644 CHEMICAL PROCESSES IN THE PLANT. 



amount of work done, as is also the case with the absorbing roots of Cuscuta, 

 Orobanche, &c. 



Sect. 6. — The Respiration of Plants ^ consists, as in animals, in the 

 continual absorption of atmospheric oxygen into the tissues, where it causes 

 oxidation of the assimilated substances and other chemical changes resulting 

 from this. The formation and exhalation of carbon dioxide — the carbon 

 resulting from the decomposition of organic compounds— may always be directly 

 observed ; the production of water at the expense of the organic substance in 

 consequence of the process of respiration is inferred from a comparison of the 

 analysis of germinating seeds with the composition of those which have not 

 yet germinated. Experiments on vegetation show that growth and the metastasis 

 in the tissues necessarily connected with it only take place so long as oxygen 

 can penetrate from without into the plant. In an atmosphere devoid of oxygen 

 no growth takes place ; and if the plant remains for any time in such an atmo- 

 sphere it finally perishes. The more energetic the growth and the chemical 

 changes in the tissues, the larger is the quantity of oxygen absorbed and of carbon 

 dioxide exhaled ; hence it is especially in quickly germinating seeds and in un- 

 folding leaf- and flower-buds that energetic respiration has been observed; such 

 organs consume in a short time many times their own volume of oxygen in the 

 production of carbon dioxide. But in all the other organs also — in every indi- 

 vidual cell — respiration is constantly going on ; and it is not merely the chemical 

 changes connected with growth that are dependent on the presence of free 

 oxygen in the tissues ; the movements of the protoplasm also cease if the sur- 

 rounding air is deprived of this gas ; and the power of motion possessed by 

 periodically motile and irritable organs is lost if oxygen is withheld from them ; 

 but if this happens only for a short time the motility returns when the oxygen is 

 again restored. 



The respiration of plants is, like that of animals, associated with a loss of 

 assimilated substance, this loss being always a great deal smaller in assimilating 

 plants than the gain of substance by the activity under the influence of light of 

 the cells which contain chlorophyll ; but when, as in the germination of seeds, an 

 energetic growth is combined with powerful respiration, no new products of as- 

 similation replacing the loss, the loss in weight of the growing plant may be very 

 considerable. Seeds which germinate in the dark may in this way lose almost 

 one-half of their weight when dry, and it would seem that this loss is occasioned 

 exclusively by the decomposition of the non-nitrogenous reserve-material and its 

 combustion into carbon dioxide and water. If the rest of the non-nitrogenous 

 reserve-material consists of oil, 2*. e. of a substance containing very little oxygen, a 

 portion of the inhaled oxygen remains in the germinating plant, carbo-hydrates 

 containing a large quantity of oxygen such as starch and sugar being formed at the 

 expense of the oil. 



* The special references for what is said on this subject will be found in my work on Expe- 

 rimental Physiology, sect. 9, On the action of atmospheric oxygen. Of more recent works may 

 be mentioned especially, Borscow, On the behaviour of plants in nitrogen (Melanges biologiques 

 tires du Bulletin de I'Acad. Imp. des Sci. Nat. de St. Petersbourg, vol. VI, 1867); also Wiesner 

 Sitzungsber. der Wiener Akad. vol. LXVIII, 1871. 



