THE SEED PLANTS 299 



The essential structures of a flower are stamens (niicrosporo- 

 phylls) and pistils, or carpels (meaasporophylls'). A carpel, 

 strictly speaking, is one megasporophyll, and when several 

 carpels are united, the result is known as a compound carpel. 

 The name pistil is used without discrimination for either a 

 simple or a compound carpel. 



The flower and the resulting seed that characterize this 

 group of plants has often led people to give to the angio- 

 sperms names which suggest these characters. The most com- 

 mon of these names are floivering plants and seed plants. The 

 name phanerogam, meaning " plants with visible reproduction," 

 was applied when botanists knew less of the intricacies of the 

 reproduction of angiosperms than is now known. In the same 

 way cryptogam, which means " plants with hidden reproduc- 

 tion," was applied collectively to the pteridophytes, bryophytes, 

 and thallophytes. These names are still used by many people, 

 but it is evident that an interchange of the names would bet- 

 ter fit the facts of reproduction in the groups of the plant 

 kingdom. 



284. Stamens and pollen. In connection with the discussion 

 of the gymnosperms the structures of the stamen are fully 

 illustrated (fig. 225). The parts of the angiosperm stamen 

 the anther and the filament are similar to the same structures 

 -in the gymnosperms, though of course many variations appear. 

 In the young angiosperm anther there are four sporangia, and 

 these, when they ripen their spores, unite in pairs, so that 

 two pollen sacs are formed from the four sporangia (fig. 108). 

 The anthers of the angiosperms may open in a variety of ways, 

 the method of opening being called the dehiscence. 



Since the pollen grains are formed by the division of cells 

 in a sporangium, it is evident that they are asexual spores. 

 When mature each pollen grain consists of a heavy outer wall, 

 an inner wall, cytoplasm, and nucleus (fig. 118). Often there 

 are starch and oil foods in the pollen grains. The single- 

 celled pollen grain sometimes begins to germinate before it 

 leaves the anther in which it was formed, and when this has 



