THE STOMATES 



299 



470. Stomate of ge- 

 ranium leaf, show- 



dermis of leaves and soft stems, allowing the passage of 



air and other gases and vapors. They are placed near the 



large intercellular spaces of the mesophyll. Fig. 470 shows 

 the usual structure. There are two guard- 

 cells at the mouth of each stomate, which 

 may in most cases open or close the passage. 

 It is commonly thought that the opening 

 and closing of the guard-cells is in response 

 to different moisture conditions of the 

 ing the guard-ceiis. atmosp h e re. When the air is dry it is 



assumed that the stomates close 



and thus retard water loss from 



the plant, and vice versa. The 



stomates have generally been 



thought to regulate transpiration. 



This is not true. In Fig. 471 is 



shown a case in which there are 



compound guard-cells, that of ivy. 



On the margins of certain leaves, as of fuchsia, impatiens, 



and cabbage, are modified stomates known as water-pores. 



508. Stomates are very numerous, as will be seen from 

 the numbers giving the pores to each square inch of leaf 

 surface: ^^ Upper 



surface surface 



Peony 13,790 None 



Holly 63,600 None 



Lilac 160,000 None 



Mistletoe 200 200 



Tradescantia 2,000 2,000 



Garden Flag 11,572 11,572 



The arrangement of stomates on the leaf differs with 

 each kind of plant. Figs. 472 and 473 show stomates on two 

 plants, and also the outlines of contiguous epidermal cells. 

 The guard-cells contain chloroplasts. 



509. Fall of the Leaf. In most common deciduous plants, 

 when the season's work for the leaf is ended some of the 



471. Stomate of ivy, showing 

 compound guard-cells. 



