358 THE SPERMATOPHYTES ^ 



plants have developed elaborate devices to insure pollination, as 

 through the visits of insects to flowers (see Chapter xxxn). 



Fertilization is effected when the pollen tube pierces the embryo 

 sac and one of its two sperm nuclei fuses with the egg nucleus. 

 When one considers the extraordinary modifications of the 

 male gametophytes of the seed plants, the process of pollination 

 and the development of the pollen tube seem quite as remark- 

 able as the retention of the female gametophyte in the mega- 

 sporangium. They are both essential features of the seed habit. 

 344. The flower. The term flower in the popular sense gen- 

 erally means some showy structure such as is only found in 

 certain groups of the angiosperms. The flower in the scientific 

 sense consists of a group of spore leaves, or sporophylls, with 

 or without surrounding envelopes, which may or may not be 

 showy. It has been defined as " a shoot beset with sporophylls." 

 Since the seed plants are heterosporous, the spore leaves are 

 either microsporophylls, called stamens (producing pollen), or 

 they are megasporophylls, called carpels (producing embryo sacs 

 in the ovules). The stamens and carpels of the gymnosperrns 

 are generally grouped in cones which resemble the cones of the 

 horsetails and club mosses. But the carpels of the angiosperms 

 form, often with adjacent tissue, closed cases called pistils. 



It should be noted that the cones of the horsetails and club 

 mosses are as truly flowers in the scientific sense as the cones 

 of the gymnosperms, and also that certain groups of angio- 

 sperms (grasses, sedges, and most trees) have flowers which are 

 not showy. 



The material of this chapter will be treated under the follow- 

 ing headings : 



Subdivision I. The gymnosperms, or Gymnospermce. 



Subdivision II. The angiosperms, or Angiospermce. 



The origin of seed plants and the seed habit. 



The evolution of the flower. 



The classification of the angiosperms. 



Summary of the spermatophytes and their relationships to the 

 pteridophytes. 



