THE SEED, GERMINATION AND THE PLANTLET 19 



Seeds of tropical plants usually require higher tem- 

 peratures for germination than those of temperate 

 plants. 



31. Free oxygen essential to germination. — If we 

 place in the bottom of each of two saucers (if flower-pot 

 saucers are used, they should first be well soaked in water, 

 so that they will not extract water from the soil) a layer 

 of puddled clay or loam, put 25 viable beans on the soil 

 in each saucer, then fill one saucer with moist sand and 

 the other with puddled clay or loam, pressing the latter 

 down very closely around the beans, cover both saucers 



Fig. 4. — Germination in respect to water. In the left-hand saucer 

 beans were planted in puddled soil. In the other, they were covered 

 with sand. They failed to germinate in the puddled soil, because 

 their contact with oxygen was cut off. 



with a bell-jar, and place iri a warm room for two or 

 three days we shall find that the beans covered with 

 the sand will sprout promptly, while those covered with 

 the puddled soil will not (Fig. 4). (Soil is said to be 

 puddled when packed until it is of the consistency of 

 putty.) In the sand-covered saucer the air between the 

 grains of sand has had access to the beans, while in the 

 other air has been shut out, which explains the sprouting 

 of one lot of seeds and the failure of the other. About 

 one-fifth of the atmosphere is free oxygen, i.e., oxygen 

 that is not chemically combined with any other sub- 

 stance. 



