114 BOTANY OF THE LIVING PLANT 



a closed space from which oxygen may be drawn. A strong solution 

 of caustic potash is passed into the tube over the mercury. As time 

 goes on the level of the mercury will rise, until the volume of the gas 

 has been reduced by about one fifth. The oxygen of the air has been 

 absorbed : in its place carbon dioxide has been given off, and as this is 

 absorbed by caustic potash the volume of the enclosed gas has been 

 diminished. The amount of the diniinution gives a measure of the 

 volume of oxygen absorbed. 



This gaseous interchange is essentially a process of combustion, 

 comparable with the consumption of fuel. A rise of temperature 

 follows, and this may be readily shown in the case of germinating 

 seeds, or living flowers. Two thermos flasks are to be prepared, 

 the one filled with living and germinating seeds, the other with 

 seeds which have been killed by boiling, or by poison. The tempera- 

 tures of the two masses of seeds are to be compared by a pair 

 of similar thermometers, the bulbs of which are plunged into the 

 middle of them. After a period of time allowed for the temperatures 

 to settle, the thermometer surrounded by the living seeds will register 

 a temperature several degrees higher than the other. The difference 

 of temperature is due to the respiration of the living seeds, while the 

 dead seeds do not respire. A similar demonstration may be given 

 with flower-buds: even simple thermometric readings will suffice in the 

 case of such a crowded mass of flowers as is found within the spathe 

 of a large Aroid. Here we know that respiration is active, and it is 

 now shown that it resembles combustion in the evolution of heat. 

 It has also been seen in Aroids that accumulations of starch pre- 

 viously present disappear during the process. This represents the 

 fuel that has been consumed. 



The ratio of the volume of oxygen absorbed to that of the carbon dioxide 

 given off in Respiration is not constant. In the case of ordinary green 

 shoots the volumes are approximately equal. This may be expressed as 



^■u ^- , f I CO, given off t n ^ ■■_ ■ ^ i 



the respiratory quotient = ^^-— f- — , — ^ = I- But it is not unusual 



O2 absorbed 



in seeds, and especially in oily seeds, that a larger proportion of oxygen 



shall be absorbed during germination than of carbon dioxide given off. 



This is explained by the extra oxygen being used m the conversion 



of the highly carbonaceous oils into carbohydrate. Conversely, 



the volume of carbon dioxide given off may sometimes exceed that 



of the oxygen absorbed, and in extreme cases carbon dioxide may be 



eliminated without any oxygen being absorbed. This can only be a 



consequence of the breaking down of complex molecules, such as 



