March, 1905.] 
The Classification 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 in 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 tarry. 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 determiiration 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 continuity of plant groups and these breaks frequently 
mark the transition to life in a different 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 verv close relation- 
ship to it, the intermediate forms having been lost. These three 
grou])s 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- 
ically 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 
