140 PLANT LIFE. 



common nor so extensive as in the other two members. The 

 formation of several to many layers of cork cells upon the 

 surface of scale leaves is not uncommon. Occasionally simi- 

 lar layers of cork are formed upon the petioles of ordinary 



Fig. 167. — Welwitschia mirabilis, a coniferous plant of Africa, showing two leaves 

 which grow at base and continue to develop throughout the many years which the 

 plant lives. The tips are dead, and become worn and frayed by winds. 5 ^ natural 

 size. — After Hooker. 



foliage leaves. In some cases the large vascular bundles 

 occupying the main ribs undergo changes similar to those de- 

 scribed for the bundles of the stem (^f 141), by which sec- 

 ondary wood and bast are produced. 



175. Leaf fall. — One of the secondary changes of most 

 importance is the preparation for the fall of the leaf. This is 

 made by the formation of a layer of secondary meristem across 

 the leaf base at or near the point where it joins the stem. 

 The cells at this point, with the exception of those constitut- 

 ing the vascular bundles, begin a series of divisions at right 

 angles to the axis. A transverse plate of cells is thus formed, 

 some of which cells may become transformed into cork, mak- 

 ing a line of weakness ; or, without such alteration, the 

 cells may round themselves off by loosening along a definite 

 line, so that the leaf is held only by the steles. The access 



