THE STEM. 53 



have taken modified forms corresponding to special func- 

 tions that they have assumed. Thus many stems, Modified or 

 a large proportion of which are subterranean, derived forms, 

 serve chiefly as reservoirs of reserve materials, and in the 

 course of time have undergone striking modifications both 

 of form and structure. The tuber of the common potato 

 shows all the essential characters of a dicotyledonous stem 

 in the formation of buds, the concentric arrangement of 

 pith, wood, and bark, and in still other respects, but the 

 fibrous tissue has almost wholly disappeared, while the 

 cellular tissue has increased to such an extent as to give 

 the tuber the appearance of a monstrosity compared with 

 the ordinary branches of the same plant. Quite as strik- 

 ing changes are seen in branches that have taken the form 

 of spines and assumed the function of protection. Good 

 examples of these are the spines of the hawthorn and 

 other familiar plants. Even more remarkable modifica- 

 tions are presented in the leaf-like organs known as 

 cladophylls. In the case of the so-called smilax of the 

 greenhouses, the true leaves are inconspicuous scales, 

 while the cladophylls so perfectly simulate foliage leaves 

 as to deceive an inexperienced eye. Much caution is 

 necessary in studying the morphology of these and other 

 modified branches. Their position on the stem, structure, 

 and mode of growth, and any tendency they may exhibit 

 to become ordinary leaf-bearing shoots, are all to be taken 

 into account. 



In their anatomical structure and mode of growth, stems 

 present well marked peculiarities, which, as already stated, 

 are sufficiently characteristic to admit of the Anatomical 

 ready determination of the great class to which ^^ e and 

 a plant belongs. The stems of a large propor- growth, 

 tion of monocotyledons are well represented by that of 



