140 BOTANY part i 



also of one' of the Maples (Acer striatum), even when a foot 

 or more thick and over forty years old, remain covered with a 

 living, growing, epidermal layer. As a rule, however, the epider- 

 mis on stems which grow in thickness becomes stretched and finally 

 ruptured. The tissue of the primary cortex, by the expansion and 

 division of its cells, can accommodate itself more easily than the 

 epidermis to the increased dimensions of the stem, arising from the 

 growth in thickness of the central cylinder. This process of cortical 

 growth is particularly noticeable in the primary medullary rays 

 (Fig. 145, pm') between the primary phloem. The formation of the 

 periderm generally begins during the first vegetative period, after the 

 secondary growth has reached a certain stage. The commencement of 

 its formation is indicated by the brown colour of the external surface 

 of the stem, which, however, remains green so long as the epidermis 

 continues alive. The periderm is derived from a secondary meristem, 

 termed the cork cambium or phellogen. This phellogen may arise, in 

 the epidermis, in a deeper layer of cells of the primary cortex, or even 

 in the pericycle itself. The cells of the phellogen divide by tangential 

 walls, and also, at times, by radial walls, in order to accommodate 

 themselves to the increasing thickness of the stems. Of the new cells 

 thus formed, those given off towards the periphery of the stem are the 

 CORK CELLS (Fig. 154, I). They usually have a tabular shape, fit 

 closely together without intercellular spaces, and possess suberised, 

 secondary, thickening layers. The cork cells are, for the most part, 

 filled with air, containing also a yellow or brown substance, and usually 

 possess brown walls. The cell walls may be thin or thick, frequently 

 thickened on one side, and occasionally to ^uch an extent that they 

 are known as stone cork. The cork tissue frequently shows an 

 alternation of thick-walled and narrow with thin-walled and larger 

 cells. These layers mark annual growths. The cork cells, being 

 impermeable to water, prevent the loss of moisture by transpiration, 

 while at the same time they shield and protect the inner tissues. An 

 example, showing how effectively cork cells retard transpiration, is 

 afforded by a potato, which, when peeled and so deprived of its 

 protecting cork covering, loses in twenty-four hours, according to Eder, 

 about sixty-four times as much water as it would otherwise have 

 done. 



The cork of the Cork-oak (Quercus Suber) is formed of broad layers of soft 

 large cells, alternating with narrow and thinner layers of cells, which mark the 

 limit of the annual growth. This may be seen in bottle - corks. The first, 

 spontaneously developed cork of the Cork-oak is stripped off, whereupon a new 

 phellogen is formed in the deeper -lying tissue. The cork thus produced is 

 removed every six or eight years, and furnishes the cork of economic value. 



In many cases the phellogen takes its origin in the epidermis (Fig. 155). 

 This is the case in the "Willow, in all Pomaceae, and in a great number of other 

 woody plants. The epidermal cells become divided into outer and inner cells, the 



