LEAVES. 43 



shoot, and consequently with the ascending sap, as we 

 may readily see, by making a vertical cut through the leaf 

 stalk and shoot. The spaces between the veins of the 

 leaf are filled up with a cellular substance similar to tho 

 pith, called parenchyma, and the whole is covered with 

 a thin skin, epidermis. The cellular substance of the 

 leaf is connected with the inner bark, and consequently 

 with the descending sap, or cambium, that forms the new 

 layers of wood. Both surfaces of the leaf are furnished 

 with small pores, through which exhalation and absorp- 

 tion are carried on ; these are most abundant on the lower 

 surface. 



This property of the leaves to receive and give out air 

 and moisture through the pores on their surface, has 

 caused them to be likened to the lungs of animals, and 

 this comparison is, to some extent, correct ; for we know 

 that, without leaves, or organs performing their offices, 

 trees do not grow ; and, in proportion to the natural 

 and healthy action of the leaves, do we find the vigor and 

 growth of the tree. 



To prove that leaves have the power, in a greater or 

 less degree, to absorb fluids, we have but to apply water 

 to the drooping foliage of a plant suffering from drouth, 

 and see how quickly it becomes refreshed. The dews of a 

 single night, we know, too, will revive plants that the heat 

 and drouth of the previous day had prostrated ; and 

 even if we put a flagging plant in a damp atmosphere, it 

 recovers. The leaves of a bouquet can be kept fresh for a 

 long time by sprinkling them with water. 



That plants exhale moisture and gases, cannot be doubt- 

 ed. It is this very exhaling process that causes plants 

 to wilt under a hot sun, or in a dry atmosphere. Plants 

 that are transplanted with their foliage on, as annuals are 

 in the spring or summer, will wilt, and even die, if ex- 

 posed to the air and sun ; but if transplanted in a moist 

 day, or covered, so that evaporation cannot take place, 



