THE BEAN LEAVES 



273 



word meaning color-bearers. The position of the palisade 



cells makes the upper surface: of the Leaf firmer than it 

 would otherwise be. The arrangement of the cells in a 



compact layer regulates the amount of light that penetrates 

 to the interior of the 

 leaf and helps to pre- 

 vent undue loss of water. 

 Below the palisade cells 

 are the loosely arranged 

 cells of a spongy layer. 

 They contain chlorophyll d! 

 and are exposed to the 



oooooooo 



spongy layer 

 chlorophyll 



lower epiaer mis 



•stoma 



air which enters through Figure 265. — Cross Siction of Bean 



i 

 the holes in the lower F * 



epidermis. Most of the How many tissues present ? 



work of the leaf is done in this green tissue. Because 

 this tissue lies in the middle of the leaf, it is known as 

 mesophyll (mez'o-fll: Greek, mesos, middle ; phyllos, leaf). 



The holes {stomata) in the lower epi- 

 dermis are more than mere holes, for they 

 can become larger or smaller according 

 to the needs of the plant. Seen from tin- 

 surface, each stoma is surrounded by two 

 bean-shaped cells, containing some chloro- 

 phyll. These cells (Figure -''>") called 

 guard cells, have the power of absorbing 

 water to a greater degree than the other 

 cells of the epidermis. When the guard 

 cells are full of water, or turgid, the 

 Opening between them is larger than 

 when they are almost empty or flaccid. The size of the 

 openings regulates the amount of air which passes in and 

 out, and of the watery vapor which passes out. 



The stomata are more numerous on the under side of 



Figure 266. — Leaf 

 Skeleton. 



Showing net veins. 



