The Plantlet. 35 
those having but one cotyledon form a class known as Mo- 
nocotyledones (mo’-no-co-tyl-e’-dones). There is also a class, 
including the pine, fir and other conifers, that have several 
cotyledons. 
47. The Hypocotyl Develops Differently in Different 
Species. In the corn and pea (Figs. 9 and 10), the cotyle- 
dons remain in the soil, while in the bean and pumpkin, they 
have been lifted bodily into the air for a considerable dis- 
tance. This striking difference is due to the fact that in the 
corn and pea, the hypocotyl lengthens very little in germi- 
nation, while in the bean and pumpkin, it lengthens com- 
paratively very much. 
48. Seeds in which the Hypocotyl Lengthens in ger- 
mination Must Not be Deeply Planted. When seeds of 
this class, which includes many 
plants beside the bean and purp- 
kin, are planted in the soil, the 
. cotyledons must be forced through 
the soil above them, an act requir- 
ing considerable energy. If such 
seeds are covered with much soil, 
the plantlet is unable to lift its 
cotyledons to the surface, and 
hence it must perish. Fig. 11 
shows two bean plantlets that tore 
off their cotyledons in the vain 
attempt to lift them through five 
Pict. Bhawing’ re bean inches of soil. The plantlet of 
plantlets that tore off their coty- wheat, barley and oats, though 
pr aa being foo deeply auch smaller and weaker than 
that of the bean, readily grows through this depth of soil, 
because the hypocotyl] does not elongate in germination, and 
