6o PLANT LIFE. 



may be divided into three zones, characterized as follows. 

 At the extreme tip new cells are being formed by division ; 

 below this the newly formed cells rapidly increase in size ; 

 still further back, the meristem is undergoing differentiation 

 to form the permanent tissue-systems of the complete 

 structure. 



In Cryptogams the meristem originates from a single cell 

 situated at the apex of the growing organ ; this is known as 

 the apical cell. The meristem is formed from the apical cell 

 in different ways, which will be noticed under the various 

 groups. Exceptions to this method occur in a few members 

 belonging to the fern group, where there is not a single 

 apical cell, but several cells of equal value. 



In Phanerogams there is no apical cell, but a number of 

 cells are present at the apex which divide and form the 

 meristem. 



Cork or Periderm is best developed in Gymnosperms and 

 Dicotyledons, where as a protective tissue it generally re- 

 places the epidermis after the first year in organs that 

 continue to increase in size, as stems and roots. Cork cells 

 usually originate from the primary cortex, rarely from the 

 inner layers of the epidermis when the latter divides. A 

 layer of cells usually some distance below the epidermis 

 divides tangentially or parallel to the surface, the outer row 

 becomes converted into periderm, while the inner row 

 remains alive and forms phello^en or cork-cambium, which 

 forms cork annually on its peripheral side, and in addition 

 often produceson its inner side a layer of chlorophyll-bearing 

 cells called the phelloderma. Phelloderma is characterized 

 by the oblong cells when seen in a transverse section, being 

 arranged in radial rows with the sides parallel to the surface 

 of the organ, and the absence of intercellular spaces. The 



