4 MORPHOLOGY OF ANGIOSPERMS 



with Pteridophytes and Bryophytes. In fact, Pteridophytes 

 and Gymnosperms together form a much more natural group 

 than do Gymnosperms and Angiosperms ; and this fact should 

 be emphasized by treating Gymnosperms and Angiosperms as 

 groups of the first rank. 



Although it is a question whether Gymnosperms and An- 

 giosperms should be so closely associated as to form the two 

 subdivisions of a great group, there can be no question that 

 Monocotyledons and Dicotyledons are naturally and intimately 

 associated. This proposition is not affected by the question of 

 their common origin, but is based upon their essential mor- 

 phological features, whatever may have been their origin. The 

 characters that separate Monocotyledons and Dicotyledons are 

 cumulative rather than specific, and although the character of 

 the embryo is held to be the decisive one in every case, there is 

 danger of using it with unnatural rigidity. When a decision 

 between two groups is reduced to a single character, there is a 

 suspicion either that the groups can only be separated arti- 

 ficially or that too much stress is laid upon the character. Mon- 

 ocotyledons and Dicotyledons are best distinguished by cer- 

 tain tendencies that involve several characters, and if these 

 tendencies are supported by the character of the embryo the 

 case is clear. A brief statement of the conspicuous differences 

 may be of service. 



1. In the embryo of Monocotyledons the cotyledon is ter- 

 minal and the stem tip lateral in origin ; while in Dicotyledons 

 the stem tip is terminal and the cotyledons lateral in origin. 

 This character seems to be fundamental, and at the present time 

 is the only one that may be regarded as decisive. That the 

 difference indicated will always be expressed in the above 

 terms is not likely, for the nature of the cotyledon is in ques- 

 tion, and the significance of this relation of parts has yet to be 

 determined. 



2. The development of the vascular bundles in the stele is 

 very different in the two groups. This difference involves not 

 only the arrangement of the bundles, but also the presence or 

 absence of fascicular cambium, and is far-reaching in its re- 

 sults upon the habit of the body. In the case of perennial 

 stems it involves the general ability to increase in diameter, 

 arrd this affects the pow T er of branching, and this in turn deter- 



