EVOLUTIONARY HISTORY OF PLANTS 257 
of great numbers of plants of different groups, it has 
become possible to draw up a sort of pedigree of the plant 
world. This is not as yet by any means complete, but 
what is already known on the subject throws much light 
on the reasons for the existence of structures in the early 
part of the life histories of many plants which would 
otherwise seem to be wholly useless and without meaning.! 
333. 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 nineteenth 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? 
334. Development of the Plant from the Spore in Green 
Algz and Mosses. — The course which the forms of plant 
1 See Engler’s Syllabus der Pflanzenfamilien, Berlin, 1907, pp. vi-xxii. 
2 See Campbell’s Zvolution 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. 
