36 



PLANTS AND MAN 



underlying tissue from drying out. In many cases this is effec- 

 tively brought about by a thickened and waxy outer layer known 

 as a cuticle. Since an entirely impervious epidermis would pre- 

 vent the necessary exchange of gases with the atmosphere, the 

 epidermis is perforated by many small pores (stomates) which 

 act as automatic valves, the size of their openings varying with 

 the dryness of the atmosphere and thus regulating the loss of 

 water by evaporation. Through the stomates the carbon dioxide 

 and oxygen can circulate freely between the photosynthetic cells 







Fig. 15. — Between the upper and lower epidermis lie the parenchyma 

 cells which contain chloroplasts (small black dots in cells) and thus carry on 

 photosynthesis. 



and the air. This water loss through the leaves, technically known 

 as transpiration, presents one of the greatest problems to land 

 plants. Openings are necessary for the gaseous exchanges asso- 

 ciated with photosynthesis and respiration; yet the plant can 

 not select which gases will pass out, so that transpiration becomes 

 a necessary evil. Reduction of transpiration loss by desert plants 

 has been brought about by various adaptations such as hairy coat 

 to the leaves, stomates in depressions, or elimination of the leaves 

 entirely as in the cacti. 



Between the upper and lower epidermis (fig. 15) lie the 

 parenchyma cells which carry on photosynthesis. The upper 



