March, 1905.] The ClassificaHon of Plants, I. 299 



include the recognition of both vertical and horizontal develop- 

 ments. In a general way the morphological characters which 

 represent the progress from unicellular to the most complex mul- 

 ticellular forms are of very great importance in placing any group 

 of organisms in the scale. But of still greater importance is the 

 character of the life cycle. 



If all tvpes of plants evolved m the past had remained to the 

 present dav, it would be possible to devise a scheme which would 

 show the natural relationships of all species and larger groups by 

 verv close connecting links. But in the evolutionary process 

 plants passed through critical stages where it was hardly possible 

 for them to tarrv. The species must either remain below the 

 critical condition or else advance farther in order to meet the 

 requirements of the newly acquired structures, habits, or func- 

 tions. The changing conditions of the earth's surface have had 

 a profound effect as well as the mere fact of a great diversity of 

 conditions. One of the most important factors in the problem 

 of classification arises out of the changing environments to which 

 plants were subjected during the geological history of the earth. 

 In finding a basis for the determination of evolutionary advance- 

 ment or retrogression, therefore, the ecological factor also be- 

 comes one of great importance. The change of functions and 

 activities in passing from one set of conditions to another is some- 

 times considerable. As one would expect, then, there are breaks 

 in the continuitv of plant groups and these breaks frequently 

 mark the transition to life in a dift'erent environment. 



There are two very important gaps which divide the entire 

 plant kingdom, as we have it at present, into three distinct 

 groups. Each succeeding group is in all essentials more highly 

 developed than the preceding and shows no very close relation- 

 ship to it, the intermediate forms having been lost. These three 

 groups may be called series and we can then say that the plant 

 kingdom is divided into three series. 



The first great hiatus occurs at the point where plants adapted 

 to a water habitat passed out to aerial conditions. It represents 

 the grand transition from water to aerial moist ground plants. 

 This transition must, however, not be confused with those cases 

 where plants having a body adapted to a water condition, typ- 

 icallv filamentous in form, merely changed sufficiently to endure 

 an aerial habitat. 



The second great hiatus marks the boundary line between 

 those plants, on the one hand, which are still dependent on con- 

 siderable moisture for one generation and on the presence of free 

 water to accomplish fertilization and the plants, on the other 

 hand, which have been practically weaned from the necessity of 

 free water during any period of their life cycle. In these highest 



