CHAPTER XV 
THE PLANT GROUPS 
147. Differences in structure. It is evident, even to the 
casual observer, that plants differ very much in structure. 
They differ not merely in form and size, but also in com- 
plexity. Some plants are simple, others are complex, and 
the former are regarded as of lower rank. For example, 
a lichen, a moss, and an oak differ very much in form and 
size, and also in complexity, and because of this last fact an 
oak would be regarded as a plant of higher rank than either 
a lichen or a moss. It must not be supposed that rank is 
measured by size, for in the highest group there are many 
small plants. 
Beginning with the simplest plants that is, those of 
lowest rank one can pass by almost insensible grada- 
tions to those of highest rank. At certain points in this 
advance notable interruptions of the continuity are dis- 
covered, structures, and hence certain habits of work, chang- 
ing decidedly, and these breaks enable one to organize the 
vast array of plants into groups. Some of the breaks ap- 
pear to be more important than others, and opinions may 
differ as to those of chief importance, but it is customary 
to select three of them as indicating the division of the 
plant kingdom into four great groups. 
148. The great groups. The four great groups may be 
indicated here, but it must be remembered that their names 
mean nothing until plants representing them have been 
studied. It will be noticed that all the names have the 
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