DEFINITIONS OF THE ORGANS 137 



has its particular features of organization and can be traced as a 

 definite and distinct structure far into the geological past of our 

 existing plants. Of the organs the root is the one usually distin- 

 guished by subserving the function of attachment to the substratum 

 and also that of absorption of nutritive substances in solution from 

 the soil. In the case of the root, also, the direction of growth is 

 normally and primitively downward. The stem has an upwardly 

 growing axis which serves as a support for the other parts of the 

 plant. The leaf is distinguished in turn by its usually flattened 

 form, which particularly qualifies it for its important function of 

 bringing the organism into relation with light and the gases of the 

 atmosphere. The sporangium has the particular office of producing 

 spores and may consequently with a certain degree of appropriate- 

 ness be designated as the organ of reproduction. The indications 

 supplied above as to the roles ordinarily played by the respective 

 parts of the higher plants by no means afford reliable definitions of 

 those organs. Very frequently stems, more rarely leaves, and 

 sometimes even sporangia serve in connection with the function of 

 attachment, so that the relation to the substratum cannot be 

 assigned as a fundamental and exclusive feature of the root. 

 Similar objections may be raised in regard to the functional defini- 

 tion of all the parts or organs of plants. In the present connection 

 it will be well to consider these important categories of structure 

 from the standpoint of organization rather than function, as that 

 procedure is most advantageous from the evolutionary point of 

 view. 



THE ROOT 



There is good reason to believe that the root is on the whole 

 the most primitive of plant organs, and it will be shown in subse- 

 quent chapters that, even if there be room for doubt as to its primi- 

 tiveness, there can be none as to its conservative character. There 

 is, in fact, no organ of plants so antique in its organization or so< 

 retentive of ancestral traits as is the root. Functionally, as has 

 been indicated above, it serves usually to connect the plant with 

 the substratum. Its unique and distinguishing features, however, 

 are supplied by its internal organization. Roots are characterized 

 by two main structural features namely, the possession of a root 



