HEATH AND LING 141 



Both the Ling and the Mediterranean Heath 

 usually grow in places more or less fully exposed to 

 the sun. Their leaves, instead of being flat, are rolled 

 at the edges, so that the lower surface lines a 

 groove. By partially closing one's hand, and imagin- 

 ing that the palm and the lower surface of the 

 fingers correspond to the lower surface of the leaf, 

 one can roughly imitate the groove formed by the 

 inrolling of the edges of the leaf. The groove is 

 filled with minute hairs, springing from the under- 

 side of the leaf, which interlock together and choke 

 the groove. The stomata or pores, through which 

 water- vapour passes, are found only on the underside 

 of the leaf, and the object of the adaptation of the 

 hair-filled groove is to prevent an excessive loss of 

 water by evaporation from the leaf through the 

 pores a matter of importance to a plant growing 

 fully exposed to the sun, and with often a limited 

 water-supply available in the soil for its roots. 



