22 GENERAL PEINCIPLES. 



a tube, similar to the woody parts of the tree or branch 

 that bears it, inside of which is a pith, similar to the pith 

 of the tree ; the leaf is thus connected with the pith and 

 wood of the shoot, and consequently the ascending sap, 

 as we may readily see, by making a vertical cut through 

 the leaf stalk and shoot. The veins of the leaf are filled 

 up with a cellular substance similar to the pith, called 

 pare7ichyma^ and the whole is covered with a thin skin 

 (ej^idermis). This cellular substance is connected with 

 the inner bark, and consequently the descending sap or 

 cambium, that forms the new layers of wood. Both sur 

 faces of the leaf are furnished with small pores, through 

 which exhalation and absorption are carried on. Absorp- 

 tion is performed principally by the pores of the imder 

 surface, and they are the largest ; exhalation principally 

 by those of the upper surface. 



This property ,of the leaves to receive and give out air 

 and moisture through the pores on their sm-face, has 

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

 this comparison is to some extent correct ; for wx know 

 that without leaves, or organs performing their offices, 

 trees do not grow. And in proportion to their natural 

 and healthy action, 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 sufiering from drought, 

 and see how quickly it becomes refreshed. Dews of a 

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

 and drought of the previous day had prostrated ; and 

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

 recovers. Even the leaves of a boquet can be kept fresh 

 for a long time by sprinkling them with water. 



That plants exJiale moisture and gases cannot be doubt- 

 ed. It is this very exhaling process that causes plants 



