THE GROWTH OF THE PLANT 87 



C. Air as a Necessary Factor in Germination 



In the course of the experiments described in this 

 book, it has been seen that a plant takes in two con- 

 stituents of the air, namely, oxygen in the process of 

 breathing, and carbon-dioxide as a food. 



Of these two gases, the carbon-dioxide is of no use 

 to the germinating seed, since it contains no chlorophyll 

 by means of which the carbon-dioxide can be assimi- 

 lated ; this, however, is compensated for by the store of 

 food within the seed itself. 



On the other hand, it has been proved that germinat- 

 ing seeds do take in oxygen (Experiment 43) ; and, 

 further, that a seedling dies if the air which surrounds 

 it is deprived of oxygen (Experiment 48). 



It is thus seen that a germinating seed does not 

 require carhon-dioxide from the air, but it dies if de- 

 prived of oxygen. 



D. Light as a Necessary Factor in Germination 



Experiment 55 



Aim. — To find out whether light is necessary to the 

 growth of a seedling. 



Method. — Seeds of various kinds are planted in pots. 

 Some of the pots are then kept in a dark room or large 

 airy cupboard, the remainder are allowed to grow under 

 the normal condition of alternating light and darkness. 

 Temperature and other factors must be, as nearly as 

 possible, the same for both sets of pots. 



Observations. — The seedlings kept in the dark grow, 

 but the growth is abnormal. The stems become long, 

 thin, and are without strength, while the leaves remain 

 small and yellow. Fig. 41 is a photograph of two broad- 

 bean seedlings, one of which has been grown in the light 

 and the other in darkness, all other conditions havmg 

 been the same for both seedlings. 



