THE MULTIPLICATION OF CELLS. 1G7 



while on the under surface they are placed loosely 

 to admit gases freely. Thus this figure ^ represents 



a cut made through the thickness of the leaf, exhib- 

 iting its edge highly magnified. 



(rt) denotes the epidermis ; 



(6) a close layer of parenchyma ; 



(c) internal cells ; 



(d) the cells of the lower surface ; 



(e) stomates, or valves through which the gases 

 enter. 



As the sap descends in the liber by gravitation, 

 it causes the multiplication of cells between the 

 bark and wood. Every cell contains within itself 

 a nucleus, from which the sap flows toward the 

 walls ; this contains several nucleoli, or cytoblasts, 

 which, when they have attained size, burst the nu- 

 cleus, and each becomes like its parent. Conse- 

 quently, if there are two nucleoli, each creates a 

 current acting counter to the other, and at their 

 point of contact a membrane is formed which divides 

 the old cell. This is very rapid work. We can 



1 Dr. Lindley's Introduction to Botany. 



