100 COLLEGE BOTANY 



modified integuments of the ovule and form a protective cover- 

 ing for the embryo, endosperm and perisperm. 



Mature seeds from different species of plants show consid- 

 erable variation in size and form, in size of the embryo, and in 

 the relative amounts of endosperm and perisperm. In some seeds 

 the perisperm is very abundant and is the most important source 

 of reserve food. In oth3rs the embryo grows very rapidly, gradu- 

 ally absorbing the endosperm. Therefore, we find seeds (1) 

 with embryo, endosperm, perisperm and seed coats, as in the 

 seeds of the pepper and water lily ; (2) with embryo, endosperm 

 and seed coats, as in the seeds of the castor-oil plant and James- 

 town weed; or (3) with embryo and seed coats, as in the bean 

 and pea. These different types of seeds will receive further 

 consideration in Chapter IX. 



The various stages in the formation of the embryo sac and 

 embryo must be studied with the microscope in order to get 

 a thorough understanding of them. They are illustrated in 

 Fig. 74. 



Secondary Effects of Pollination and Fertilization. Al- 

 though the primary function of pollination and fertiliza- 

 tion is the formation of a seed containing an embryo from 

 which a new plant will be produced, there are several sec- 

 ondary physiological effects. The first is the production of 

 fruits; many plants, such as melons, cucumbers and fruit 

 trees, will not produce normal fruits unless fertilization has 

 occurred. Partly developed cucumbers and one-sided apples 

 usually result when only a part of the ovules have been fertil- 

 ized. Many of the plants will not set fruit unless fertiliza- 

 tion has occurred. Other plants, such as the banana and the 

 seedless orange, will produce excellent fruits without fertiliza- 



