256 BOTANY: PRINCIPLES AND PROBLEMS 



and leaves, and rapidly developing into a new plant. The many 

 advantages of reproduction by seeds over the old method of 

 wind-blown spores and independent gametophytes are obvious, 

 and it is easy to see why the seed-plants have become so dominant 

 and successful. 



Plant Classification. — As the result of these slow, progressive 

 changes, which have been working themselves out gradually 

 through millions of years, we see around us the plant kingdom of 

 today, occupying almost every corner of surface on land and sea 

 and consisting of an enormous variety of species. The task of 

 the science of taxonomy is not merely to list and describe these 

 species, but to classify this great array by distinguishing and 

 bringing together groups of species which resemble each other, 

 thus reducing our knowledge of the plant kingdom to that orderly 

 arrangement which is the aim of all science. Ever since the dawn 

 of botanical study, men have been endeavoring to construct 

 such a classification and the results are very diverse. One of 

 the earliest attempts used the resemblance in growth-habit as 

 a basis of classification and divided plants into three groups, — 

 trees, shrubs, and herbs. As botanists learned more about the 

 vegetable kingdom, such crude systems were seen to be wholly 

 inadequate, and resemblances of a much more deeply-seated 

 kind began to be noted, based on a larger number of characteris- 

 tics. Thus the conception of plant "families" began to take 

 form, and the Rose family, the Carrot family, the Legume 

 family, and many others were distinguished and described. 

 There were still wide differences of opinion as to what the groups 

 should be and how they should be subdivided, and there were 

 almost as many "systems" as botanists. Indeed, on the theory 

 which assumed that all plants had been created at the same time, 

 it was difficult to see why these well-marked groups of similar 

 species should exist at all, and there was really no rational foun- 

 dation for any system of classification. 



The establishment of the theory of evolution in the latter part 

 of the nineteenth century, however, threw a flood of light on the 

 whole problem, for it showed that resemblance among members of 

 a plant group was not an arbitrary or chance one but was due to 

 the fact that all the members had descended from a commoji ancestor. 

 Classification became thereupon a definite effort to work out a 

 genealogy or "family tree" for the plant kingdom, or for a given 

 group within it, similar in its type to that which we might construct 



