regard to season and old age. Their repose, too, after the activity of 

 the day, and their revival on the appearance of light, are not less 

 remarkable than with man, or lower animals, under like circum- 

 stances. 



The breathing of plants is their absorption and exhalations, phy- 

 siological facts as notable as any other in the vegetable or animal 

 economy. This is performed by, and is especially observable in the 

 leaves. A plant growing under ice constantly emits bubbles of pure 

 oxygen, which rise to escape. Placed also in a tumbler of water, 

 exposed to the sun, it is soon seen to be covered with air-bubbles, 

 which rise to the surface and burst. The inspiration of carbonic acid 

 through the leaves of plants is as constant, and in quantity still more 

 abundant. By this they live and furnish their organs with nourish- 

 ment ; and, by their expirations during the day they afford the vital 

 gaseous principle, oxygen, which is as necessary to the life of man and 

 the animal world as to that of plants withdrawing at the same time, 

 carbonic acid, which is most hurtful to animal life. Besides gases, 

 they also exhale liquids which, in a common sized tree, amounts to 30 

 pounds a day. 



Their odor, thus exhaled, consists of volatile oils which, in 

 quantity is proportionate to their volatility, their nature, light, 

 heat, etc. Their taste depends on like circumstances, the chemi- 

 cal character of their constituents and the nature of the soil. 

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