THE DICOTYLEDONES 



1657 



which are bent towards one another. When the flower opens the stamens 

 gradually become erect and dehisce successively from the outside inwards. 



Fig. 15 17. — Spiraea sp. Pollination. See text. 



Insects are attracted to the flowers by nectar which is secreted in abundance 

 by an annular, orange-yellow thickening on the inner wall of the concave 

 receptacle, internal to the insertion of the stamens. At the beginning of 

 anthesis, therefore, insects perform cross-pollination whereas later they 

 may perform self-pollination, since the stigmas remain receptive until the 

 innermost anthers have dehisced. Finally automatic pollination may occur. 

 The most specialized condition is found in the genus Riibus. The 

 flowers (Fig. 15 18) are either pink or white in colour and when the buds 



Fig. 15 18. — Rubus fniticosus. 

 Longitudinal sections of 

 flowers illustrating pollin- 

 ation. 



