Anatomy and Physiology. 87 



Leaves, 



These epitomes of vegetable nature, containing within them- 

 selves the germs of independent existence, are indispensable to 

 the growth of the plant. They are its lungs ; by their means 

 the ascending sap is separated from its superfluous oxygen, and 

 a supply of carbonic acid taken in, and combined with it to fit it 

 for its offices of nourishing the various parts. The quantity of 

 water they perspire is almost incredible. Hales placed a sun- 

 flower, SI feet high, presenting a surface of 5.616 square inches, 

 in the air in favorable circumstances, and found that it perspired 

 from twenty to thirty ounces avoirdupois every twelve hours, or 

 seventeen times more than a man. Many experiments have 

 been tried since on the same«subject. It is found that some will 

 perspire even more in the same ratio than the sunflower ; the 

 amount varying of course with the warmth and dryness of the 

 air and exposure to light ; in damp evenings it is not at all per- 

 ceptible. This can only be understood by explaining the pecu- 

 liar structure of the leaf, which consists of a series of cells, mostly 

 six-sided, covered with two membranes. Looking at these 

 through a compound microscope, we discover an immense 

 number of pores communicating with each other by open lines 

 like network ; these are called stomata; they vary in shape in 

 different plants, but most commonly incline, like the cells, to 

 a six-sided form. They are always most numerous on the 

 lower surface of the leaf, only enough being on the upper, as a 

 general rule, to act as safety valves, and prevent the accumulation 

 of too much watery matter ; they are also so arranged as to close 

 when there should be a limit to the perspiration. Leaves, which 

 exist entirely under water, have none whatever. They vary in 

 number. The Rhevm Palmatum — Rhubarb, has 1,000 on the up- 

 per surface, and 40,000 on the under; the Dianthus Caryophyllus — 

 Carnation, has 38,500 on both surfaces ; and lastly, the Hydrangea 

 Quercifolia — Oak-leaved Hydrangea, has none on the upper, 

 and 100,000 on the lower. This common hot-house plant re- 

 quires a constant supply of water, and if not assiduously sup- 

 plied, the leaves soon wither and become crisp. The whole 

 apparatus of the leaf is nothing more than a contrivance to ex- 



