306 STRUCTURAL BOTANY 



classes are mainly distinguished by vegetative characters, 

 the reproductive phenomena being the same in both. On 

 both these questions, however, we may hope for further 

 light, especially from palaeontological research, for the first 

 appearance of both Monocotyledons and Dicotyledons falls 

 within a geological period from which abundant fossil 

 remains have come down to us. 



This brief summary has had one main object, to 

 indicate how complicated and difficult, questions as to 

 the affinities of plants really are. Most naturalists in 

 these days are agreed that the different forms of plants 

 and animals arose from one another by descent. If this 

 be so, a natural classification of the vegetable kingdom 

 would take the form of a genealogical tree, just like the 

 pedigree of a family. The genealogical tree of plants 

 must have been complex beyond all power of conception, 

 with boughs, branches, and twigs of every degree starting 

 from each other at every possible point, some long and 

 some short, a few reaching on to our own day, while the 

 immense majority came to an end in the long past 

 geologic ages. 



If we attempted to construct such a tree, even for. our 

 twenty-six types, almost every branch would be marked 

 with a query. If the reader has gained some idea of the 

 difficulty and complexity of the profoundly interesting 

 problems which the comparative study of plants presents 

 to us, the object of this concluding chapter will have been 

 attained. 



