196 APPLIED BIOLOGY 



FLOWERS OF SEED-PLANTS 



189. The Functions of Flowers. Our studies of the bean 

 flower ( 75) may now be applied to flowers in general. The 

 petals and sepals of flowers serve the purpose of inclosing 

 and protecting the stamens and pistil, which are the parts 

 of flowers essential to development of seeds. The important 

 parts of the stamens are the anthers or pollen-cases which 

 contain pollen-grains. There may be one or more pistils. 

 If we open the ovary of the pistil in a common flower, we 

 see one or more rounded bodies, called ovules; and by 

 comparing flowers of different ages it is made evident that 

 from each ovule a seed develops. Inside of each ovule is 

 an egg-cell, the part from which the embryo of a new plant will 

 develop. The ovules are usually easily seen by the unaided 

 eye, but egg-cells are microscopic. An ovary in some species 

 contains a single ovule, with a single egg-cell; and hence 

 a flower with one such ovary could form only one seed and 

 one plant. Examples are buckwheat and " four-o'clock." 

 In most flowers each ovary contains more than one ovule, 

 and hence could produce as many seeds as there are ovules. 

 Examples are the bean pod, which grows from a single 

 flower, and has six to ten seeds, and many flowers (e.g., 

 poppy) with numerous seeds from one flower. 



The pollen-grains form cells which are often called male re- 

 productive cells or fertilizing cells, and the egg-cells are called 

 female. Neither kind of cell alone is able, as a rule, to pro- 

 duce a seed ; and usually there must first be a union of the 

 two kinds of cells as a preliminary to development of seeds 

 (see 75, on bean flower) . There are a few cases of flowering 

 plants in which the egg-cells have the peculiar power of 

 developing without the aid of the pollen-grains. This con- 

 dition is parthenogenesis. 



Pollination. The fertilizing cells produced by pollen- 

 grains must be brought near the egg-cells' in the ovary. 



