SEEDLINGS OF THE COMMON BEAN TYPE 



107 



Onion Seedling. The seedling of the Onion represents 

 another type of monocotyledonous seedlings. In this type the 

 hypocotyl elongates and pushes the cotyledon above ground. 

 (Fig. 99.) As in the Grass seedlings, the primary root system 

 is temporary a feature quite common in Monocotyledons, al- 

 though in some it lives much longer than in others. 



Seedlings of the Common Bean Type. The seedling of the 

 Common Bean is representative of those dicotyledonous seedlings 

 in which the cotyledons through the elongation of the hypocotyl 

 are carried above ground, sometimes several inches or even a 

 foot in some Beans. Squashes, 

 Cucumbers, Pumpkins, Melons, 

 Radishes, Turnips, Castor Bean, 

 Maples, Ashes, Clover, Alfalfa, 

 etc., besides many of the Beans 

 have this type of seedling. In 

 seedlings of this type the first root 

 system is usually the permanent 

 one and soon firmly anchors the 

 hypocotyl which then by an arch- 

 ing movement pulls the cotyle- 

 dons out of the ground in such a 

 way that they offer the least re- 

 sistance in passing through the 

 soil and afford the most protec- 

 tion for the delicate plumule. 

 (Fig. 100.) In some cases, as in 

 the Melons and Pumpkins, the 

 hypocotyl also assists in casting 

 off the seed coat, in which case the arch of the hypocotyl pulls 

 the cotyledons out of the seed coat while the latter structure is 

 held in place by a peg-like structure of the hypocotyl. (Fig. 101.) 

 In most cases, however, the seed coat is torn and gradually 

 pushed off by the growth of the seedling. Since the first root 

 system is usually the permanent one, its depth is closely related 

 to the depth of planting. 



The plumule remains small and enclosed between the coty- 

 ledons until pulled out of the soil. Then by a straightening of 

 the hypocotyl arch and the spreading of the cotyledons, it is fully 

 exposed to the light, where it develops all of the plant above the 



FIG. 101. Squash seed germi- 

 nating, showing the peg by which 

 the seed coat is held while the 

 cotyledons are pulled out of the 

 seed coat by the arch of the hy- 

 pocotyl. Somewhat reduced. 



