MORPHOLOGY OF HIGHER PLANTS. 



323 



other hand, the shoot-axis may be modified so as to increase the 

 assimilating surface, as by a flattening of the axis, as in some of 

 the Cacti, the leaves being suppressed or considerably reduced. 



FIG. 181. Bryonia dioica. a, young, spirally coiled tendril; b, expanded and irritable 

 tendril; c, tendril which has grasped a support; d. tendril which has not grasped a sup- 

 port, and has undergone the old-age coiling. After Pfeffer. 



Branches are not infrequently modified to hard, pointed, and 

 spiny structures, as in the Japanese quince, when they are spoken 

 of as thorns. Leaves and even flowers may arise upon thorns, 

 which shows that they are modified branches. 



