70 LEAVES. 



the pupil may be surprised at this fact, it is not unaccountable 

 to those who know, that although we may have the picture of 

 an object in the mind, it is not always easy to convey our con- 

 ceptions to the minds of others. To give correct definitions of 

 terms, is one of the greatest difficulties in science. 



The manner in which different persons describe objects, va- 

 ries with the degree of knowledge possessed respecting their 

 properties. For example ; in attempting to describe common 

 salt, if a person knew nothing more of it than his unassisted 

 senses had informed him, he would speak of its colour, taste, 

 and other obvious properties ; one familiar with the principles 

 of chemistry, would first speak of the materials which compose 

 salt ; he would describe it as a compound substance, consisting 

 of chlorine and sodium; and then might enumerate its proper- 

 ties. In the first definition, given without any reference to 

 scientific principles, there is nothing so definite as to afford a 

 certain mark of distinction between salt and other substances ; 

 in the chemical definition, we have a test for salt, Sn a know- 

 ledge of its composition, which distinguishes it from all other 

 substances. 



In botanical definitions, we do not include the constituent 

 elements of the vegetable substance ; this belongs to the de- 

 partment of chemistry, but we consider the external forms and 

 uses of the various parts of the plant. 



One botanist* says, " leaves are organs of an expanded form, 

 almost always of a green colour, internally vascular, and more 

 or less pulpy." Another botanistf says, "the leaf may be de- 

 fined to be a temporary organ of plants, which performs nearly 

 the same function in the economy of vegetable life, as the 

 lungs perform in the animal ; or, they are the respiratory or- 

 gans of plants." 



We might go on quoting from different authors, each of 

 whom describe according to their own manner of considering 

 the subject ; but as our object is, to express botanical facts in 

 the most simple terms, we will merely add to^ the definitions 

 above given, one which, with your present knowledge, is as 

 scientific as you can understand. 



The leaf is generally a thin, flat organ, consisting of an ex- 

 pansion of the fibres of the bark, connected by a substance 

 which is called the cellular tissue ; the whole is covered with 

 a green coat or skin called the cuticle. Leaves are furnished 



Withering. t Nutall. 



Descriptions of objects vary with our knowledge of their properties Differ, 

 cat definitions of leaves General description of a leaf. 



