1656 A TEXTBOOK OF THEORETICAL BOTANY 



The common genera fall naturally into the following classes in regard 

 to their pollination mechanism. 



1. Anemophilous. 



Poterium. 



2. Entomophilous: pollen flowers, devoid of nectar. 



Rosa, Ulmaria, Arunciis, Kerria. 



3. Entomophilous with exposed nectar. 



Alchemilla, Sibhaldia, Amelanchier . 



4. Entomophilous with partly concealed nectar. 



Primus, Geum, Potentilla, Spiraea, Crataegus. 



5. Entomophilous with concealed nectar, pollinated chiefly by hive bees. 



Rubus, Fragaria. 



In species of the genus Rosa the flowers are homogamous and often 

 fragrant, with large, brightly coloured petals but devoid of nectar. When 

 the flower opens the stamens curve outwards and an insect wiU alight on 

 the apices of the carpels which occupy the centre of the flower. These 



Fig. 1 5 16. — Alchemilla vulgaris. Flowers in longitudinal section, showing marked 



protandry. 



visits are primarily for the purpose of collecting the abundant pollen but 

 in the process cross-pollination is made possible. Should insect visits fail, 

 automatic self-pollination generally occurs by pollen falling directly into 

 the stigmas, for the quantity of pollen produced is very great. In Alchemilla 

 (Fig. 15 16), the flowers are small and apetalous and the nectar is secreted 

 by a fleshy ring on the rim of the receptacle cup. As a rule the flowers are 

 either protandrous or protogynous. Automatic pollination is therefore 

 rendered diflicult. Moreover in many flowers in which the stigma develops 

 normally the stamens are greatly reduced and their filaments remain much 

 shorter than the style. Most of the species are apomictic. 



A more specialized condition is found in the genus Spiraea (Fig. 15 17). 

 In S. sorbifolia, a Siberian species often grown in gardens, the flowers are 

 large and fragrant. These flowers are protogynous and even in the bud the 

 stigmas are provided with receptive papillae and project beyond the stamens, 



