300 FOUNDATIONS OF BOTANY 



by his own observations, are much alike. As the egg-cell 

 grows and develops, the sporophyte of a liverwort, which 

 proceeds from the egg-cell, is extraordinarily unlike the 

 " fern" or asexual generation (gametophyte) among Filices. 

 Now this progressive unlike ness between liverworts and 

 ferns, as they develop from the fertilized egg-cell, points to 

 the conclusion that both groups of plants have a common 

 origin or that the more highly organized ferns are direct 

 descendants of the less highly organized liverworts. 



371. Plants form an Ascending Series. All modern 

 systems of classification group plants in such a way as to 

 show a succession of steps, often irregular and broken, 

 seldom leading straight upward, from very simple forms 

 to highly complex ones. The humblest thallophytes are 

 merely single cells, usually of microscopic size. Class 

 after class shows an increase in complexity of structure 

 and of function until the most perfectly organized plants 

 are met with among the dicotyledonous angiosperms. 

 During the latter half of the present century it first 

 became evident to botanists that among plants deep-seated 

 resemblances imply actual relationship, the plants which 

 resemble each other most are most closely akin by descent, 

 and (if it were not for the fact that countless forms of plant 

 life have wholly disappeared) the whole vegetable kingdom 

 might have the relationships of its members worked out by a 

 sufficiently careful study of the life histories of individual 

 plants and the likeness and differences of the several groups 

 which make up the system of classification. 1 



1 See Campbell's Evolution of Plants and Warming's Systematic Botany, 

 Preface and throughout the work. In the little flora of the present book, the 

 families are arranged in the order which, according to the best recent German 

 authorities, most nearly represents their relationships. 



