MORPHOLOGY 



143 



merit of a secondary periderm is accompanied by the formation of new 

 lenticels. By means of the lenticels the intercellular spaces of the 

 inner tissues are kept in com- 



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munication with the outer atmo- 

 sphere. The air enters the 

 intercellular spaces of the 

 medullary rays through the 

 lenticels, and is thence dis- 

 tributed throughout the living 

 tissues of the whole plant. In 

 stems in which the periderm 

 is free from lenticels, provision 

 is made for securing the free 

 passage of gases through open- 

 ings left by the overlapping 

 margins of the periderm layers. 

 The Falling of Leaves. — 

 Preparatory to the falling of 

 leaves an absciss layer is formed, 

 by means of which the separa- 

 tion of the leaves from the 

 stem is effected. This layer 

 arises through the division of 

 all the living cells in the plane 

 of separation, including even 

 those of the vascular bundles. 

 At a later stage, a layer of cells 

 in the middle of the absciss 

 layer becomes absorbed, and 

 the separation of the tissues of 

 leaf and stem is completed by 

 the rupture of the tracheal 

 elements and sieve-tubes. The 

 absciss layer is usually formed 

 just before the leaves fall, al- 

 though frequently much earlier. 

 The wound left on the stem 

 either simply dries up, as is the 

 case in the Ferns, or it is closed 

 by a layer of cork, which is 

 formed just below the surface 

 and joins the periderm of the stem. This cork layer may be formed 

 before the fall of the leaves, but in that case it does not extend through 

 the living elements of the vascular bundle, and does not become 

 complete until after the leaves have fallen. The ends of the tracheal 

 elements at the leaf-scars become filled with a protecting gum, and in 



Fig. 157. — Transverse section of the peripheral 

 tissues of the stem of Qltercus sessil iftoro . 1, 2, 3, 

 .Successively formed layers of cork ; pr, primary 

 cortex, modified by subsequent growth ; in- 

 ternally to pc, pericycle ; .*/, sclerenchymatous 

 fibres, from the ruptured ring of sclerenchy- 

 matous fibres of the pericycle ; s, subsequently 

 formed sclereids ; s', sclereids, of secondary 

 growth ; cr, bast fibres with accompanying crystal 

 cells ; l; cells, with aggregate crystals. All of 

 the tissue external to the innermost layer of cork 

 is dead and discoloured, and has become trans- 

 formed into bark, (x 225.) 



