16 EVOLUTION OF PLANTS 
by external influences, and as they represent presuma- 
bly primitive conditions, the importance of a study of 
these early stages of the plant’s existence is evident. 
When we consider the manifold sources of error, it 
is not to be wondered at that botanists have not yet 
been able to establish a perfect system of classification, 
and that it must be a long time before anything ap- 
proaching this can be hoped for. 
The plant kingdom is usually divided into a num- 
ber of primary divisions, “branches,” or ‘sub-king- 
doms,” as to whose limits there is not complete accord 
among botanists. Excluding a number of groups of 
doubtful affinity, sometimes put together under the 
name Protophyta, botanists usually recognize the fol- 
lowing sub-kingdoms: 1. Algwe (green plants below 
the Mosses); 2. Fungi (a group parallel with the 
Alge, but destitute of chlorophyll); 3. Archegoniate 
(Mosses and Ferns); 4. Spermatophyta (seed-plants— 
the “flowering plants” of the older botanists). Of 
these divisions, the Fungi and Algz are often united 
into a single great division, Thallophyta, and the Arche- 
goniatz divided into two sub-kingdoms, Mosses (Bry- 
ophyta) and Ferns (Pteridophyta); but the arrangement 
here given seems to the writer more in accordance with 
what we know of the relationships of the different 
members of these groups. 
