590 CONSERVATIVE ORGANS. [LECT. X. 



figure (35.), and not round 3 as might be sus- 

 pected from their longitudinal aspect. Sprengel 

 and some others have stated that the cells in all 

 , leaves are elongated near the upper surface ; but 

 the simple inspection of many leaves is sufficient 

 to refute this statement. Among the succulent 

 leaves, even, in which the tubular cells are more 

 frequently met with than in membranaceous 

 leaves, they are not always present ; as, for in- 

 stance, in the leaf of Hoya carnosa, the only dif- 

 ference between the upper and under disk of which 

 is in the structure of the cutis, which on the su- 

 perior surface (a. fig. 16, Plate 10) is a simple 

 transparent pellicle, while on the inferior (b. ibid.) 

 it is cellular ; and in the cells near the upper 

 disk being filled with a greener and more opaque 

 juice than those near the under disk. 



With regard to the individual structure of the 

 cells constituting the parenchyma of leaves, we 

 find it is the same as that of the cells in the other 

 parts of the plant. Each cell appears to be a dis- 

 tinct, transparent, membranous vesicle, formed 

 into the figure it displays by the pressure of the 

 contiguous cells, and thence, the partition sepa- 

 rating each cell, must be a double membrane. 

 This is more evident in the microscopic examina- 

 tion of the cellular substance of some leaves than 

 of others; thus, in this minute portion (36. page 

 2 



