LEAVES AND LEAF STRUCTURE 91 



layer, starch making and all the other leaf functions are carried 

 on. The passing off of water to the air spaces is part of its work. 

 The air spaces are usually large, irregular cavities among the 

 spongy cells. They open through the lower epidermis by way of 

 the stomates, their function being to receive water vapor from the 

 spongy cells and to pass it out through these openings. They 

 also permit oxygen and carbon dioxide to pass to all the cells of the 

 spongy layer. They are very important, since through them food 

 making, respiration, and transpiration go on. They occupy about 

 three-fourths of the bulk of the spongy layer. The veins or duct 

 bundles are scattered through the spongy layer transporting water 

 and food stuffs and supporting the blade of the leaf. 



The Lower Epidermis. Like the upper, the lower epidermis 

 usually has but one layer of cells. Through it open many stomates 

 which regulate the passage of air and water vapor to and from the 

 inside of the leaf. 



The Stomates. These have been referred to as openings through 

 the epidermis. They are minute slit-like holes, about one-twen- 

 tieth as wide as the thickness of this paper. On each side of the 

 slit is an oval guard cell which regulates the opening and closing 

 of the stomate. Controlled by the needs of the plant, the sto- 

 mates open when there is an excess of water to be passed off, and 

 close in a drought. They open when carbon dioxide is required 

 for starch making or air for breathing, and close when either process 

 stops, thus regulating, in a remarkable degree, the activities of the 

 leaf. The function of the stomates is threefold, 



1. To admit carbon dioxide for starch making. 



2. To regulate transpiration of water vapor. 



3. To admit oxygen and liberate carbon dioxide in respiration. 

 However, this elaborate mechanism would be of little use were 



it not for the extensive system of air spaces in the spongy tissue of 

 the leaf into which the stomates open, and by means of which all 

 parts may have access to air for starch making, respiration, and 

 transpiration. Their number may vary from 60,000 to 450,000 

 per square inch and is usually greatest on the lower surface where 

 they are best protected from dust and rain. Floating leaves have 



