138 



All Food Is Made by Green Plants unit hi 



Close study of the leaf blade. Althouijh 

 the \ariation in leaf shape and size is 

 interesting, we can learn very little about 

 the activities of a leaf from this kind of 

 study. To learn more it is necessary to 

 study the internal structure. This can be 

 done by examining a thick fleshy leaf 

 from a plant such as sedum. We can 

 break or cut the leaf crosswise and, with 

 the aid of a knife, pull off^ some of the 

 "skin." When we hold this to the light, 

 we discover that it is thin and trans- 

 parent. This "skin" is found on both the 

 lower and upper sides of leaves. It is a 

 tissue called the epidermis (ep-i-der'mis). 

 You will notice that the exposed part 

 under the epidermis feels moist and soft. 



If \'ou use a microscope to examine a 

 thin slice made across an ordinary leaf 

 you will have no trouble in identifying 

 the parts of the leaf. You can see the 

 upper and lower cpidciyf/is, the spongy 



Fig. 163 (above) Water lily. The leaf blade 

 floats on the water. Its long petiole is attached 

 to a stem at the bottom of the pond, (new york 



BOTANICAL GARDEN) 



Fig. 162 (left) Spanish vioss hafiging on live oak 

 branches. It is not a moss but a flowering plant. 



(NORTH CAROLINA DEPARTMENT OF CONSERVATION 

 AND development) 



cells with air spaces between them, the 

 palisade cells, and the veins. See Figure 

 164. You can easily study the epidermis 

 by doing Exercise i . Study of many dif- 

 ferent leaves will be interesting. 



Leaf epidermis. Every leaf, thin as it 

 may be, is covered above and below with 

 epidermis. This tissue consists of cells 

 closely fitted tooether. Fitrure 16c is a 

 drawing of lower epidermis of a sedum 

 leaf. In addition to the ordinary trans- 

 parent and usuall\' colorless cells, there 

 are at frequent intervals pairs of green 

 cells, each shaped like a slender kidney 

 bean. These cells lie in such a way that 

 there is an opening left between them. 

 This opening is called a stoma (stoh-ma). 

 Each stoma connects an air space within 

 the leaf with the air outside. The upper 

 epidermis of most plants has few or no 

 stomata (stoh'ma-ta), plural of stoma; 

 its cells, then, are mostly all alike. 



