The Plantlet. 



37 



bean and pumpkin, are planted in soil, the cotyle- 

 dons 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 must perish. 

 Fig. 12 shows two bean plantlets that tore off their 

 cotyledons 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, because the 

 tiny pointed shoot (plumule 

 (55) of these plants readily in- 

 sinuates itself between the soil 

 particles and comes to the sur- 

 face with little expenditure 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 cotyle- 

 dons through one inch of soil, while those of the pea, 

 though much more slender, 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. 



Fig. 12. Showing two 

 bean plantlets that tore 

 offi their cotyledons from 

 being too deeply planted. 



* Eicinus. 



