THE LIFE AND GROWTH OF SEEDLINGS 19 



the conditions of growth in plants are similar to those under 

 which animals and human beings thrive. 



(i.) Necessity of Air. It is a matter of common experience 

 >ven to a child that no human being, no animal, can live without 

 air. Is this true of plants ? By experiment, it can be shown 

 that air consists mainly of oxygen and nitrogen, the former 

 being about one-fifth of its volume, the latter four-fifths. To 

 exclude oxygen from seeds and to see the effect of doing so is 

 the object of the following experiments. 



1. Germinating seeds are grown on a sponge suspended in 

 a bottle with some pyrogallic acid, dissolved in a strong solution 

 of potash, at the bottom. The processes of germination are 

 stopped and the seeds die, because the oxygen has been absorbed 

 by the solution, and seeds cannot germinate without oxygen. 



2. If germinating seeds are put in a glass bottle, tightly corked 

 with an indiarubber bung, they will, in about twenty-four hours, or 

 even less, have absorbed all the oxygen. A lighted taper plunged 

 into the bottle at once goes out, owing to the absence of oxygen, 

 and supposing the seedlings were left an indefinite time instead 

 of twenty-four hours, they would die. 



These experiments clearly show that seeds require air, for 

 they must have oxygen. It is because germinating seeds need 

 oxygen, if they are to live, that care has to be taken in the pre- 

 paration of the soil of a seed-bed. It should be well separated 

 and well drained, in order that air may make its way easily 

 between the particles, so that the radicle as it grows into the 

 root may get the oxygen necessary to its life. 



The taking in of oxygen from the air is not the only chemical 

 process going on in germination. It is a matter of everyday 

 experience that the air of a room in which people are living 

 requires renewing, otherwise it gets stuffy and gives headache 

 to those who are obliged to remain in it. If some of the air in 

 a stuffy room is collected in a glass jar in which some lime-water 

 has been placed, the lime-water will turn milky, owing to the 

 presence of carbon dioxide. To show a class the source of the 

 carbon dioxide, it is merely necessary to breathe into a test- 

 tube containing lime-water, then pressing the thumb tightly 

 gainst the mouth of the test-tube to shake it well. The lime- 



