Modified Parts. 



137 



having roots, stems, and leaves, they seem to have found in 

 these a sufficient number and kind of members for the suc- 

 cessful nutrition of the individual (we may consider struc- 

 tures of the nature of hairs and prickles as outgrowths of 

 roots, stems, and leaves without grouping them as distinct 

 morphological elements). We find very few plant members 

 which may not be classified certainly as either roots, stems, 

 or leaves, and for this reason we 

 may speak of these members as the 

 morphological elements, without im- 

 plying that no other members may 

 sometimes occur. Thus, when for 

 any reason plants have need of new 

 structures, it is their habit to modify 

 one of these elements to meet the 

 new demand. When Solanum jas- 

 minoides, for instance (Fig. 67), was 

 acquiring the climbing habit it put 

 into its petioles sensibility to contact, 

 and power to respond in such a way 

 as to twine about the object with 

 which it came in contact. Or, when 

 the turnip began to store up food 

 for use in the succeeding year, in- 

 stead of producing a new member 

 as a storehouse for reserve food, its tap root was incited 

 to increase in size sufficiently for this purpose. So, too, 

 when buds were to be protected, internodes were kept 

 short, and the leaves were so modified as to enwrap the 

 tender parts in the form of tough resistant scales. Ex- 

 amples of this kind might be cited at great length. In 

 acting in this way, plants have shown themselves to be 

 wise economists, for it is certainly moving along lines of 



FIG. 67. 



Shoot of Solanum jasminoi- 

 des, showing the leaf peti- 

 oles acting as tendrils. 

 After GRAY. 



