THE STEM 



249 



the bark becomes smooth as in the Banyan tree. In some trees, 

 e. g., the Mango, layers of cork are formed deeper inside the 

 cortical tiss«e with the result that any rind tissue lying outside 

 such cork is starved and perishes, and thus increases the thick- 

 ness of the cork. 



The Outer Bark, as the dead cork-cells together with the 

 dead tissue of the inner bark is called, comes off it may be in 

 flakes, as in the Jack, or in long vertical stripes, as in the Euca- 

 lyptus or Casuarina, or in horizontal shreds, as in the Rubber 

 tree, or in rounded and hollow shell-like pieces, as in the Awla 

 tree, or in thick rugged pieces, as in the Mango tree. 



{c) The Cortical Pores. — The in- 

 ner part of a stem requires aeration 

 no less than a leaf. Hence the epi- 

 dermis in young stems is supplied 

 with air pores (stomata). When the 

 epidermis gives way to a cork-coat, 

 new ways of air communications 

 must be opened. Such are the corti- 

 cal pores or lenticels, formed by 

 loosely arranged tissue, through 

 which the air can pass in and out. 

 Such cortical pores are very distinct 

 in the stems of Alstonia as small 

 brown or gray pustules. (See also 

 page 221.) 



{d) The Covering of Wounds. — 

 When leaves become old and unfit 

 for their functions, they are dropped. 

 But before they fall, a corky layer 

 is formed across the leaf-stalk at its 

 base, and when the leaf is disrupted, 

 there remains no wound on the sur- 

 face of the stem. If a stem is wounded, the living cells adjoin- 

 ing the wound form cork-cells to close it as soon as possible. If 

 the wound is so deep as to lay bare the wood, the cortical tissue 

 round the w^ound grows vigorously and covers the wound. This 



Fig. 229. — Cortical pores in a 



young stem of Elder. (Nat. size.) 



To the right a magnified pore. 



