THE PLANT KINGDOM 



257 



for any family (Fig. 139). The problem confronting the taxono- 

 mist today, therefore, is not the recognition of certain rather 

 vague "affinities" between plants, but simply the determination 

 of what might be called their "blood relationships"; and during 

 the past fifty years particular emphasis has been placed on the 



Ancient Coniferous Stock 



Fig. 139. — A suggested "family tree" for the Conifers. According to this 

 hypothesis, the ancient coniferous stock long ago divided into two groups, each of 

 which has given rise to three modern families or sub-families. Two of these (the 

 Taxineae and Podocarpineae) are more closely related to each other than they are 

 to the other families. Within the Abietineae there are evidently two distinct 

 groups of genera. The branch stumps represent extinct groups. The twigs are 

 genera of coniferous trees which are living today. Such a diagram as this makes 

 it possible to show graphically the inter-relationships between the various 

 members of a group of plants. 



science of Phylogeny, which endeavors, through a study of fossil 

 history, comparative anatomy, and other sources, to trace out 

 the complicated problem of ancestry and descent throughout 

 the whole plant kingdom. The findings of this science are of 

 great importance in providing a basis for classification. While 

 there are still differences of opinion as to facts, everyone is agreed 

 that the ideal to be attained is a system of classification which 

 is truly a "natural" one, or based on descent. As our knowledge 

 of the evolutionary history of the plant kingdom becomes more 



