218 FLOWERS AND THEIR WORK. 



only animals concerned in carrying pollen from flower to 

 flower, but there are foreign flowers which are pollinated by 

 birds (especially humming-birds), bats, and even snails. 



248. Mechanism of Broad Bean Flower. Here cross- 

 pollination is brought about by bees, which may be seen at 

 work on the flowers if you watch plants growing in the 

 open during fine weather. Wind-pollination is out of the 

 question in this case, since the stamens remain enclosed in 

 the keel. 



When a bee alights on the flower it stands on the two 

 wing-petals, which bend down under the weight of its 

 body. Since the wings are jointed with the keel, the latter 

 is also pressed down. If you carefully watch the bee at its 

 work, or imitate its action by pulling down the tip of the 

 keel, you will see that when this happens a mass of pollen is 

 brushed out of the keel by the tuft of hairs on the style, and 

 the stigma projects from the keel as well. Some pollen is 

 deposited on the bee's hairy body, which also rubs against 

 the stigma ; if the bee has visited another Broad Bean flower 

 previously, some pollen will probably be left on the stigma. 

 When the bee leaves the flower the keel rises to its former 

 position, and the process may be repeated a few times when 

 other bees visit the same flower. 



Notice the long " tongue " (proboscis) which the bee pokes 

 into the lower part of the stamen-trough below the free 

 stamen. It is easy to see this if you pull off the standard 

 and carefully watch the next bee that comes to the flower. 

 What does the bee find here ? If you take a newly opened 

 flower, remove the calyx and corolla, and pull apart the 

 stamen- trough and the pistil, you will notice the liquid which 

 has collected at the bottom of the trough. Sugar is present 

 in this liquid (the honey or nectar) which is produced by 

 gland-cells at the base of the stamen-trough. 



The bee is guided to the flower, in its search for honey, by 

 the white corolla (easily distinguished among the foliage, and 

 made more conspicuous by the velvety black spot on each 

 wing-petal) and also by the fragrant odour of the petals. 



The coloured streaks on the standard help to guide the 

 bee to the opening below the free stamen ; but its removal 

 does not prevent a bee from finding this opening, as can 



