510 ORGANOLOGY. 



off, or is evaporated into the atmosphere, whilst another part sinks into 

 the earth and feeds the springs. There are but few observations upon the 

 quantity of water needed by plants, but the facts supplied by Hales and 

 Schiibler show that rain, after making allowances for that which flows 

 away and is evaporated, does not supply more than a tenth part of what 

 is necessary. It is unaccountable and inexcusable, that not a single 

 botanist since the time of Hales should have taken up and carried on his 

 experiments. If we take the previous calculations (page 501.) of the 

 quantity of water required by plants in England, which is deduced from 

 Hales' experiments, and which agree with those of Schiibler on Poa 

 annua, we shall obtain the following approximative results. According 

 to Schiibler *, there falls in England, upon the acre of 40,000 D F., at 

 the utmost 1,600,000 Ibs. of water during 120 days of summer. According 

 to the researches of Dalton, Miiller, Berghaus f , and Dausse J, at least a 

 third part of this water flows away into the rivers, but it is probably 

 more than this, as the great rapidity of the flow of the water in steep, 

 hilly, and mountainous regions is not sufficiently taken into consideration. 

 A considerable, but not accurately to be estimated quantity of water 

 evaporates immediately after the fall of rain, as the vaporous state of the 

 atmosphere indicates. From this it would appear that at the most there 

 is left disposable for plants and future evaporation 800,000 Ibs. of water on 

 the acre. Now this quantity of water, according to the preceding expe- 

 riments, would cover not more than two-thirds of the demand of an acre 

 of Cabbages, half of the demand of an acre of Sunflowers or of the 

 Jerusalem Artichoke (Helianthus tuberosus) ; the fourth of a fruit- 

 garden, the fifth of a hop-garden, and about the seventh or eighth of a 

 meadow. It must be recollected, that here only the water is taken into 

 calculation which is given out from the plants and weeds growing in a 

 meadow, but that which is afforded by the evaporation of the soil itself 

 cannot amount to less than 2,000,000 of Ibs. for an acre. Thus much is 

 very evident from these calculations, that the quantity of rain that falls 

 upon the earth is no more a measure of the quantity needed or consumed 

 by the plant, than is the quantity of humus an index to the fertility of the 

 soil. We may learn from this that the quantity of rain which falls in a 

 given region is not a measure of its fruitfulness, but the quantity of 

 moisture, the absolute and relative quantity of vapour, which yearly, and 

 especially during those months which are most important for vegetation, is 

 contained in the atmosphere. 



Thus much then is certain, that the soil, in order that it may nourish 

 plants, must absorb a large quantity of water from the atmosphere, and 

 must possess the necessary properties for that purpose. This property 

 is only possessed to a great extent amongst the original constituents of 

 the soil by clay, so that a soil free from clay is unfruitful. But the 

 primitive vegetation of the earth enriched the soil, by its death, with a 

 substance (humus), which also possesses this property, and which in 

 proportion to its abundance produces a luxurious vegetation without 

 affording from its own substance any part of the nourishment of the 



* Meteorologie, p. 130. 



f Berghaus Lander und Vblkerkunde, vol. ii. pp. 24, 227. 



| Studer Lehrb. d. physikal. Geog. p. 85. 



The calculations of Berghaus, for the flow of atmospheric water into the Rhine, 

 gives a result of three-fourths ; those of Studer for the same river, four-fifths. But the 

 calculations of Berghaus for the Weser show a larger quantity of water carried away 

 than falls from the atmosphere. 



