The Plantlet. 



37 



class, which includes many plants beside the beau and 

 pumpkin, are planted in soil, the cotyledons must be 



forced through the soil above 

 them, an act requiring consider- 

 able energy. If such seeds are 

 coA'ered with much soil, the 

 plantlet is often unable to lift its 

 cotyledons to the surface, and 

 hence must perish. Pig. 12 

 shows two bean plantlets that 

 tore off their cotyledons in the 

 vain attempt to lift them through 

 five inches of soil. The plantlet 

 of wheat, barley and oats, though 

 much smaller and weaker than 

 that of the bean, readily grows 

 through this depth of soil, be- 

 cause the tiny pointed shoot (plu- 

 mule (56)) of these plants 

 readily insinuates itself between the soil particles and 

 comes to the surface 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, be- 

 cause the plantlets are unable to lift their cotyledons 

 through a baked surface soil. 



Fig. 12. Showing two bean 

 plantlets that tore off their 

 cotyledons from being too 

 deeply planted. 



* Ricinus. 



