Page 141 - Fundamentals of Air Pollution

I. Injury versus Damage 111

Fig. 8-1. Biological response spectrum for plants and air pollution.

Air pollutants may enter plant systems by either a primary or a secondary
pathway. The primary pathway is analogous to human inhalation. Figure
8-2 shows the cross section of a leaf. Both of the outer surfaces are covered
by a layer of epidermal cells, which help in moisture retention. Between
the epidermal layers are the mesophyll cells—the spongy and palisade
parenchyma. The leaf has a vascular bundle which carries water, minerals,
and carbohydrates throughout the plant. Two important features shown
in Fig. 8-2 are the openings in the epidermal layers called stomates, which
are controlled by guard cells which can open and close, and air spaces in
the interior of the leaf.

Fig. 8-2. Cross section of leaf showing various components.