210 HEKliUlTY AND INVOLUTION IN PLANTS 



fossil forms the direct ancestors of those now living, although 

 a study of their structure is of the greatest value in ena- 

 bling us to understand the genetic relationships of the great 

 groups of plants. 



143. Ancestors of the Angiosperms.^ — Just as the Cyca- 

 dofilicales indicate the ancestry of the cycads, so fossil 

 types of Cycadophyta have been discovered which are 



Fig. 94. — Cycadcoidea dacotensis (?). Photomicrograph of a young 

 seed (X 15), showing a sterile scale on either side. Between them pro- 

 jects the entire length of the tube through which the micropyle extends. 

 The partially collapsed nucellus is distinctly shown in the center. (After 

 Wieland.) 



interpreted by some paleobotanists as ancestors of the 

 modern angiosperms. Other investigators, however, 

 dissent from this view and consider that we have not yet 

 sufi&cient knowledge of fossil forms to be justified in desig- 

 nating the ancestors of the Angiosperms. This differ- 

 ence of opinion is largely due to the meagerness of the 

 available evidence. As one writer has stated it, "A 



