400 PART IV. VEGETABLE TAXONOMY. 



frozen Greenland the remains of a temperate flora closely related 

 to our own, but there is a close resemblance between the arctic 

 floras of all the northern continents, and an increasing diver- 

 gence of forms as we pass along the line of the continent south- 

 ward. For example, many species of the extreme north of 

 Europe are identical with those of northeastern North America ; 

 the flora of central Europe and that on the corresponding iso- 

 thermals of eastern North America still resemble each other, but 

 the species are mostly different ; still greater differences exist 

 between the floras of southern Europe and the southern United 

 States, while the floras of Africa and South America are quite 

 widely different, not only in species but in many of the genera 

 and in some of the natural orders. 



In our study of Organography we found the same organ exist- 

 ing under a great variety of modifications, now adapted to one 

 function, now to another, the rhizome, tuber and runner were 

 modified stems, and the bud-scale, petal and stamen were modi- 

 fied leaves ; in some plants we found no differentiation of organs, 

 in others it was partial, in still others, complete. Vegetable His- 

 tology taught us that all plants had for their structural unit the 

 cell, and that the cell is essentially the same in all plants ; some 

 plants, we found consisted of a single cell, others of simple aggre- 

 gates of like cells, still others of aggregates of cells of different 

 kinds, together forming a complex structure possessing various 

 organs. We found, moreover, that every plant, however complex 

 it may be at maturity, begins its life as a single cell. In our 

 study of Vegetable Taxonomy, also, we were able to trace deep- 

 seated relationship between forms apparently quite distinct. In 

 fact, all our studies of plants lead us irresistibly to the thought of 

 their genetic relationships — to the belief that, in the course of 

 time, all of the immense wealth of plant forms that populate the 

 modern world, were derived by gradual modification from one or 

 a few simple forms in the past. This view, the geological history 

 of plants, fragmentary though it be, tends strongly to confirm 

 and establish. 



