244 BOTANY: PRINCIPLES AND PROBLEMS 



That record of remote events which has survived to our day in 

 the form of fossil plants makes it clear that the panorama of the 

 earth's vegetation has repeatedly changed, that group after group 

 has arisen, flourished and disappeared, and that thousands of 

 species have evolved only to become extinct. Plants of today 

 are the product of a long period of evolutionary development, 

 and to understand the vegetable kingdom and its relationships 

 we must therefore know something of the main events in its 

 history. 



Plants and Animals. — Plants and animals constitute the two 

 great branches of the organic world. In their lowest represen- 

 tatives they are often hard to distinguish from one another, and 

 certain simple forms exist which clearly combine the characters 

 of both kingdoms. The earhest of living things would perhaps 

 have been difficult for us to classify, but as evolution progressed, 

 animals and plants became clearly distinguishable through the 

 development among their members of certain characteristic 

 traits. The animal tends to be motile, to ingest its food through 

 a mouth, and to depend on other organisms as sources of food 

 supply; the plant, to be stationary, to absorb its nutrient mate- 

 rials in solution over a considerable area of the body, and (except 

 in the fungi and a few others) to manufacture its own food from 

 simple inorganic substances. Many other differences in struc- 

 ture and function are associated with these fundamental ones. 



Forward Steps in Plant Evolution. — During the divergent 

 history of the two great groups, certain notable events took place 

 in each with which the student of biology should become familiar. 

 Before discussing the classification of the plant kingdom which 

 follows in the succeeding chapters, we shall therefore consider 

 briefly a few of the important steps which have marked its 

 development. 



The causes which led to the appearance on our earth of the 

 first living things, and the characteristics which these primitive 

 organisms displayed, are buried so deeply in antiquity that we 

 shall probably never discover them. There is good reason to 

 beheve, however, that among the earliest of all plants, thriving 

 in the warm primeval seas, were simple, single-celled forms which 

 multiplied by simple division or fission, possessed chlorophyll or a 

 similar substance, and in their general characters were not 

 greatly unlike some of the simplest of our living algae. For 

 ages they doubtless were the only vegetable life on the globe. 



