THE GENERAL LIFE PROCESSES 331 



formation of flowers. It is impossible to say always just 

 what a flower is, but for our purpose it is easy enough. 

 The flowers of our tree contain stamens and pistils, either 

 both in the same flower or separated in different flowers. 

 It is these stamens and pistils that contain the structures 

 essential to reproduction. The stamen produces pollen 

 grains, and within the pistil there are one or more ovules 

 which will become seeds, if fertilization .is secured. Asso- 

 ciated in the flower with these stamens and pistils there 

 may be petals and sepals, or only one of them. These are 

 protective structures and perhaps attractive, so that in the 

 work of reproduction they are entirely subordinate and 

 hence not always present. 



The pollen is carried from the stamens to the pistil 

 of the same flower, or of some more or less distant flower, 

 the usual agents of transfer being the wind and insects. 

 After lodging upon the receptive surface (stigma) of the 

 carpel, the pollen grain sends out a tube which penetrates 

 the carpel, reaches an ovule, and entering the ovule, finally 

 comes in contact with the egg contained in the ovule. 

 Then the tip of the pollen tube discharges its contents, the 

 egg is fertilized, and this fertilized egg develops the embryo 

 of a new plant. 



While the embryo is developing within the ovule, 

 changes are taking place in the outer region of the ovule, 

 by which a hard coat is formed. This hard coat •finally 

 hermetically seals the growing embryo within, which then 

 stops growth; the whole structure being now called the 

 seed. The conditions under which the embryo can resume 

 growth and develop into a complete plant were considered 

 at the beginning of this chapter. 



