se °t. li PHYSIOLOGY 217 



ihrer Anwendung auf Agricultur unci Physiologie), showing the applica- 

 tion of organic chemistry to agriculture and physiology, refused to 

 believe in the respiration of plants. Although the question had 

 already been thoroughly investigated by Saussure in 1822, and by 

 Dutrochet in 1837, and its essential features correctly interpreted, 

 Liebig pronounced the belief in the respiration of plants to be opposed 

 to all facts, on the ground that it was positively proved that plants 

 on the contrary decomposed carbonic acid and gave off the oxygen. 

 He asserted that it was an absurdity to suppose that both processes 

 were carried on at the same time ; and yet that is what occurs. 



Assimilation and respiration are two distinct vital pro- 

 cesses CARRIED ON INDEPENDENTLY BY PLANTS. WHILE IN THE 



process of assimilation green PLANTS alone, and only IN THE 



LIGHT, DECOMPOSE CARBONIC ACID AND GIVE OFF OXYGEN, all PLANT 

 ORGANS WITHOUT EXCEPTION BOTH BY DAY AND BY NIGHT TAKE UP 



OXYGEN AND GIVE OFF carbonic acid. Organic substance, obtained by 

 assimilation, is in turn lost by respiration. A seedling grown in the dark 

 so that assimilation is impossible, loses by respiration a considerable part 

 of its organic substance, and its dry weight is considerably diminished. 

 It has been found that during the germination of a grain of Indian Corn, 

 a full half of the organic reserve material is consumed in three weeks. 

 That green plants growing in the light accumulate a considerable 

 surplus of organic substance, is due to the fact that the daily production 

 of material by the assimilatory activity of the green portions is greater 

 than the constant loss which is caused by the respiration of all the 

 organs. Thus, according to Boussingault's estimates, in the course 

 of one hour's assimilation a plant of Sweet Bay will produce material 

 sufficient to cover thirty hours' respiration. 



The question may be asked, why then is respiration essential to 

 life 1 It cannot be that its importance for plants arises from the 

 loss of substance ; that would be but a mere waste of material 

 which had been previously elaborated by the plant. A means of 

 judging of the importance of respiration is afforded by the behaviour 

 of the plants themselves when deprived of oxygen. By placing them, 

 for example, under a jar containing either pure nitrogen or hydrogen, or 

 in one from which the air has been exhausted, it will then be found that 

 all vital activity soon comes to a stand-still ; plants, previously growing 

 vigorously, cease their growth ; the streaming motion of the protoplasm 

 in the cells is suspended, as well as all external movement of the 

 organs. Motile organs of plants become stiff and rigid and sink into 

 a death-like condition. If oxygen be admitted, after not too long an 

 interval, the interrupted performance of the vital function is again 

 renewed. A longer detention in an atmosphere devoid of oxygen will, 

 however, irrevocably destroy all traces of vitality ; as in every condi- 

 tion of rigor internal chemical changes take place, which, by a pro- 

 longed exclusion of oxygen, lead to the destruction and disorganisation 



