POLLINATION AND FERTILIZATION 155 
5. Some willows, as the common yellow willow (Salix alba, var. vitel- 
lina), 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, 
