162 A TEXTBOOK OF BOTANY [Ch. IV, 8 



mode of growth for roots is fully explained by facts in their 

 physiology and structure, as will later be noted. 



8. The Respiration of Plants 



This process has received frequent mention in the foregoing 

 pages, and in ways which imply much importance. Since 

 its phenomena are more clearly manifest in connection with 

 growth than elsewhere, it may best be considered at this 

 place. Respiration and photosynthesis are without question 

 the two most important of all physiological processes. 



A demonstration of typical plant respiration is the fol- 

 lowing. In the chamber of a respiroscope (a " respiration 

 demonstrator"), like that pictured in Figure 115, there is 

 placed a handful of germinating seeds, and the instrument 

 is stood in a warm, darkened place for twenty-four hours ; 

 then water is poured down the thistle tube, and the air of 

 the chamber thus forced out goes bubbling up through 

 limewater placed in the cylinder. The limewater speedily 

 turns milky, thus proving by this familiar test that carbon 

 dioxide was present abundantly in the chamber. If the 

 student should ask the natural question whether the carbon 

 dioxide known to be present in air would not account for 

 this result, the answer is given by the other or " control" 

 chamber, the air of which, sent through similar limewater, 

 leaves it quite clear. In fact the quantity of carbon dioxide 

 present in so small a volume of air is insufficient to show any 

 effect by this method. It is therefore evident that carbon 

 dioxide has been produced by the germinating seeds. If, 

 further, a sample of the gas is withdrawn from the chamber 

 containing the seeds and chemically analyzed, as can be 

 done very easily, these additional facts appear. First, some 

 of the oxygen originally present in the air has disappeared, 

 and various evidence shows it has been absorbed by the 

 seeds. In starchy seeds, like Oats, the oxygen absorbed and 

 the carbon dioxide produced are approximately equal in 

 volume, though often this respiratory ratio is different. 



