ENDOGENS. 65 



shape, and compound when divided into distinctly 

 separate parts, or leaflets, connected with the petiole 

 by secondary petioles. 



Leaves may be lanceolate, or narrow and tapering ; 

 oblong -, or narrow and not tapering ; cordate, or heart- 

 shaped; sagittate, or arrow-shaped; ovate, or egg- 

 shaped, etc. A compound leaf having leaflets placed 

 laterally is called pinnate. If the leaflets are them- 

 selves divided, it is bipinnate, and a further division 

 of the leaflets is tripinnate. Sometimes a compound 

 leaf is triple, or . ternate, etc. When a ternate leaf 

 divides twice it is biter nate; when thrice, triter- 

 nate. 



It is not uncommon to find on the same plant leaves 

 of different forms. The radical leaves, or those 

 which grow from the lower part of the stem, are often 

 different from the upper ones. 



The function, or use, of leaves is to expose the 

 juices of the plant to light and air, and thus aid in 

 forming the woody matter of the stem and the vari- 

 ous secretions. If the leaves are excluded from air 

 and light, as is the case in crowded plantations, the 

 wood is not properly formed. The same may be said 

 of all the substances formed by the plant. Thus, 

 potatoes grown in the shade, which impedes the ac- 

 tion of the leaves, become watery, and produce little 

 starch in their tubers. 



Leaves also exhale watery fluid, and by decompos- 

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