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 ege. 
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 flura, 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 tn which 
they live is known as plant ecoloyy. 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 fairly 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 times, 
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 
