U8S9 OP LEAVES 14S 



teaches us that plants droop, in consequence of the ex- 

 cess of the former, and are to be revived by diminish- 

 ing their discharge, or increasing their absorption. 

 The former is accomplished by confining the air around 

 them ; and the latter, by sprinkling water over the 

 leaves ; and when plants have recently been removed, 

 such management is frequently required. 



Leaves are Organs of Respiration. By the aid of the 

 microscope, Dr. Grew ascertained the existence of air 

 cells in the substance of leaves. He also observed the spi- 

 ral coated vessels, which are particularly numerous in the 

 leaf, as well as in the substance of the alburnum. His dis- 

 coveries led to the conclusion, that leaves absorb air, that 

 their cells receive, and their vessels convey it, to every 

 part of the growing plant. The next experiments were 

 performed with an Air-pump ; by which abundance of air 

 was procured from the leaves, and other parts of vegeta- 

 bles, and the truth of former speculations, more firmly 

 established. It was also demonstrated by Bonnet, whose 

 experiments on absorption, we have already mention- 

 ed, that when leaves are placed in water, and exposed 

 to the influence of the sun, air escapes from their sur- 

 face. But we owe to the labours of Priestly more than 

 to all others. Aided by the resources of modern chem- 

 istry, this distinguished veteran of science, first exhi- 

 bited in its true light, the influence of vegetation on the 

 air we breathe, and presented to the scientific world, 

 a simple yet satisfactory explanation of phenomena, 

 which had eluded the investigations of all his prede_ 

 cessors. 



It is well known, that when animals cease to breathe, 

 they immediately perish. The air by which they are 

 sustained, is no less essential to the healthy existence 

 of plants. But to comprehend and explain this subject. 



