IO2 



PHYSIOLOGY. 



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

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

 best understood in a straight ovule. Upon the 

 opposite side of the scar and close to it can be 

 K> seen a minute depression, the micropyle. Under- 

 A neath the seed coat and lying between this point 

 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 the beans in water and as 

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





; faster than the inner portion of the seed, which 

 a 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 the 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 part 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, becomes strongly 

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

 th<j soil. As the cotyledons become exposed to the light they 

 assame 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 bean. This is not a true bean, since it 

 belongs to a very different family of plants (Euphorbiaceae). In 

 the germination of this seed a very interesting comparison can 

 be mad? with that of the garden bean. As the "bean" swells 

 the very hard outer coat generally breaks open at the free end 

 and slips off at the stem end. The next coat within, which is 



Fig. 92. 



Bean seed split 

 open to show plan.-- 

 let. 



