ABSORPTION OF FOOD, AND EXCRETION. 503 



5. Succulent fruits also, in the period after ripening, absorb oxy- 

 gen gas, and give out carbonic acid gas. 



6. Almost all the parts of plants absorb, under some circum- 

 stances, nitrogen from the atmosphere, and exhale it again. 



7. Hydrogen also is exhaled by plants, as we know from Hum- 

 boldt's experiments on fungi. 



The experiments which have been made to ascertain the relation of 

 plants to the atmosphere, we owe chiefly to Hales*, Bonnet f, Priestley J, 

 Ingenhousz , Sennebier ||, Woodhouse ^[, Th. de Saussure**, Link if, 

 and Grischow. J f They may be arranged in three groups, except the 

 old experiments on some water plants, which require to be repeated with 

 extreme accuracy. The first group embraces the experiments in which 

 leaves or stalks, cut from the plant, have been observed. These are 

 worthless ; for it has never been ascertained how much air, and what 

 other matters these parts contained within themselves, nor what changes 

 were being carried forward in the interior, nor how long the experi- 

 ments were continued, &c. 



The second group of experiments contains those in which entire 

 plants, vegetating in water or soil, were enclosed in a receiver. These 

 experiments also, the greater part of which were performed by De Saus- 

 sure, are quite useless, since we have no data whence we may learn how 

 much of the changes produced in the atmosphere were wrought by the 

 leaves, and how much through the soil and roots. The third group is 

 that alone which can yield useful results. In these experiments the 

 green parts of plants were enclosed in a receiver, without the plant 

 being withdrawn from its natural place, and the air in the receiver was 

 that of the atmosphere. To this group belong the experiments of Wood- 

 house, Saussure, Link, and Grischow. 



These last experiments gave the rare result, that the plants, after long 

 vegetation in confined air, neither changed the quality nor quantity of 

 the air by their green parts. Hence there must have existed some fault 

 in connection with them, since the result was an impossibility. The 

 principal mass of the matter in the plants contained less oxygen than the 

 mixture afforded by the soil could have enabled them to take up. In 

 whatever way vegetation goes on, the final result must be the disengage- 

 ment of oxygen gas, which, when exceeding in quantity the measure which 

 can be held in solution by the fluids of the plant, must escape. The 

 most recent experiments of Boussingault go to prove, what was before 

 known, that plants absorb carbonic acid through their green parts in 



* Vegetable Statics. 



f Rech. sur 1' Usage des Feuilles dans les PI. (1754), p. 24, et seq. 



j Experiments and Observations relating to various Branches of Natural Philosophy, 

 with a Continuation of the Observations on Air (1779), vol. i. p. 1, et seq. 



Versuche mit Pflanzen, wodurch entdeckt ward u. s. w. A. d. Engl. 1780; und: 

 Ueber die Ernahrung der Pflanzen u. s. w. A. d. Engl. von G. Fischer, 1798, p. 53 ff. 

 Experiments upon Plants, and an Essay upon the Food of Plants and the Renovation of 

 Soils. London, 1796. 



|| Physiologic vegetale (1801), vol. iii. pp. 104 148. 



If Gilbert's Annalen, 1803, vol. xiv. p. 351. 



** Chemische Untersuchungen liber die Vegetation; ubersetzt von Voigt, 1805. 



ft Grundlehren der Anatomic und Physiologic. Gbttingen, 1807. P. 283. 



|i Physikalisch-chemische Untersuchungen iiber die Athmungen der Gewachse 

 und deren Einfluss auf die gemeine Luft (1819). 



K K 4 



