CHAPTER IV. 



THE LEAP. 



1. The leaves of plants ordinarily present a large surface 

 expansion. They are perforated by minute pores, called Sto- 

 mates. Through these oxygen and carbon-dioxide enter the 

 leaf, and water in the form of vapor escapes from it. Nearly 

 all of the water that is taken up by the roots, thence conveyed 

 through the stem and branches to the leaves, passes out 

 through the stomates. The leaf retains the minerals that 

 were dissolved in the water. It also decomposes the carbon- 

 dioxide (CO2) which it takes from the atmosphere, retaining 

 the carbon but liberating the oxygen. These several elements 

 of inorganic matter are formed into organic or vegetable 

 material, which the plant uses in building up all its parts. 

 The leaf, then, has evidently ver}' important functions to per- 

 form, for which it is specially adapted by its structure. Only 

 the ordinary foliage leaves fully perform these functions ; they 

 may be said to exhibit the typical form. Besides these, there 

 are many modified forms, some of which have departed so far 

 from the type that their true nature can be understood only 

 when we see all the intermediate forms or gradations connect- 

 ing the two extremes. Such are cotyledons, scales, spines, 

 tendrils, pitchers and fiy-traps. 



Illustrations of everything (or with few exceptions) to which reference is 

 made in this chapter can be obtained everywhere and in great abundance. 

 The pupils should examine, compare, make outline figures and thoroughly 

 study this illustrative material, which of course will be work for many days. 

 Then the text can be studied to advantage. 



2. The two halves into which a pea, bean, etc., readily divide 

 are called the Cotyledons, or seed-leaves. If they be observed 



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