OBOUPa OP TISSUES, OR TISSUESTSTEMS. yg 



ally be readily removed as a thin transparent sheet of col- 

 orless cells. 



69. The change in the form of the epidermal cells is due 

 to the mode of growth of the organ of which they form a 

 part; the lateral and longitudinal growth of an organ causes 

 a corresponding extension and consequent flattening of the 

 cells; if the growth has been mainly in one direction, as in 

 the leaves of grasses, or if the growth in two directions has 

 been regular and uniform, the cells are quite regular in 

 outline; where, however, the growth is not uniform the 

 cells become irregular, often extremely so (Fig. 24, page 38). 



70. The thickening of the walls is greatest in those plants 

 and parts of plants which are most exposed to the drying 

 effects of the atmosphere. It consists of a thickening of 

 the outer walls, and frequently of the lateral ones also. 



'11. The outer portion of the thickened walls sometimes 

 separates as a continuous pellicle, the so-called cuticle, 

 which extends uninterruptedly over the cells, and may be 

 readily distinguished from the other portions of the outer 

 epidermal walls. It is insoluble in concentrated sulphuric 

 acid, but may be dissolved in boiling caustic potash. 

 Treated with iodine it turns a yellow or yellowish-brown 

 color. A waxy or resinous matter is frequently developed 

 upon the surface of the cuticle, constituting what is called 

 the hloom of some leaves and fruits. 



72. The protoplasm of the epidermal cells generally dis- 

 appears in those cases where there is much thickening of 

 the walls; it is always present in young plants and parts 

 of plants; it is also frequently present in older portions, 

 which are not so much exposed to the drying action of the 

 atmosphere, as in roots, and the leaves and shoots of aquatic 

 plants and of those growing in humid places. In few 



