14 STRONG DRINK AND TOBACCO SMOKE. 



in so doing partically close or open the slit, and so 

 regulate the passage of moisture and gaseous matter 

 from or to the interior of the leaf. Though very- 

 minute, they must, from the fact of thousands of them 

 occurring within a square inch of surface, play a most 

 important part physiologically in the life of the plant. 



It is also on the skin of the leaf that the peculiar 

 cellular appendages, hairs, are placed. These are found 

 sometimes in two or three different forms on the same 

 leaf, and although frequently invisible to the naked eye, 

 are, from their excessive transparency, very beautiful 

 objects under the microscope. Their forms and charac- 

 ters are as constant for each leaf as they are diversified 

 in the leaves of different plants ; and, as our investiga- 

 tions proceed, it w411 be seen how objects so simple 

 as a hair may be made available in the analysis of 

 tobacco leaves. 



The plants and leaves, the structure of which we 

 have now to study, are nothing more than variously 

 disposed groups of cells and vessels held firmly together ; 

 I have now to describe briefly the manner in which they 

 may be most profitably studied. 



An architect, desirous of conveying an idea of the 

 form and construction of the house he intends building, 

 prepares very carefully a series of drawings, exhibiting 

 ground plans and sections of the various parts of the 

 proposed building. We, on the other hand, have to take 

 to pieces the fabric which Nature has constructed, in 



