THE BEAN 



143 



white ; those of other varieties are colored in various ways. 

 The uppermost petal (the standard) is broad and erect, ex- 

 cept that its lower part is bent so as to be horizontal and 

 to cover the lower parts of the other petals. At its upper 

 end the standard is two-lobed, so that it looks as though it 

 might be composed of two petals ; but this is not the case. 

 On the sides of the flower 

 and below the standard are 

 two other petals, called 

 wings; they are broad and 

 rounded, but each is much 

 narrowed at the base. Be- 

 low the wings are the two 

 remaining petals ; they are 

 narrow and are united, ex- 

 cept at their ends, into a 

 structure like a trough with 

 its opening upward ; this 

 trough-shaped structure is 

 the keel. Its outer end is 

 spirally twisted. 



FIG. 86. A, a pollen grain of the 

 bean, as seen from the outside. B, a 

 cross section of a pollen grain ; a, one 



167. Stamens. Just of the germ poreS) where th ' e outer 



within the petals are the layer of the wall is lacking, and where 



stamens or micros pore leaves, 

 which here are not at all 

 like foliage leaves. Of the 

 ten stamens in the bean 



the pollen tube, surrounded by the 

 inner layer of the wall, may push out 

 when the pollen grain germinates; 

 b, the vegetative cell; c, the genera- 

 tive cell. C, a pollen grain (of the 

 lily) which . has germinated ; d, the 

 flower, nine are united by two male gamete nudei . e> the pol]en 



their filaments into a partial tube ; /, the nucleus of the vegetative 



tube that is open on the cell > which has moved to the f nd of 

 . 1 _ 1 . . , the tube. B after a preparation by 



upper side. This partial M iss Mabel Brown, 

 tube really forms a second 



trough that lies inside the trough-shaped keel. When all 

 the parts of the flower are in their usual positions, the one 

 free stamen covers the open part of the trough made by the 



