218 EVOLUTION OF PLANTS 



In passing from the lower to the higher types there 

 is a reduction in the number of parts, accompanied by 

 their cohesion. The carpels are first affected, and then 

 the sepals, and finally the corolla. Reduction in the 

 number of stamens is common in certain groups, espe- 

 cially the Labiatiflorse, and the cohesion of the stamens 

 among themselves occurs regularly in the Compositse, but 

 is much less perfect than that of the other floral parts. 

 Most of the more specialized types, both of Choripetalse 

 and Sympetalse, have inferior ovaries. 



The Sympetalse are unquestionably the highest of the 

 Angiosperms. Whether the group is a homogeneous 

 one, or, as seems more likely, sympetaly has originated 

 more than once, must be determined by further re- 

 searches. The radially symmetrical (actinomorphic) 

 Isocarpse are probably nearer the Choripetalse, as shown 

 by the occurrence of forms like Monotropa, which have 

 sepals and petals quite distinct. It has recently been 

 suggested that the Isocarpse have perhaps been derived 

 from the Centrospermse among Choripetalse and have 

 given rise to the Tubiflorae and Labiatiflorse, whose 

 highest members are the mints and figworts. Another 

 line, originating from the Calyciflorse, has through the 

 Umbelliferse developed the Rubiacese (madder family), 

 and through these the Compositse. There are several 

 less important lines of development which cannot be 

 taken up here, and it must be remembered that the 

 suggestions given here as to the origin of the different 

 groups of Dicotyledons are likely to be essentially 

 modified when we are in possession of data more com- 

 plete than we now possess. The accompanying diagram 

 illustrates graphically the arrangement of the principal 

 groups of Dicotyledons adopted here. 



