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A TEXTBOOK OF BOTANY [Ch. VIII, 4 



already within them in ex-albuminous kinds, is ready for 

 translocation, and use in the growing parts of the embryo. 

 Fourth, the end of the hypocotyl of the embryo, lying next 

 the micropyle, now pushes forth, and as soon as clear of the 

 seed coats, grows geotropically over to point downward, 

 developing meantime the root at its tip. This root is a 

 new growth, and not a transformation of the hypocotyl; as 

 students are prone to suppose. Then, if the seed, as is 

 usual with wild plants, lies on the sur- 

 face of the ground, the root begins to 

 enter the earth. No sooner does the root 

 start into the soil than (from small seeds 

 at least) it sends out a radiating ring or 

 collar of root hairs which take firm hold 

 on the rock particles. Thus is provided 

 a resistance, without which further 

 growth might rather lift the seed from 

 the ground than force the root into the 

 soil. In some other seeds, such as Flax, 

 such resistance is provided by a muci- 

 laginous coat which gums it, so to speak, 

 to the ground. Practically all embryos, 

 as the first act of their development, 

 thus secure access to the water supply 

 which is indispensable to their further development. 



Fifth, on the basis of the anchorage secured by the pene- 

 tration of the root into the earth, the hypocotyl now begins 

 to make such growth movements, too complex for easy de- 

 scription but readily shown in our pictures (Fig. 274), as 

 cause the withdrawal of the cotyledons from the seed coats, 

 and their subsequent elevation, when they open out to the 

 light. In cases, however, like Peas and some Beans, where 

 the cotyledons are apparently too thick to serve later as 

 effective foliage leaves, they remain in the ground, while 

 the plumule issues from between them, and grows geotropi- 

 cally upward. 



Fig. 273. — Germi- 

 nating seed of Pump- 

 kin, showing the "peg" 

 or "heel" by the devel- 

 opment of which the 

 seed coat is forced open. 

 (From F. Darwin.) 



