42 



C H A P T E K I V. 



MERISTEM. 



Thk various organs and adult tissues of which a plant is made up 

 arise from young undifferentiated tissue which occurs in certain 

 positions, notably at the apices of young shoots and roots, and 

 in the form of zones of dividing cells in stems and roots, and 

 at times in other positions ; this rudimentary tissue is known as 

 meristem, and the first few cells of this tissue as promeristem, 

 A meristematic tissue from which the primary tissues arise 

 (viz., that producing the first wood and bast, and the epidermis, 

 pith, and cortex) is known as a primary meristem, whereas that 

 arising from previously differentiated cells (viz., that producing 

 cork, secondary wood and bast) is known as secondary meristem. 

 Amongst the lattei' \vould be classed the various cambiums met 

 with (cambium proper, cork-cambium, pericycle) and certain 

 layers known as intercalary meristem, which arise in such organs 

 as young leaves towards the base, and which, in this ease, 

 function in the trans^'erse growth of the leaf. 



Meristem may thus be defined as " a tissue which, during 

 some part of the existence of a plant, is, as regards its component 

 cells, either in a condition of active cell-formation or else remains 

 capable of renewed activity after periods of quiescence." The 

 cambiums may be described as zonal meristem. 



A. DIVIDING CKLLS OF ANY KUDIMENTARY TISSUE AND 

 THEIR MODE OF GROWTH. 



Cells of embryonic tissues are in structural details similar to 

 the type of young undifferentiated cell which was examined in 

 Chapter ii., B., where it was seen that protoplasm almost filling 

 the cell-cavity, large nucleus, cell-sap, and plastids were the main 

 cell-contents. 



In a tissue where rapid cell-formation is in progress (meristem), 



