RESPIRATION. 233 



form as reserve material, to be afterward transformed and assimilated 

 by the plant, or consumed by animals. In vegetables, as well as in 

 animals, a true respiration also takes place, marked in both instances 

 by the absorption of oxygen. The deoxidizing "process, by which 

 organic matter is produced, occurs only in green vegetables, under the 

 influence of solar light ; but the absorption of oxygen is a constant 

 phenomenon, taking place in both green and colorless plants, in dark- 

 ness as well as in the light. 



The active phenomena of vegetation, moreover, are dependent on 

 the absorption of oxygen, and cannot go on without it. When the 

 starch stored up in a seed becomes liquefied and converted into sugar, 

 and germination begins, the absorption of oxygen is necessary to its 

 continuance. This is the case not only in germinating seeds, but also 

 in expanding leaf and flower buds, all of which consume in a short 

 period several times their volume of oxygen. The processes of germi- 

 nation, growth, and flowering, as well as the intra-cellular movement 

 of the vegetable plasma, the motions of the sensitive-plant in response 

 to stimulus, and certain periodical movements of the leaves in other 

 species, all cease in an atmosphere deprived of oxygen.* The function 

 of respiration is accordingly essential to every form of vital activity. 



Organs of Respiration. 



Eespiration is very active in the mammalians and birds, less so in 

 reptiles and fishes ; and in different classes the organs by which it is 

 accomplished vary in size and FlG 



structure according to the activity 

 of the function. Its requisite con- 

 ditions are that the circulating 

 fluid be exposed in some way to 

 the influence of the atmosphere 

 or of an aerated fluid. The respi- 

 ratory apparatus consists essen- 

 tially of a moist and permeable 

 respiratory membrane, with blood- 



~els on one side and air or an HEAD A - VD GlLLS OF MEXOBRAXCHUS. 

 aerated fluid on the other. The blood and the air, consequently, do 

 not come in direct contact with each other, but absorption and exhala- 

 tion take place through the intervening membrane. 



In most aquatic animals, the respiratory organs have the form of 

 gills; that is, vascular prolongations of the integument or mucous 

 membrane, which are bathed in the surrounding water. In Meno- 

 branchus (Fig. 50) the gills are external feathery tufts on the sides of 

 the neck, connected, through lateral fissures, with the mucous mem- 

 brane of the pharynx. Each filament consists of a fold of mucous 



* Mayer, Lehrbuch der Agrikultur-Chemie. Heidelberg, 1871, Band i., pp. 91, 95. 

 Hoppe-Seyler, Physiologische Cheinie. Berlin, 1877, p. 171. 



