138 LEAVES AND BUDS. 



activity of such leaves is nearly over, and they have been 

 partly emptied of the food- substances they contain, a layer of 

 cork-tissue forms across the base of the leaf, and by inter- 

 posing an impermeable layer between leaf and stem, cuts off 

 from the former all supplies of water and mineral salts. 



The exact structure and development of this absciss-layer 

 cannot be fully gone into here, but what occurs in the actual 

 " fall " of the leaf is the splitting of this layer in such a way 

 that the leaf only hangs on by the veins which pass from it 

 into the stem, and then these snap, leaving the scar of 

 attachment of the leaf already healed and covered with cork. 

 The process is a vital one due to the activity of the living 

 plant, for if a branch is killed by means of hot water the 

 leaves wither upon it without being shed. On the other hand, 

 leaves can be made to " fall " in summer by wrapping them in 

 a damp cloth. 



"Whenever the plant requires to throw off certain organs at 

 a definite time (petals and other floral parts after fertilisa- 

 tion ; fruits and seeds when ripened) it generally makes use 

 of similar absciss-layers. 



172. Buds. Before the young stem branches, it is in 

 most cases easy to see that between the base of each leaf 

 and the upper portion of the stem (i.e. in the axil of the leaf) 

 there is a young bud (axillary bud or side-bud), which may 

 grow into a shoot bearing leaves only, or flowers only, or both 

 leaves and flowers. The apex of the stem is usually occupied 

 by a terminal bud (end-bud). 



Buds appear as little prominences on the outer surface of 

 the stem, and are produced by outer tissue, whereas secondary 

 roots arise from within. Each bud consists of a short " con- 

 densed" stem in which the " internodes " are undeveloped, 

 so that the young leaves are closely packed together. The 

 formation of a bud depends on the fact that the slowly 

 growing end of the shoot elongates less than the leaves which 

 it has produced, and these bend over its tip and protect it. 

 The apex of the stem of the bud is occupied by a growing- 

 point, which grows and elongates when the bud opens ; since 

 it is not exposed to friction against the soil, there is no need 

 for any such structure as the root-cap, which covers and 

 protects the growing apex of the root. 



