I02 



PHYSIOLOGY. 



(see Chapter 36), and at the outer end of the raphe is the cha- 

 laza, the point where the stalk is joined to tlie end of the ovule, 

 best understood in a straight ovule. Upon the 

 opposite side of the scar and close to it can be 

 o -' \ "• seen a minute depression, the mkropyle. Under- 

 ■'* neath the seed coat and lying between this point 

 r and the end of the seed is the embryo, which gives 

 „ greater prominence to the bean at this point, but it 

 is especially more prominent after the bean has been 

 soaked in water. Soak tlie beans in water and as 



Fig. Qi. 



Garden bean, they are swelling note how the seed coats swell 

 ™'hiium"r'scar', faster than the inner portion of the seed, which 

 whe?e'^'chaSSa causes them to wrinkle in a curious way, but finally 

 '^'" the inner portion swells and fills the seed coat out 



smooth again. Sketch a bean showing all the external features 

 both in side view and in front. Split one lengthwise and sketch 

 the half to which the embryo clings, noting the young root, 

 stem, and tlie small leaves which were lying 

 between the cotyledons. There is no endo- 

 sperm here now, since it was all used up in 

 the growth of the embryo, and a large \)a.ri of 

 its substance was stored up in the cotyledons. 

 As the seed germinates the young plant gets its 

 first food from that stored in the cotyledons. 

 The hypocotyl elongates, l^ecomes strongly 

 arched, and at last straightens up, hfting the cotyledons from 

 the soil. x\s the cotyledons become exposed to the light they 

 assume a green color. Some of the stored food in them goes 

 to nourish the embryo during germination, and they therefore 

 become smaller, shrivel somewhat, and at last fall off. 



211. The castor-oil hean. — This is not a true bean, since it 

 belongs to a very different family of [jlants (Euphorbiaceff). In 

 the germination of this seed a very interesting com])arison can 

 be mad? with that of the garden l)ean. As the "hean" swells 

 the very liard outer coat generally lireaks open at the free end 

 and sHps off at the stem end. The next coat within, wliich is 



Fig. 92. 



Bean seed split 



pen to shuw plant"' 



let. 



