152 Plant Life and Evolution 



The microsporangia, or pollen-sacs, are borne upon 

 much modified sporophylls called stamens, and the 

 leaves known as carpels, from which are developed 

 the ovules, or megasporangia, are also greatly al- 

 tered. The carpels form a closed cavity, the ovary, 

 within which the ovules are placed so that they are 

 effectively protected during the development of the 

 seed. The flowers also possess in many cases showy 

 leaves, forming the floral envelopes. The ovules 

 are not strikingly different from those of the gym- 

 nosperms, but there is usually a second integu- 

 ment. 



Origin of the Angiosperms Doubtful. There are 

 undoubtedly certain points in common between the 

 Gnetales and the angiosperms, this being especially 

 the case in the genus Gnetum; but as we have al- 

 ready seen, the Gnetales are very doubtfully related 

 to the other gymnosperms, so that even if it 

 should be shown that there is a relation between 

 the Gnetales and the angiosperms, this would not 

 help much in explaining the relation of the latter 

 to lower forms. Some of the monocotyledons, es- 

 pecially the palms, bear a superficial resemblance 

 to the cycads; but this is probably only a paral- 

 lelism, and does not indicate any real relation- 

 ship. 



Gametophytes of the Angiosperms The pollen- 

 spores of the angiosperms are not materially dif- 

 ferent in structure from the microspores of the 

 heterosporous pteridophytes or the pollen-spores of 



