PAET II 



CHAPTER XVII 



THE PRINCIPLES OF MORPHOLOGY, EVOLUTION, AND 

 CLASSIFICATION 



187. Morphology. Morphology treats of the form and struc- 

 ture of a plant or animal. The lowest organisms have a simple 

 morphology, but the higher plants and animals are made up 

 of many parts or organs, and consequently their morphology 

 is very complex. Organs are structures set apart or developed 

 for a definite kind of work. Thus the roots of a plant are organs 

 usually employed to attach the plant to the ground in order that 

 it may absorb soil moisture. 



One department of morphology (comparative morphology) 

 deals with the various forms or disguises which the same sort 

 of organ may take in different kinds of plants, and compares 

 these structures with one another. For example, the foliage leaf 

 is a well-defined organ which can be recognized at a glance ; but 

 it requires some study to understand that the scales on the bud 

 and around an onion, and also some forms of spines and tendrils 

 are morphologically leaves, that is are leaves variously modified. 

 Because all of these structures are related to one another they 

 are called homologous, and morphology studies the homologies, or 

 relationships, of organs. Comparative morphology is one of the 

 most interesting subjects of biological study, since it furnishes 



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