es 
et 
HAIRS, AND STOMATA 51 
72. Not to be confused with hairs are those outgrowths 
called emergences. These are not epidermal in nature 
but are projections produced by the development of 
cells beneath the epidermis. Often such emergences 
are found bearing, and as it were, forming the support 
for a stout hair, as in the sunflower or nettle. 
73. The presence of hairs seems to be advantageous 
to plants in many ways. They make it difficult for small 
insects to ascend the plant, especially if the hairs are 
pointed downward or are sticky-glandular. Stinging 
hairs like those of the nettle, and also merely sharp- 
pointed stiff hairs, such as abound on many plants, 
are deterrents for animals that would otherwise feed 
on the plant. The same is probably true of various 
evil-smelling substances secreted by some glandular 
hairs. Finally, it has been shown that the presence of 
hairs and scales reduces the loss of water from the plant 
by forming an entanglement for a layer of air, thus 
preventing the air currents from coming into direct 
contact with the epidermis. 
74. Stomata (singular, stoma), or breathing pores, 
are definite openings through the epidermis to air 
cavities beneath, through which an exchange of gases 
takes place. These cavities (‘‘substomatal chambers’’) 
are connected with the intercel- 
lular air spaces throughout the 
plant. 
75. Except in the Liverworts 
(Hepaticae), where the stomata 
are of different structure, the 
typical stoma consists of an open- F's. 22.—Stomata, surface and 
cross-section. 
ing, slit-shaped or narrowly ellip- 
tical, bordered by two, usually chlorophyll-bearing, epi- 
dermal cells, somewhat kidney-shaped, and in contact with 
