344 



GENERAL SCIENCE 



SECTION OF A LEAF 



e, epidermis; c, 

 cells containing 

 chlorophyll bodies; 

 p, intercellular pas- 

 sages; g,g,guard cells, 

 of stoma. Magnified. 



the leaf during a wet season or during rain; it may 

 also help to keep out disease bacteria and parasitic 

 plants. If we dip a green leaf in water, we can see that 

 a thin layer of air is held on the leaf by 

 this extra covering, and when the leaf 

 is removed from the water, the water 

 does not adhere to the surface of the leaf. 

 The blade of the leaf has a thin layer 

 of cells forming the upper and lower 

 covering or skin of the leaf; this layer 

 is called the epidermis. Just under the 

 upper epidermis is a layer of long cells 

 with their ends against the epidermis 

 and arranged in column form ; hence the 

 layer which they form is called the pali- 

 sade layer. Between this palisade layer 

 and the lower epidermis is a layer of loosely arranged cells 

 with large air spaces between them ; this layer is the spongy 

 tissue. The air in these air spaces in the spongy tissue 

 can pass in and out through very small 

 openings, which are in the lower epi- 

 dermis of most leaves. These open- 

 ings in the lower epidermis are called 

 stomata (plural of stoma). There may 

 be many thousand stomata to each 

 square inch of surface. The air passes 

 in and out of the stomata by filtration. 

 There are two almost semicircular cells ^ ordinary epider _ 

 around each stoma, which can easily be malcell; g, guard cell. 



. , . .1 n j Magnified. 



seen with a microscope ; they are called 

 guard cells. By changing their shape they control the 

 amount of air that can pass through and also control the 

 amount of evaporation of water from the leaf. 



LOWER SURFACE OF 



LEAF 



