THE LEAVES. II3 



may be flat, but broad leaves are commonly folded or rolled 

 in various ways. , 



141. 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 compara- 

 tively limited. Roots arise from leaves in precisely the same 

 way as lateral roots arise from stems (^ 84), that is, they are 

 internal in their origin, and begin to develop always near the 

 surface of a stele. 



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

 from the green tissue underlying it, never from a stele. 

 Shoots thus arise from the part of the leaf correspond- 

 ing to that from which branches arise upon the parent 

 shoot. 



142. Leaf fall. — Leaves, like roots and stems, undergo 

 certain secondary changeSj but these are neither so common 

 nor so extensive as in the other two members. 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 

 transverse plate of cells, some of which may become trans- 

 formed into cork, making 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- of water to this crevice, and its 

 freezing, serve to rupture the remaining tissues, and thus 

 allow the leaf to fall by its own weight, or to be torn off by 

 the wind. 



The scar left by the fall of the leaf is protected either by 

 the cork already produced, or by mere drying of the exposed 

 tissues. The leaflets of compound leaves fall in like manner. 

 Sometimes provision for the leaf fall is begun as early as 

 June, as in the Kentucky coffee-tree. In other plants pro- 

 vision for leaf fall is begun late in the season, and in some, 

 such as the oaks, it is very imperfect, so that the leaves are 



