CHAPTER VII. 

 MODIFIED PARTS. 



PROVIDING MATERIALS. 



Material for the study of the morphology of spines can be procured 

 dt any time of the year, but branches for the study of the spines of bar- 

 berry should be gathered while in leaf, and either pressed or preserved 

 in 2 % formalin. The wild smilax, whose tendrils are excellent material 

 for morphological study, should be gathered during the summer and 

 preserved as suggested for barberry. The greenhouse smilax can, of 

 course, be obtained at any time of the year from greenhouses ; and 

 nothing could be better than this for testing the ability of students in 

 seeking out morphological evidence. Utricularia should be gathered 

 from ponds and shallow lakes during the summer and preserved in 2 % 

 formalin. 



OBSERVATIONS. 



Most plant members are either roots, stems, or leaves, 

 and for the sake of classification these may be taken as the 

 morphological elements, although some structures have a 

 different origin. To determine the origin of a modified 

 structure, that is, whether it is a root, stem, leaf, etc., 

 evidence along the following lines should be sought : 

 (i) arrangement; (2) relation to other structures; (3) tran- 

 sitional forms; (4) construction. Thus, under i, is there a 

 definite arrangement, a fixed angular divergence .'' Under 2, 

 does the structure have a definite and constant relation to 

 a leaf or bud.? Under 3, can forms be found which are 

 less and less like the modified form in question, and more 

 and more like a typical morphological element .' Under 4, 



134 



