THE CLASSIFICATION OF PLANTS. 217 



263. Plants form an Ascending Serii's. — 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 ichole 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 viake up> the system of classification} 



264. Order of Appearance of Types of Plant Life on the 

 Earth. — Fossil remains of plants are foufid 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 



I See Warming's Systematic Botany, Preface and tltroughont 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. 



