48 Plant Life and Evolution 



leaves are in no sense homologous organs and do not 

 point to any close relationship between the plants 

 which possess them. A proper study of comparative 

 morphology must take into account all the organs of 

 the forms compared, but it also is evident that some 

 of these organs are much more important in hered- 

 ity than others. It is also necessary to distinguish 

 between structures which are readily affected by ex- 

 ternal conditions and those which show evidence of 

 being more permanent in character. The repro- 

 ductive parts are as a rule much more stable than 

 the vegetative organs, and are rightly considered to 

 hold the first place in indicating affinities between 

 plants. For example, the vegetative organs, espe- 

 cially the leaves, are rudimentary in such a plant as 

 the Indian pipe (Monotropa), which has entirely lost 

 the power of the photosynthesis and feeds on organic 

 substances. The flower, however, is but little altered, 

 and shows its unmistakable relationship to the rho- 

 dodendron, huckleberry, and other heath-like plants. 

 Embryology. The study of the development of 

 the plant and of its different organs, or embryology, 

 in its wider sense, is of great importance. Here 

 also great care must be taken in determining what 

 are primitive characters and what are merely adapta- 

 tions. Of late, much attention has been given to 

 Experimental Morphology, or the critical study of 

 the direct effects of various stimuli upon the de- 

 veloping organism. While there is good reason to 

 suppose that much valuable information as to the 



