242 



Practical Plant Biology. 



cortex is limited by the endodermis. This is a single layer of cells 

 with cutinised cell-walls which fit together closely and seem to form 

 a physiological barrier between the cortex and the conducting tract. 

 The air passages in the cortex are cut off by the endodermis from 

 coming into contact with the elements of the wood and bast. 

 Here and there a cell of the endodermis is not thickened or cutinised 

 like its fellows. It seems to have the function of transmitting 

 water from the cortex to the wood and is in consequence of this 



FIG. 87. Tradescantia virginiana (a typical monocotyledon), root-apex, 

 longitudinal section, x 80. a, procambial tract ; b, cortex; c, root-cap. 



called a passage-cell. The passage-cells occur radially opposite the 

 protoxylem-filaments. 



The root grows at its tip, which is formed of a rounded cone of 

 undifferentiated cells. The apex of this cone forms, and is covered 

 over by, a cap of cells in various stages of disintegration. The 

 inner layers of this root-cap next the growing tip are rounded cells 

 in active life ; those further out loosen from one another, and the 

 outermost part connection with one another easily and are cast off. 



