270 BOTANY PART i 



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. 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. If assimilation is sup- 

 pressed by keeping the plant in darkness, it loses considerably in 

 dry weight. 



Plants produce in twenty-four hours about five to ten times their own volume 

 of carbonic acid. In shade plants this is usually reduced to twice the plant's 

 volume, while the commonly-cultivated Aspidistra produces only one-half of its 

 own volume, and can therefore succeed even under conditions which are unfavour- 

 able to assimilation. 



In order to demonstrate the existence of respiration either the 

 absorption of oxygen or the giving off of carbon dioxide by the plant 

 may be employed. If a handful of soaked seeds is placed at the 

 bottom of a glass cylinder, the top of which is closed for a day by a 

 glass plate, the oxygen in the space is used up by the germinating seeds; 

 a candle will be extinguished if it is introduced into the cylinder. If 

 germinating seeds or flower- heads of Compositae (B, Fig. 254) or 

 young mushrooms are placed in a flask and prevented from falling 

 out when the flask is inverted by means of a plug of cotton-wool (W), 

 the mouth of the flask can be dipped under mercury (S) and some 

 solution of caustic potash (K) be introduced above this. The carbon 

 dioxide formed is then absorbed by the caustic potash and the 

 mercury rises (Fig. 254). When this experiment is carried out 

 quantitatively it is found that a fifth of the volume of air disappears, 

 so that all the oxygen has been absorbed. Since, however, when no 

 potash is present, the volume of gas is not altered by the respiration 

 of the plants, an equal volume of carbon dioxide must be formed for 

 each volume of oxygen that is absorbed. The respiratory coefficient 

 or ratio between the absorbed oxygen and the excreted carbon dioxide 



is equal to unity ( Q 2 = 1 \ If we assume that sugar is the substance 

 respired, this must take place according to the formula 

 6 H i26 + 60 2 = 6C0 2 + 6H 2 0. 



This is an exactly opposite process to the assimilation of carbon dioxide. 

 It is not so easy to demonstrate the formation of water in typical 

 respiration as it is to show the utilisation of oxygen and the pro- 

 duction of carbon dioxide. Quantitative estimates of the loss of dry 



