GEOGRAPHICAL DISTRIBUTION l8l 



Angiosperms in earlier geological periods were almost all 

 woody. The number of herbaceous forms increases as 

 we pass from older to more recent strata. Fossils of 

 herbaceous plants are rarely found in Cretaceous rocks 

 but become increasingly abundant throughout the Ter- 

 tiary. Caution is necessary here, however, for the foliage 

 and other parts of herbs are more tender and delicate 

 than those of woody plants, and therefore less liable to 

 be preserved as fossils. This evidence is significant only 

 in connection with evidence derived from other sources. 



2. A study of the comparative anatomy of stems indi- 

 cates that the continuous ring of wood, which character- 

 izes the stems of all trees and shrubs, is a more primitive 

 character than the separate nbro-vascular bundles of 

 herbaceous stems. It is suggested that a change from a 

 woody to an herbaceous type may have resulted from 

 regional decrease in the activity of the cambium layer, 

 from which the wood is formed by cell-division followed 

 by lignification. 



3. Evidence from phylogeny shows that the more 

 primitive groups of Angiosperms and their probable an- 

 cestors are composed overwhelmingly of woody plants. 

 In more than half of the families of Dicotyledons there are 

 no herbaceous species, and the few families which are 

 entirely herbaceous are almost all insectivorous plants, 

 water plants, parasites, or monotypic families, 1 and hence 

 can lay no claim to great antiquity. Also, there is a 

 much larger proportion of woody plants in the lower 

 groups of Angiosperms (Apetalae and Polypetalae) than 

 in the higher groups (Sympetalae.) 



4. From a study of plant geography we learn that 



1 A monotypic family is a family having only one genus. 



