POLLINATION AND FERTILIZATION 137 



and, together with the embryo, they constitute the seed. Under 

 favorable circumstances the seed may germinate and the em- 

 bryo grow into an adult plant. One pollen grain may suffice 

 to fertilize an ovule, but the result is more certain if there are 

 several germinating pollen grains for each ovule, though but 

 one male cell unites with one egg. 



128. Pollination an ecological subject. Before considering 

 the ecology of flowers it is necessary to explain what is meant 

 by plant ecology. It is impossible to study plants in any all- 

 round fashion without paying a good deal of attention to the 

 way in which they are influenced by their surroundings. Any 

 change in the temperature, light, water supply, or supply of 

 raw materials for food-making is sure to affect the plant in 

 some way. Hillside, plain, swamp, lake, or sea each has its 

 own flora, or set of plant inhabitants, which can thrive under 

 the conditions found in their growing place. Plants are also 

 greatly affected by the favorable or unfavorable influences 

 exerted on them by their animal and plant neighbors. The 

 whole subject of the relation of plants to the environment in which 

 they live is known as plant ecology. In earlier chapters much 

 has been stated, and still more suggested, that concerns this 

 side of botany, but the subject was not treated in detail, because 

 it is one of the most difficult departments of botanical science 

 and its study is as yet only f ably begun. In this chapter most 

 that is to be said of the processes of pollination should be 

 classed as pertaining to the ecology of flowers. 



129. Relation of types of floral structure to pollination. 

 Probably most students of plants, from the earliest tunes, 

 were somewhat puzzled over the great variety in form, colora- 

 tion, odor, and other characteristics shown by flowers, but until 

 about a hundred and fifty years ago no botanist seems even to 

 have reasoned about the facts that some flowers are over a 

 yard in diameter, while others are almost microscopic; that 

 some spread the perianth widely and others are narrowly 

 tubular or urn-shaped; that some are sweet-scented, others 

 carrion-scented, and others odorless ; that some have extremely 



