268 FIRST YEAR COURSE IN GENERAL SCIENCE 



FIG. 142. CELLS FROM 

 THE SURFACE OF A LEAF. 

 (magnified about 250 

 times) 



300. Leaves and their Functions. The examination of 

 a bean plant will show that what we might at first take to be 

 separate leaves of the plant are joined in groups of three. 

 These leaflets, .as they are called, are 

 really parts or divisions of one leaf. 

 Such a leaf is called a compound 

 leaf. The clover, the rose, and the 

 locust have compound leaves. Most 

 of our commonest plants bear simple 

 leaves. The elm, the geranium, and 

 the lettuce have simple leaves. 



The leaves of a plant are so placed 

 as to shade one another as little as 

 possible. The leaves at the lower 

 end of the twig may have longer 

 stems than those above. In some 

 plants leaves grow in pairs on oppo- 

 site sides of the stem, the second 

 pore or pair at right angles to the first. If 

 a leaf is held up to the light, many 

 fine branching veins can be seen. 

 The veins of a leaf are not like veins 

 in an animal tubes solely for carry- 

 ing a liquid. They furnish a frame- 

 work for the softer parts of the leaf. 

 (LABORATORY MANUAL, Exercise 

 XXVI.) 



work of the leaf. i. Why Microscopic examination of a leaf 

 would the pores be smaller would show us a great number of 

 minute pores which open into tiny 

 cavities or air spaces in the interior 

 of the leaf. These pores furnish the 

 means of entrance for the air. They 

 also provide an exit for those constituents of the air which 

 are not of use to the plant, and for the waste products of 



of a leaf, p is 

 opening into one of the cells 

 below, fir is a guard cell; 

 two of these regulate the 

 size of the opening p, ac- 

 cording to conditions, ch is 

 one of the few chlorophyll 

 grains in guard cells; the 

 cells in the interior of the 

 leaf contain many such 

 grains. The pores admit 

 air and give out the water 

 vapor and oxygen that are 

 released by the chemical 



living matter are illustrated 



of 



