3o8 



ECOLOGY, 



the pea are broader and shorter than the leaves of the corn, and cannot well 

 form a long pointed covering for the stem. If the stem remained straight 

 the friction of the leaves against the soil would tear them while they are so 

 tender. But lifted out as they nre, suspended from the bent stem, they are 

 unharmed. 



584. The common garden bean. — The bean also in swelling cracks open 

 the outer coat, the root emerges from underneath the coat in the region of the 

 scar (hilum) on the concave side, while the minute plumule lies curved between 

 the edges of the cotyledons near one end. In the case of the bean, the part 

 of the stem between the cotyledons and the root (called the hypocotyl in all 

 seedlings) elongates, so that the cotyledons are lifted from the soil. The hypo- 

 cotyl is the part of the stem here which becomes strongly curved, and the large 

 cotyledons are dragged out of the soil as shown in fig. 401. The outer coat 

 becomes loosened, and at last slips off com- 

 pletely. The plumule (the young part of the 

 stem with the leaves) is now pushing out from 

 between the cotyledons. As the cotyledons 

 are coming out of the ground the first pair of 

 leaves rapidly enlarge, so that before the stem 

 has straightened up there is a considerable leaf 

 surface for the purpose of car- 

 bon conversion. The leaves 

 are at first clasped together, 

 but as the stem becomes erect 



Y\%. 402. 

 Germination of castor-oil bean. 



they are gradually parted and come to stand out nearly in a horizontal posi- 

 tion. Fig. 401 shows the different positions, and we see that the same pro- 

 vision for the protection of the leaves is afforded as in the case of the pea. 

 As the cotyledons become exposed to the light they assume a gi-een color. 

 Some of the stored food in them goes to nourish the embryo during germina- 

 tion, and they therefore become smaller, shrivel somewhat, and at last fall off. 

 585. The castor-oil bean. — This is not a true bean since it belono-s to a 

 very different family of plants (euphorbiaceas). In the germination of this 

 seed a very interesting comparison can be made with that of the o-arden bean. 

 As the " bean'' swells the vtry hard outer coat generally breaks open at the 



