PLANT TISSUES 123 
loss of water. In old stems and roots of secondary growth it is 
ultimately replaced by cork. Its cells are living, containing a 
thin marginal layer of cytoplasm, a nucleus and a large central 
vacuole and frequently leucoplastids. Non-protoplasmic inclu- 
sions as crystals, mucilage, etc., may occur in the lumen. They 
may be brick-shaped, tabular or polygonal, the vertical wall being 
equilateral or wavy in outline. Their outer walls are frequently 
cutinized (infiltrated with a waxy-like, waterproof substance called 
cutin). The radial walls also are sometimes cutinized. 
Fic. 64.—Upper epidermis of Sweet Fern (Comptonia asplenifolia) leaf (surface view) 
showing epidermal cells and two non-glandular trichomes. 
Stomata.—Among the epidermal cells of leaves and young 
green stems may be found numerous pores or stomata (sing. 
stoma) surrounded by pairs of crescent-shaped cells, called guard 
cells. The guard cells contain chloroplasts and are capable of 
undergoing changes in size and shape dependent upon changes 
in their turgor. Increased turgor causes them to open. A 
decrease in turgor causes them to close. The stomata are in 
direct communication with the sub-stomal air chambers beneath 
them which in turn are in communication with intercellular 
space systems of the tissues of the entire plant. Through the 
stomata an exchange of gases between the tissues of the plant 
and the external air takes place. The function of the stomata is. 
- 
