THE ANGIOSPERMS 375 



constitute the original endosperm nucleus from which the cells 

 of the endosperm or trophophyte take their origin. The high 

 degree of constancy in the essential features of organization of 

 both sporophyte and gametophyte in the angiosperms mark them 

 as a monophyletic group in which the two great divisions, dicoty- 

 ledons and monocotyledons, have had a common origin. 



There seems to be no reasonable doubt that both divisions of 

 the angiosperms the dicotyledons and monocotyledons have 

 originated from gymnospermous ancestors and not directly from 

 any existing or extinct group of vascular cryptogams, since they 

 entirely lack cryptogamic features of organization in all their 

 organs with the sole exception of the root. The radical organ, 

 however, is without significance as indicating cryptogamic deriva- 

 tion, for cryptogamic or centripetal primary wood is present in 

 all roots without exception. It further seems obvious that the 

 angiosperms in neither of their two divisions can have originated 

 from the Archigymnospermae, since they present the siphonoga- 

 mous mode of fertilization in contrast to the zoidogamy or fertiliza- 

 tion by antherozoids present in the lower gymnospermous tribes 

 which are more nearly related to the Filicales. 



The general features of the angiosperms indicated above 

 characterize the two great divisions, dicotyledons and monocotyle- 

 dons, in common; and it is now necessary to specify the distinguish- 

 ing structures which separate these from one another. Since the 

 dicotyledons are with the greater probability the older and more 

 primitive of the two main groups, they will first be considered. 

 In the dicotyledonous angiosperms the seed is distinguished by 

 an embryo possessing paired cotyledons or seed leaves. This 

 feature is perhaps the most constant characteristic of the group. 

 In the fibrovascular structures the wood is distinguished by well- 

 marked secondary growth which becomes feeble only in forms in 

 which the herbaceous habit has become distinctly developed. The 

 tissues belonging to the conductive category are, moreover, typi- 

 cally arranged in the form of a cylinder which is continuous in 

 woody forms but becomes broken up into separate strands in 

 stems with herbaceous texture. When the fibrovascular organiza- 

 tion consists of isolated bundles, these are ordinarily arranged in 



