138 Introduction to Botany. 



least resistance to modify old members rather than to pro- 

 duce entirely new ones. 



110. Characteristics of Morphological Elements. — Since 

 there is such diversity in form as well as in function of 

 roots, stems, and leaves, how are we to tell when we are 

 dealing with these members ? By a comparative study, we 

 find that there are certain characters which appear so fun- 

 damental as to furnish reliable evidence for recognizing 

 the members to which they belong. Thus, roots are out- 

 growths from stems or from other roots, and do not, as a 

 rule, arise in definite order or definite angular divergence, 

 except in the case of the secondary roots of seedlings; 

 and although they may bear adventitious buds, they do not 

 directly bear leaves. Stems directly bear roots, leaves, and 

 buds ; and most lateral stems arise either in, or just above, 

 the axils of leaves. Leaves are borne directly on stems ; 

 they have a definite angular divergence, and they commonly 

 bear buds in their axils. Mere outgrowths of the epidermis, 

 or bark, such as hairs and prickles, differ structurally from 

 the morphological elements in containing none of the parts 

 of a vascular bundle, such as wood fibers and tracheal tubes. 

 Whatever form, size, structure, color, or function a member 

 may have, if it possess a set of characteristics as above 

 stated we may classify it accordingly. 



111. Modified Roots. — Roots are very commonly modi- 

 fied to serve as storehouses for nourishment ; in such cases 

 they consist for the most part of thin-walled tissues, to and 

 from which the reserve materials can readily pass in solu- 

 tion. The dahlia and sweet potato afford familiar exam- 

 ples of roots of this kind. In our western plains, Ipomcea 

 leptopliylla has a storage root weighing from 10 to 100 

 pounds. The cHmbing roots of the trumpet creeper occur 

 in clusters at the nodes, while those of the poison ivy occur 



