Modified Parts. 135 



is it composed of bark, wood, and pith, or of one or two of 

 these ? The evidence under 4 is not entirely reliable, since 

 the modification of a structure may involve the suppression 

 of some of its tissues. Before proceeding with the study 

 of modified structures, write out the characteristics under 

 I, 2, and 3, of a typical root, stem, and leaf (see Discussion 

 no). It must be remembered that form and physiological 

 function do not furnish reliable evidence as to the mor- 

 phology of a modified structure ; this should be tested as 

 you proceed with the studies outlined in this chapter. 



109. Make a drawing of a spine of wild plum, showing 

 its form, size, mode of attachment to the stem, and position 

 on the stem with reference to nodes and internodes. Show 

 its relation to buds, branches, leaves, or leaf scars. To 

 show the attachment to the stem, make a median longitu- 

 dinal section through the spine and the branch which bears 

 it, and draw the cut surface. 



no. Give the morphology of the spine, that is, state 

 what one of the morphological elements has been modified 

 to form this structure, and give in your notes all of the 

 evidence for your decision, as suggested under i, 2, 3, etc., 

 of the introductory note. Refer to the details of your 

 drawings by letters or numbers for each point of evidence. 

 Determine the probable function of the spine, and give 

 reasons in full for your conclusion. It is not possible to 

 demonstrate a useful function for every structure. Some 

 modified structures seem to have arisen either from lack of 

 nutrition or from abundant nutrition, and in either case 

 they may, or may not, be useful. The student should not 

 attribute a function to a structure without good evidence. 



III. In a similar manner, work out the morphology of 

 the spines of the honey locust, black locust, prickly ash, 

 and osage orange. 



