The seed, germination and the plAntlet 29 



cotyledons must be forced through the soil above them, 

 an act requiring considerable energy. If such seeds are 

 covered with much soil, the plantlet is often unable to lift 

 its cotyledons to the surface, and hence nuist perish. 

 Fig. 12 shows two bean plantlets that tore off their coty- 

 ledons in the vain attempt to lift them through five inches 

 of soil. The plantlets of wheat, 

 barley and oats, though much 

 smaller and weaker than that 

 of the bean, readily grow 

 through this depth of soil, be- 

 cause the tiny pointed shoot or 

 plumule (55) of these plants 

 readily insinuates itself between 

 the soil particles and comes to 

 the surface with little expendi- 

 ture of energy, even when deeply 

 planted. Plantlets of the larger 

 beans usually fail if the seeds 

 are planted three inches deep 

 in a clay soil that bakes above 

 them. Those of the castor bean, 

 though very robust, can hardly 

 lift their cotyledons through 

 one inch of soil, while those of 

 the pea, though much more slen- 

 der, readily grow through four to six inches. Apple seeds 

 planted in autumn on clay soil, usually fail to germinate 

 the following spring unless covered with sand or humus, 

 or carefully mulched, because the plantlets are unable 

 to lift their cotyledons through a baked surface soil. 



48. The vigor of the plantlet is generally in proportion 

 to the size of the seed. This is true not only between 



Fig. 12. — Loss of cotyle- 

 dons. Showing two bean 

 plantlets that tore off their 

 cotyledons from being too 

 deeply planted. 



