THE CLASSIFICATION OF PLANTS. 217 
263. 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 relation- 
ship, 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 likenesses and differences of the several groups 
which make up the system of classification.’ 
264. Order of Appearance of Types of Plant Life on the 
Earth. — Fossil remains of plants are found preserved in the 
rocks in so many places that much is known about the early 
history of plant life. Thallophytes of some kind were 
undoubtedly the first plants, and more highly organized 
groups appeared gradually afterward. It is nearly as certain 
that the more complex and highly specialized forms descended 
by gradual modifications and improvements from the simpler 
ones as it is that elm trees a hundred feet in height, with all 
their complicated structures of root, stem, leaf, and flower, 
grow from seeds not nearly as large as one’s finger-nail. But 
1See Warming’s Systematic Botany, Preface and throughout the work. In the 
little flora which accompanies Part II of the present book, the families are arranged 
not in the order in which they occur in Gray’s Manual, but in one which according 
to the best recent German authorities more nearly represents their relationships. 
