146 GERMINATION, GROWTH, TISSUE TENSION. 



suggest that germinating seeds cause some change in the air, that 

 they use the air up ? 



After three or four days carefully remove the cork from one of 

 the jars and lower a lighted taper or match into it : note what 

 happens. Open another of the jars, and dip into it a glass rod 

 which has been dipped into clear lime-water (or baryta- water) ; note 

 the white precipitate indicating the presence of carbon dioxide. 



These experiments show that germinating seeds respire they 

 absorb oxygen and release carbon dioxide, thus changing the com- 

 position of the air around them in the same way that animals do by 

 their breathing or respiration. 



188. Growth of Seedlings in Light and in Dark- 

 ness. Experiments on the respiration of germinating 

 seeds show that the seedling loses carbon, which is re- 

 leased in the form of carbon dioxide. To estimate this 

 loss we must dry the seeds and the seedlings before weigh- 

 ing them, since the water present must not be taken into 

 account. Does this loss in dry weight occur both in light 

 and in darkness ? 



(a) Take about forty Beans as nearly alike in size and 

 weight as possible; select four of them as samples, and 

 find their weight after thoroughly drying them on a water 

 or sand bath or in a slow oven. Take the dry weight of a 

 seed, found in this way, as the average. Sow half of the 

 seeds in sifted garden soil in a box which is kept in dark- 

 ness, the other half in a box kept in full light ; water both 

 lots about equally. 



At the end of each week measure and record the average 

 height of the shoot in each lot of seedlings ; remove three 

 seedlings from each box, wash the roots in running water 

 (do not leave any in the soil or lose them in any way), and 

 dry them thoroughly without charring any part. When 

 quite dry and brittle, weigh each lot and obtain the average 

 weight of the solid matter in each plant. G-et a piece of 

 squared paper, as in Fig. 36 (spaces representing inches 

 need not, of course, ~be inches) . As the weekly observations 

 proceed, trace two lines across the sheet, one (a continuous 

 line) to show the weight, the other (a dotted line) the 

 height of the seedlings grown in light; draw two other 

 lines in red ink to show the dry weight, and the height, of 

 the seedlings grown in darkness. 



