2.Again: physiological causes, though in a less degree than the physical, are still very largely influential in causing this watery circulation. Thus the dew and rain which fall sink in part into the soil, and are thence drunk in by the roots of growing plants. But these plants spread out their green leaves into the dry air, and from numberless pores are continually exhaling watery vapour in an invisible form. From the leafy surface of a single acre in crop, it is calculated that from three to five millions of pounds of water are yearly exhaled in the form of vapour, in Britain; while, on an average, not more than two and a half millions fall in rain. Whether the surplus thus given off be derived from dews or from springs, it is plain that this evaporation from the leaves of plants is one of the most important forms which the circulation of water assumes.
Animals take into their stomachs another portion of the same water, and, as a necessary function of life, are continually returning it into the air from their lungs and their insensibly reeking hides. About two pounds a day are thus discharged into the air by a full-grown man; and larger animals probably giveoff more, in proportion to their size. Multiply this quantity by the number of animals which occupy the land surface of the globe, and the sum will show that this also is a form of watery circulation which, though less in absolute amount than the others I have mentioned, is yet of much importance in the economy of nature.
3.But water circulates also, in consequence of unceasing chemical operations, in a way which, if less obvious to the un instructed, is, if possible, more beautiful and more interesting than the mere physical methods above described. The main substance of plants-their woody fibre-consists in large proportion of water. The same is true of the starch and sugar which are consumed by an animal. When the plant dies and decomposes in the air, the water is again set free from its woody stem. When the animal digests the starch and sugar, the water which these contain is discharged from its lungs and skin. Thus the living plant works up water into its growing substance, which water the decaying plant and the breathing animal a gain set free; and thus a chemical circulation continually goes on, by which the same water is caused again and again to revolve. Within a single hour it may be in the form of starch in my hand, be discharged as watery vapourfrom my lungs, and be again absorbed by the thirsty leaf to add to the substance of a new plant!
- JAMES F. W. JOHNSTON
NOTES
① Physiological, that is, pertaining to the functions of organic bodies, both vegetable and animal. Thus evaporation and condensation in the air constitute a physical cause, inhalation and exhalation by plants and animals form a physiological cause, decomposition of matter forms a chemical cause, of the circulation of water.
QUESTIONS
What proves the frequency with which the rise and fall of watery vapour must take place? To what three causes is the ascent of water into the atmosphere due? Give an example of the circulation of water due to purely physical causes. Show how physiological causes operate. Give an example of the chemical operations which cause water to circulate.