152 



TEXTBOOK OF BOTANY 



Fig. 93. — Cross sec- 

 tion through a young 

 bean fruit and a. seed ; 



a, groups of vascular 

 bundles (" strings ") ; 



b, outer layer of the 

 pod; c, inner layer; 



d, outer seed coat; 



e, inner seed coat; 

 /, seed leaf; g, stalk 

 by which the seed is 

 attached to the pod. 



are the ' ' strings ' ' that are familiar to 

 every one who has shelled or cut up 

 green beans. The strings and the inner 

 tough layer of the pod were present in 

 all the earlier varieties of beans ; but in 

 more recent years improved varieties 

 have been produced in which the thick- 

 walled cells are either lacking altogether 

 from the vascular bundles and the inner 

 layer of the pod, or else are late in 

 developing. 



174. Germination of the Seed (Fig. 

 94) . — The conditions under which seeds 

 will germinate have been carefully 

 studied in connection with the bean 

 and some of its relatives. One neces- 

 sary condition is the presence of water, 

 a good deal of which is absorbed by the 

 seed coat and by the embryo. Bean seeds germinate more 

 quickly if they are first placed in cold water and left there 

 overnight. Another necessary condition is a suitable tem- 

 perature. Bean seeds will not 

 germinate, no matter how 

 well they are supplied with 

 water, below 32° F. nor above 

 about 122° F. Another con- 

 dition is the presence of oxy- 

 gen. Seeds will not germinate 

 in an atmosphere from which 

 the oxygen has been removed ; 

 nor will they germinate well 

 in soil which is so thoroughly 

 water-soaked that there is 

 little or no air in its pores. 

 Germination begins by the 



Fig. 94. — Stages in the germina- 

 tion of a bean seed and the devel- 

 opment of the seedling. 



