202 



A YEAR IN SCIENCE 



stomates. (The Greek word stoma means mouth.) On 

 each side of a stomate there is a kidney-shaped cell 

 called a guard cell. The shape of these cells can be 



changed and in so do- 

 ing the openings into 

 the leaf can be made 

 smaller or larger. It 

 is not exactly clear of 

 just what service this 

 is to the leaf. There 

 are thousands of sto- 

 mates on each leaf, 

 usually only on the 

 under surface. Gases 

 pass in and out of 

 the leaf through the 

 stomates. A cross- 

 section of a leaf shows 

 that, immediately below the epidermis, there is a row of 

 green cells closely packed together. Below these are 

 the large, green, loosely-arranged cells of the meso- 

 phyll. The stomates open directly into the spaces, air 

 chambers, formed between these cells. 



Chlorophyll. Some things in nature are so common 

 that we do not stop to study them; we take them for 

 granted. We know that most plants have a great 

 many leaves and that these leaves are green. "We per- 

 haps have never considered the nature or importance 

 of this green color. Inside of all the cells of the leaves, 



Fig. 79. Epidermis of a leaf of 

 geranium showing stomates ; c, cell ; 

 p, opening of stomate ; gc, guard cell. 



