MORPHOLOGY OP LEAVES AND FLOWERS 



597 



Pig. 452 I 



Fig. 452 II 



Fig. 452. I. Wedge-shaped leaflet of Horse Chestnut. 



the Wall Pellitory. 

 (After Thome.) 



II. Pointed leaf of 



All of the above marginal characters may characterize the lobes 

 of a simple leaf or the leaflets of a compound leaf. 



LEAF STRUCTURE. 



Before considering the work of the leaf it will be necessary to 

 become acquainted with its structure. 



The leaf is covered with an epidermis which is composed of com- 

 pact layers of cells, so modified as to protect the more delicate 

 inner parts. The epidermis may be peeled off as a delicate trans- 

 parent skin. A microscope shows that this transparent skin is 

 made up of many cells, so closely fitted together as to make a contin- 

 uous sheet or covering. Many slitlike openings between two cres- 

 cent-shaped cells (guard cells) appear quite evenly distributed in 

 the epidermis. The opening and guard cells constitute the stoma 

 (plural stomata) which really means mouth. These numerous 

 openings are passage ways into the interior of the leaf and per- 

 mit interchange of gases between outside air and the air in the 

 leaf interior. The guard cells can change their shape and so vary 

 the size of the opening. In horizontal leaves the stomata are chiefly 

 and sometimes exclusively on the lower surface, a fair average 

 number being about 62,500 to the square inch. 



