176 PRACTICAL BOTANY 



epidermis : that when fairly cut through the middle 

 they differ in section from the other epidermal cells : 

 the cell-wall separating them from the adjoining epi- 

 dermal cells is relatively thin, while that adjoining the 

 pore of the stoma is thick, excepting at one point, 

 where the guard-cells are near to, or in contact with, 

 one another. There are external and internal thick- 

 ened ridges on each guard-cell, which appear in section 

 as sharp projecting teeth : these respectively define and 

 partially inclose the front cavity and back cavity of the 

 pore. The cuticle is seen to be continuous through the 

 pore, to the lower surface of the epidermis. Note also 

 the large air-chamber leading into the system of inter- 

 cellular spaces of the cortex. The contents of the 

 guard-cells are conspicuously stained blue with chlor- 

 zinc-iodine. 



The above are the main characteristics of the guard- 

 cells of most stomata, but they can seldom be so readily 

 observed as in this case. 



VII. To observe the ^toma in surface view, and in 

 the living condition, take leaves of Hyacinth, or better 

 of some species of Lilium, in which the stomata are 

 of unusually large size, on a bright day, and after full 

 exposure to the light, strip off a piece of the epidermis, 

 and mount it in water, with the outer surface upper- 

 most, and examine under a low power. It may then be 

 readily seen that the pores of the stomata are widely 

 open, the guard-cells being strongly curved. 



Having seen this, irrigate with a 2 per cent, solution 

 of common salt, keeping watch upon a stoma which 

 has been seen to be open : when the salt solution 

 reaches it, the stoma will be seen to close, the guard- 



