STRUCTURE OF A LEAF 



77 



Study of the leaf in cross section shows that the stomata 

 open directly into air chambers which penetrate between and 

 around the loosely arranged cells of spongy tissue composing the 

 under part of the leaf. The upper surface of leaves sometimes 

 contains stomata, but more 

 often it does not. The under 

 surface of an oak leaf of or- 

 dinary size contains about 

 2,000,000 stomata. Under the 

 upper epidermis is a layer of 

 green cells closely packed to- 

 gether (called collectively the 

 palisade layer) . These cells are 

 more or less columnar in shape. 

 Under them are several rows of 

 the loosely placed cells called 

 collectively the spongy tissue. If we happen to have a section 

 cut through a vein, we find this composed of a number of tubes 

 made up of, and strengthened by, thick- walled cells. The veins 

 are evidently a continuation of the fibrovascular bundles of the 



stem out into the blade of 



'4r 



Opening 



Guard cell 



[p'idermalcdl 



Stoma opening into yr 



air spdce in leaf / C 



Stomata open 



Stomata and guard cells, greatly 

 magnified. 



Eplderms 

 Palisdde layer 



Vein 

 Spongy tissue 



Lower epidermis 



Stoma 



the leaf (figure, page 47). 



Evaporation of Water. — ■ 

 During the day an enor- 

 mous amount of water is 

 taken up by the roots and 

 ' passed out through the 

 leaves in the form of vapor. 

 So rapid is this evaporation, 

 or transpiration, in a small 

 grass plant, that the water 

 evaporated in a day may 

 weigh more than the plant. It is estimated that nearly half a ton 

 of water may be delivered to the air during twenty-four hours by 

 a grass plot 25 by 100 feet, the size of the average city lot. It is 

 estimated that a corn plant in the Central West passes out from 

 its body more than forty gallons of water during its lifetime. 

 Fields of wheat transpire nearly 20 per cent of the total rainfall on 



Diagram of section through the blade of a leaf, 

 seen under a compound microscope. 



