THE BEAN 141 



become almost as thick as the primary root, and in many 

 cases they are longer. Some short adventive roots also 

 grow from the lower part of the stem. The root system of 

 a bean bears many small swellings, in which are great num- 

 bers of bacteria. Something was said in Chapter II about 

 the bacteria that live in these swellings on the roots of the 

 bean and of many of its relatives (see 41 and Fig. n). 

 We have seen that these bacteria can use the nitrogen of 

 the air, and that they are of great value to the host plant 

 (in this case the bean) because they provide it with the 

 nitrogen-containing substances that it needs for food. 



165. Flowers. These are borne in clusters. The stalk 

 of the cluster (the peduncle) is in each case a branch or the 

 end portion of a branch (or occasionally the end of the main 

 stem) . Each flower has a short stalk of its own (a pedicel) 

 which grows from the axil of a very small leaf (a bract) that 

 is borne on the peduncle. Each pedicel, therefore, is also 

 a branch. The flower cluster of the bean is of the sort 

 known as a raceme. Each flower is so attached to the 

 peduncle that it stands out horizontally, or nearly so. For 

 this reason we can speak of the upper and lower sides of a 

 flower. 



A bean flower (Fig. 85) seems at first sight entirely dif- 

 ferent from anything that we have seen in the pine or in 

 any of the lower plants. Really, however, the flower of the 

 bean corresponds very closely to the cones of the pine. One 

 difference lies in the fact that a single pine cone bears only 

 one kind of spore leaves, whereas the same bean flower con- 

 tains both microspore leaves (stamens) and a macrospore 

 leaf (which forms the pistil). Therefore-, while the pine 

 has two kinds of cones, the bean has only one kind of flower. 1 

 The flower has probably developed from a structure not 



1 Many angiosperms, however, like the cucumber, bear stamens and pistils in 

 separate flowers. In such cases the staminate and pistillate flowers correspond 

 respectively to the staminate and carpellate cones of the pine. 



