RESPIRATION. 495 



4677 units of heat. The mechanical equivalent of this is 1,987,725 gramme- 

 millimetres. When a carbohydrate is respired, for every cubic centimetre of 

 carbon dioxide exhaled a store of energy is obtained which is equal to 1,987,725 

 gramnie-niillimetres, and therefore by it a gramme weight can be raised to a 

 height of 1987 metres. It has, however, been ascertained that seedlings of poppy 

 (which, when subsequently dried, weighed 0-45 gramme) exhaled 55 cubic centi- 

 metres of carbon dioxide in 24 hours, and seedlings of mustard (which, when 

 dried later, weighed 0'55 gramme) 32 cubic centimetres in the same length of time. 

 It can thei-efore be easily imagined what a large store of energy is obtained by 

 respiration, even although the result, in consequence of various interruptions and 

 obstructions, should fall far behind this estimate. 



In comparing the living plant to a machine heated by coal, and trying to 

 measure the work performed by it numerically, we are justified by the analogy 

 of the transactions in the two cases, which are obvious. The comparison suggests 

 itself naturally from the fact that in both cases similar impelling forces come into 

 play, and that in both the necessary store of vital force is obtained by the com- 

 bustion of carbon. Yet, on the other hand, combustion in a machine and respira- 

 tion in a living plant are widely different. The peculiarity of plant respiration 

 lies in the fact that materials are combined with the oxygen of the atmospheric 

 air which would not enter into combustion with it at ordinary temperatures 

 outside the living plant. Neither carbohydrates, fats, nor albumins, which are 

 either directly or indirectly affected in respiration by the process of combustion, 

 undergo, outside the plant cell, the alterations and decompositions which are 

 carried on within it, and it may be taken as an established fact that oxygen only 

 operates on them when conveyed to them by means of the living protoplasm. 

 The effect of the transmitted oxygen is also restricted by the living protoplasm 

 to the carbohydrates and other non-nitrogenous compounds which it incloses. 

 Nitrogenous compounds are not respii-ed directly, and the quantity of nitrogen in 

 breathing plants is not lessened. We can only imagine these remarkable correla- 

 tions as occurring in the following manner. The starch grains and droplets of oil 

 are first rendered soluble, and are then oxygenated by the oxygen brought by the 

 protoplasm; the albumins, on the other hand, are first split up into asparagin and 

 a carbohydrate. The latter alone becomes oxidized, for the nitrogenous asparagin 

 is not only not burnt, but is reconstructed into albumin, with the co-operation of 

 the sun's rays, by attracting the newly-formed carbohydrates of the green cells 

 and combining with them. 



If we adhere to this view, it at once becomes evident how important is the 

 co-operation of respiration and the formation of fresh carbohydrates in the green 

 cells. If, in a plant, the production of new carbohydi-ates should be suspended, the 

 reconstruction of albumins cannot ensue. At first all the respirable materials 

 which yet remain in the plant are used up for the continuance of action, but if 

 the formation of fresh carbohydrates remains unaccomplished, and even the last 

 reserves are consumed, then the plant becomes exhausted, and ceases to breathe 



