POLLINATION AND FERTILIZATION 155 



5. Some willows, as the common yellow willow (Salix alba, var. vitel- 

 Zma), are usually represented in this country only by pistillate trees. 

 Can they bear seed? Can they be reproduced? 



6. Of the various methods by which self-pollination is rendered diffi- 

 cult, unisexual flowers borne on the same plant, unisexual flowers 

 borne on different individuals, dichogamy, dimorphism, which is the 

 least effective? Which is the most effective? Give as many instances 

 as you can of plants which exemplify each method (not citing any of 

 those mentioned in this chapter). 



7. Describe as well as you can the qualifications of each of the fol- 

 lowing flowers for securing pollination : hazel, birch, box elder, butter- 

 cup, nasturtium, horse-chestnut, dandelion, locust, sweet pea, primrose. 



8. Explain why the subject of pollination is an ecological one. Can 

 you give any reasons why plant ecology is one of the most difficult 

 divisions of the science of botany? 



9. Give some practical applications of the principles of pollination 

 to agriculture and horticulture. 



