THE LEAVES. 



139 



the basal growth of a single pair of persistent secondary 

 leaves is continued throughout the long life of the plant, 

 while the tips die and are frayed out. 



173. Production of the other members. — Leaves give rise 

 under certain conditions to roots or to shoots. The number 

 of plants, however, in which this occurs is comparatively 



Fig. 166. — Development of the pinnately compound leaf of the locust {Robinia Pseud- 

 acacia). A, young stage, showing on one flank the first lateral growing-point a, 

 which is to produce the lowest leaflet. B, an older stage with the fifth growing-point 

 x just showing. A sixth is still to be developed. The hairs in A and B are on the 

 back (under side) of the leaf, and drop off early. C, nearly mature leaf. A, B, 

 magnified ; c, about i natural size.— After Frank. 



limited. Roots arise from leaves in precisely the same way 

 as lateral roots arise from stems (^f 95), that is, they are en- 

 dogenous in their origin, and develop always near the surface 

 of the steles. 



When a leaf produces a shoot, it is from the epidermis or 

 from the green tissue underlying it, never from the steles. 

 Shoots thus arise from the part of the leaf corresponding to 

 that from which branches arise upon the parent shoot. 



174. Secondary changes. — Leaves, like stems and roots, 

 undergo certain secondary changes, but these are neither so 



