134 LECTURES ON 



air-tight to the mouth of the large globe, and, the apparatus being 

 disposed in a suitably lighted place, was left to itself until the plants 

 began to turn yellow and show signs of decay. Then they were re- 

 moved, separated from the soil, and, by chemical analysis, the amount 

 of nitrogen in them was ascertained. It was found in every instance 

 (the experiment being several times repeated) that the nitrogen in the 

 plants thus raised was no more than that contained in the seed from 

 which they had grown. Our ingenious countryman, Dr. Evan Pugh, 

 now president of the Farmers' College of Pennsylvania, while resi- 

 dent in England a few years since, made an elaborate investigation 

 of this subject, with results confirming those of Boussingault. 



So tar from the external free nitrogen being assimilated, it appears, 

 especially from the researches of Dr. Draper, of New York, that 

 plants constantly evolve this substance in the gaseous form; al- 

 though, according to the investigations of linger and Knop, made 

 more recently, and with more exact methods, the nitrogen found by 

 various observers in the exhaled air of plants comes only from the 

 atmospheric air absorbed by them. 



It thus appears that the two gases which, together, make up 

 ninety-nine per cent, or more of the atmosphere, do not constitute 

 in any way the direct food of vegetation. It is, in fact, in the small 

 quantity of other and somewhat variable ingredients that we must 

 look for the atmospheric nutriment of the vegetable kingdom. 



Water in the vaporized form we find never absent from the air, 

 and it is especially abundant in the warm period of the year when 

 vegetation is active. Its presence is made evident by its deposition 

 in the states of dew, fog, rain, and snow, when the temperature of 

 the atmosphere is reduced. 



It has been universally taught that the watery vapor which is thus 

 in perpetual contact with the leaves of plants is readily and largely 

 absorbed by them. According to Unger and Duchartre, however, it 

 is never imbibed by foliage in even the slightest degree. On the 

 contrary, under all circumstances there occurs a constant loss of water 

 by evaporation from the leaves, which does not wholly cease even 

 when they are confined in an atmosphere saturated with moisture. 

 Duchartre admits that liquid water in contact with the leaves is 

 slightly absorbed; but it would appear that the root is the organ of 

 absorption for water, and that the soil must perform the function of 

 supplying this indispensable body to the plant. 



It has long been known that water is absorbed by the roots in 

 large quantity, and exhaled through the leaves into the atmosphere. 

 The well-known trials of Hales prove this. He found, in one in- 

 stance, that a single cabbage exhaled 25 ounces of water in 24 hours. 

 We owe to Mr. Lawes, of Rothamstead, England, a series of experi- 

 ments on the transpiration of water through wheat, barley, beans, 

 peas, and clover, continued throughout nearly the whole period of the 

 growth of these plants. The result was, that for every grain of solid 

 matter added to the mass of the plant 150 to 270 grains of water 

 passed through it. Prom these, and especially from very recent in- 

 vestigations of Knop and Sachs, it is seen that the transpiration is 



