46 BIOLOGY FOR BEGINNERS 



by setting up experiments in which several seeds are given 

 similar treatment, except that one of these factors is changed in 

 each. 



To prove the necessity of air, place several seeds in each of two 

 bottles, give them moist moss to grow in, and keep in places of 

 similar temperature. Seal one tightly and leave the other open. 

 The results show that the sealed seeds, though they start growth, 

 cease as soon as the air in the bottle is used up, while those in the 

 open bottle grow naturally. In this, as in all experiments, several 

 seeds should be used, so as to prevent drawing a false conclusion 

 from incomplete evidence. Using many seeds and repeating the 

 same experiment increases the accuracy of the test. Emphasis 

 must also be placed upon giving the same conditions, with the one 

 exception, in every case. In the above experiment, if the seeds 

 are not kept in places of similar temperature and moisture, the 

 result of the experiment might be attributed to the differences in 

 these factors and not to the presence or absence of air. 



In the same way, it can be proved that seeds require a definite 

 amount of moisture for germination. If none be supplied, or if 

 they be completely covered with water, most seeds will not grow 

 even when the air supply and temperature are properly regulated. 



A similar experiment may be used to show the effect of tempera- 

 ture on seed growth. Arrange several seeds in each of three or 

 four bottles; give the same amount of moist moss to grow in, and 

 expose all to free air supply. The one condition to be varied is 

 the temperature. It will be found that those in extreme cold 

 usually do not start growth at all, those in very warm places usually 

 decay, and only those in a moderate temperature germinate 

 naturally. 



Suppose some of these last sets of seeds had been given vary- 

 ing amounts of moisture as well as different temperatures, what 

 ob'ection could be raised to the conclusion given? 



Experiments like those above in which no air or water or warmth 

 were supplied and in which no results occurred are sometimes 

 called " check " experiments. They are very important, as show- 

 ing that a certain result will not occur without certain conditions. 



