300 FOUNDATIONS OF BOTANY 
by his own observations, are much alike. As the egg-cell 
grows and deyelops, 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 unlikeness 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 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. 
