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. — Tlie four great groups may be 

 indicated here, but it must be remembered that their names 

 mean nothing until plants representing tliem have been 

 studied. It will be noticed that all the names have the 



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