1678 A TEXTBOOK OF THEORETICAL BOTANY 



tube. Insects can only obtain nectar legitimately by alighting on the left 

 wing petal and thrusting their probosces under the opening on the right 



Fig. 1535. — Phaseohts multiflonis. Flower in longitudinal 



section. 



side of the tip of the keel. Only large humble bees can do this. When 

 the keel is depressed the style springs out. It is a coiled brush-like structure 

 and it is pushed out of the opening in the keel. The free stamen retains its 

 position but the other nine are depressed. Since the proboscis of the 

 insect touches the stigma before the pollen, cross-pollination is regularly 

 effected and unvisited flowers remain sterile. 



The final sub-family is the Dalbergioideae, which contains twenty- 

 seven genera. They are mostly trees or shrubs and many of them produce 

 woods of economic importance. Pterocarpus santalinus yields red Sandal- 

 wood, P. marsupiiim. Bastard Teak, Dalbergia nigra, Rosewood, D. melan- 

 oxvloN, Blackwood, and D. latifolia, East Indian Rosewood. Many are 

 lianas, climbing by short lateral shoots which are sensitive to contact, as 

 in D. variabilis. They are mostly tropical plants occurring especially in 

 Africa and southern Asia. There are no British representatives. 



From the above account it may be noted that the pollination mechanisms 

 are distinct in each of the sub-families. The following scheme in which 

 some common genera are grouped according to the mechanisms employed 

 serves to bring out this point more clearly. 



1. Simple valvular arrangement. Stamen and stigma project from the 

 keel as long as the pressure of the insect continues and then return to their 

 former positions. Such flowers allow a number of effective visits. 



(a) Nectar present. E.g., Melilotiis, Trifoliiim, Galega, Onobrychis, 



Astragalus, Oxytropis, Phaca, Ornithopus, Hedysarum. 

 {h) Enclosed sap reached by boring. E.g., some species of Cytisus. 



2. Explosive arrangement. Stamens and stigma suddenly spring out of 

 the keel. Such flowers allow of only one effective visit. 



