230 STRUCTURAL BOTANY 



assimilation ceases, and then it is easy to prove that 

 carbon dioxide is given off. The same is the case, 

 whether in light or darkness, with any organs which 

 are not green, such as flower-buds or seeds just begin- 

 ning to germinate. In order to prove that carbon 

 dioxide is evolved, a number of seeds are allowed to 

 germinate under a bell-glass, which is fitted with 

 tubes through which a current of air can be drawn. 

 The air which enters the apparatus is deprived of all 

 its carbon dioxide by passing through a solution of 

 caustic potash. The same air, after it has passed over 

 the seeds, is drawn through lime water, in which it 

 throws down a precipitate, owing to the formation of 

 calcium carbonate (CaC0 3 ). By drying and weighing 

 the precipitate, we can tell how much carbon dioxide has 

 been formed by the germinating seeds in a given time. 



As a rule, subject, however, to many exceptions, 

 the volume of carbon dioxide given off in respiration 

 is about equal to that of the oxygen taken in. 

 Organs which are altogether deprived of oxygen can, 

 however, go on breathing for some time. This is 

 called internal, as distinguished from normal, respiration. 

 It is probable that in both kinds of respiration the 

 oxygen as well as the carbon is derived from the proto- 

 plasm itself. In normal respiration, however, the 

 loss of oxygen is made good by fresh supplies from 

 the atmosphere. In internal respiration, where this 

 is not the case, the process can only go on for a 

 limited time. 



It cannot be too strongly insisted upon that all 

 parts of plants breathe at all times so long as they 



