RESPIRATION 205 



of the organism come to a standstill — growth and the phenomena of movement 

 as well as the transport of food material (compare Lecture XIV, p. 169) from 

 cell to cell, the movements of protoplasm and of entire organs. We must not 

 forget, too, that oxygen is also an essential food-stuff of the plant, and hence 

 we have for the first time to deal with an element as a nutritive material, 

 whilst the nutrients hitherto spoken of were compounds. We are still far from 

 a complete understanding of the significance of respiration in the mainte- 

 nance of vital phenomena, but we can at least claim to possess an ap- 

 proximate conception of it by studying the energy relations of the process. 

 In burning wood or coal energy is liberated which, as every steam-engine 

 demonstrates, is capable of doing work. A transformation of the energy origin- 

 ally present in the material must take place ; it must be changed from the 

 potential into the kinetic condition. Similarly, in the physiological combustion 

 of starch or sugar in the plant-cell, kinetic energy is evolved, obviously essential 

 for carrying on the manifold activities of the organism. When organic materials 

 are broken down in the process of intra-molecular respiration energy is also 

 released, although no free oxygen be added, just as when in the breaking down 

 of certain chemical compounds a re-arrangement of their atoms only takes 

 place without the addition of any other element. In the higher plants the energy 

 arising from intra-molecular respiration is insufficient to carry on all the vital 

 phenomena ; in our next lecture, however, we shall get to know of organisms 

 where this is so. The heat of combustion of the respiratory materials gives 

 us an approximate idea of the amount of energy released in respiration. If 

 these materials be oxidized down to the final products, water and carbon-dioxide, 

 we thereby obtain bodies which have no heat of combustion, and the whole of 

 the energy is released by respiration. When, however, organic acids or alcohol 

 arise as primary products it is only the difference between the heat of com- 

 bustion of the materials produced, and the sum of the heats of combustion 

 of the final products that is available for carrying on the work of the plant. 

 In the production of heat which appears to be inseparable from respiration 

 (compare Lecture XXXI) we have the evidence required to show that the 

 chemical energy of the respiratory materials is transformed, still we must 

 remember that the heat so produced must obviously be reckoned as lost to 

 the plant. If the production of heat were the chief end of respiration then 

 respiration might be compensated for by heat introduced from without, and 

 we could reduce it by heating the plant ; this, however, is quite incorrect, for 

 with every increase in temperature respiration also increases. Under these 

 circumstances it is almost inconceivable that, as Rodewald thinks (1888), the 

 total energy of the respiratory materials in certain cases appears as heat. One 

 would not expect this to be generally true ; one would rather expect, that in 

 addition to the heat produced in physiological combustion, other forms of 

 energy would appear which might be of service in the plant economy. 



We cannot close this lecture without summarizing, if only in a sentence, 

 the history of our position in relation to this subject. The fact that in illumi- 

 nated green parts respiration is masked by assimilation makes the demonstration 

 of universal respiration extremely difficult. Although Saussure had a clear 

 perception of the fact that respiration was continuous in chlorophyll-bearing 

 regions exposed to light, we should not have been indebted to Sachs (1865) for 

 the first expression of the modern conception of the matter, if Liebig had not 

 explicitly denied respiration in plants. The service rendered by Sachs lay 

 essentially in correcting the phraseology previously in use, seeing that Garreau 

 (1857), had already shown the probability of the existence of respiration in all 

 green parts of plants ; before Sachs's time it was customary to speak of a ' day 

 and night respiration', but Sachs, by introducing the terms ' assimilation ' and 

 * respiration ' for these two antagonistic processes, did away with many un- 

 ending misconceptions. 



