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 ])y the develo]:>ment 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 otlierwise 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 0]:)en- Fig. 22.— Stomata, surface and 



crosa-scctiou. 



ing, slit-shaped or narrowly elli])- 



tical, bordered by two, usually ('hlur()j)liyll-l)(niring, e])i- 



dermal cells, somewhat kidney-shaped, and iiicontact with 



