42 BIOLOGY FOR BEGINNERS 



water supply is essential, too much causing decay, and too little 

 precluding growth altogether. 



As to temperature, a maple seedling will germinate on a cake 

 of ice and many other seeds grow in extreme cold, while a smaller 

 number tolerate high temperatures. The majority, however, 

 germinate most freely between 60 and 80 F. 



Air from some source is essential to growth, for seeds, like all living 

 things, must breathe. Many can obtain the needed supply even 

 from the air dissolved in the water in which they maybe submerged. 



In addition to these external conditions, the embryo must also 

 have a supply of stored food for immediate use while the roots 

 and leaves are developing. This food may be stored in the coty- 

 ledons, as in the bean and pea, or outside the embryo, as in the 

 case of the endosperm of the corn and other grains. 



Stages in Germination. Germination consists of three steps, 

 emergence from the seed coats, penetration of the soil, and the 

 obtaining of first nourishment. 



In getting out of the seed coats, the hypocotyl appears first, 

 emerging by way of the micropyle. The rest of the embryo follows 

 by various ingenious schemes, all apparently planned by Nature 

 to enable the seedling to escape uninjured from the testa, on whose 

 protection it has so long depended. 



Penetration of the soil may be either from above or from below. 

 When seeds are scattered on the surface of the soil they are enabled 

 to gain a foothold in the earth by various contrivances so that the 

 roots may be sent down into the soil. In the case of buried (planted) 

 seed the process of penetration not only has to do with sending 

 down roots, but the seed must find a way out of the earth, un- 

 harmed by its passage. This latter problem is solved most often 

 by the plantlet being started from the seed in an arched position. 

 One end of the arched stem takes hold of the ground and sends 

 out roots, while the other, attached to the wide cotyledons or the 

 delicate plumule leaves, gently pulls these through the ground 

 after the growing arch has broken away to the surface. If forced 

 directly upward these bulky appendages would be stripped off 

 by soil pressure. 



