28 



DOMESTIC BOTANY. 



forms of the human face, whence it is known as the 

 caricature plant. Many leaves are lucid and reflect 

 light ; some are also in imitation of flowers, as in the 

 common red and blue clary. Leaves are attached to 

 the stem by a footstalk, called the petiole, which is 

 short or long, and in some plants entirely wanting ; the 

 leaf is then called sessile. They generally have a mid- 

 rib, which is a continuation of the footstalk, and are 

 traversed by veins (see Structure) containing vascular 

 vessels ; the whole is enclosed by a skin called the cuticle^ 

 the upper surface being at a greater or less angle with 

 the zenith, and the under facing downwards. The 

 microscope shows both sides of the leaf, but more espe- 

 cially the under, to have pores, in some instances so 

 numerous that a square inch contains many thousands ; 

 these pores are round or oblong, and are called stomata, 

 being the inhalers and exhalers of the atmosphere, and 

 equivalent in their action to the breathing pores and 

 lungs of animals. The diff'erence between the upper 

 and under surfaces of leaves is familiar to common obser- 

 vation, but there are exceptions to this rule. 



In a great number of the trees and shrubs compos- 

 ing the vegetation of Australia, by a twist of the petiole, 

 the leaves stand vertical, that is, the one edge towards 

 the sky and the other towards the earth, both sides in 

 this case having stomata. In the pretty flowering lily 

 genus, Alstromeria, the twist is so great that the true 

 upper side becomes the under. 



Leaves in general are thin and soft, being easily 

 crumpled, soon withering when separated from the plant; 

 or tough like leather ; or they are hard and stiff, as in 

 the cycad family, and many Australian woody plants ; 

 or thick and fleshy, as in aloes and other succulents. 



