198 SAP AND SECRETIONS. 



wliich are seen to best advantage in the centre. These 

 cells, which are unusually large in the Elder, are filled with 

 fluids when young, but in old branches the fluids are gone 

 and the cells are empty. Of its uses in the economy of 

 vegetation, but little is known. 



Questions. — 1. What is the cuticle of a plant ? 2. How is it de- 

 scribed and what are its uses ? 3. Describe the cellular integument. 

 4. Tho bark. 5. The wood. 6. The pith. 7. What chiefly resides 

 in the bark of plants .'' 8. What is said of the circular layers of wood ? 

 9. How has R been shown that the bark produces the wood .' 



LESSON 89. 



Sap mid Secretions. 



Odoriferous, fragrant, perfumed. Propnl'sion, the act of driving 

 forward. Es'culent, good for food, eatable. 



That the whole vegetable body is an assemblage of tubes 

 and vessels is evident to the most careless observer ; and 

 those who are conversant with the microscope and book* 

 relating to it, have frequent opportunities of observing how 

 curiously these vessels are arranged, and how different spe- 

 cies of plants, especially trees, differ from each other in 

 the structure and disposition of them. It is familiar to every 

 one that plants contain various substances, as sugar, gum, 

 acids, odoriferous fluids, and others, to which their various 

 flavours and qualities are owing f and a little reflection will 

 satisfy ua that such substances must each be lodged in proper 

 cells and vessels to be kept distinct from each other. They 

 are extracted, or secreted, from the common juice of the 

 plani, and called its peculiar or secreted fluids. Various 

 experiments and observations prove also that air exists in 

 the vegetable body, and must likewise he contained in ap- 

 propriate vesaels. Besides these, we know that plants arc 

 nourished and invigorat,ed by water, which they readily ab- 

 sorb, and which, by proper tubes or vessels, is quickly con- 

 veyed through their stalks and leaves. It is observed, 

 moreover, that all plants, as far as any experiment has been 

 raade, contain a common fluidj which at certain seasons of 

 the year Is to be obtained in great quantity, and this is proper- 



