THE GREAT GROUPS OF PLANTS 159 



liverworts. The lowest group is the thallophytes, the lowly or 

 prostrate plants, which group includes the fission plants, the 

 fungi and the algce. It is the last-named group, the thallo- 

 phytes, which we shall first consider. 



145. Some aspects under which plants are studied. The 

 study of the great groups, their subdivisions, and the proper 

 classification of plants, is known as taxonomy, or systematic 

 botany. The study of plant structures, their similarities, differ- 

 ences, and relationships is known as morphology. Special study 

 of the cell is cytology. Plant activities or work and their rela- 

 tions to the immediate surroundings of the plant are included 

 in physiology, while the relationships of plants to one another 

 and to the environment in general is ecology. One phase of 

 ecology deals with the distribution of plants over the earth 

 and is known as ecological plant geography. The study of 

 plant diseases is known as phytopathology, or plant pathology. 

 A study of the bacteria constitutes bacteriology. A considera- 

 tion of the useful or harmful aspects of plants is included 

 under the general term economic botany, and under this head 

 there are such subdivisions as agricultural and horticultural 

 botany. These are but the leading aspects under which plants 

 may be studied. 



It is evident that these divisions have no sharply marked 

 lines between them, and that they are not all made upon the 

 same basis. For example, we might study the bacteria as 

 shown in their structure, which would be morphology ; or as 

 shown in their relation to disease, which would be pathology ; 

 or in their relation to farm and garden crops, which would be 

 economic botany. 



146. Names of plants and groups not most important. It is 

 impossible to study plants in any extended way without hav- 

 ing definite names for them and their parts, as well as for the 

 different kinds of work that they do. We need names of the 

 different people whom we know in order that we may speak 

 of them in a definite way. If we had not these names we 

 should constantly have to use long descriptions that would 



