ANATOMY OF LEAVF^. 308 



ed, that the fluids may he transmitted from cell to 

 cell, even when imperforate, hy the exercise of the 

 alternate functions of secretion and absorption ; but 

 these functions imply the existence of either glands 

 or vessels, connected with the absorbing and secreting 

 surface, which are, however, even less demonstra- 

 ble than the pores. Upon the whole, the question 

 still remains unanswered ; and all that we certainly 

 know of the subject is, that the fluids are transmitted 

 from cell to cell, through every part of the vegetable 

 system, although the structure by which this is ac- 

 complished remains undiscovered. 



Whatever may be the mode in which the cells com- 

 municate with one another, their contents are more or 

 less fluid or solid, according to their situation in the 

 thickness of the leaf. Thus, in thin leaves, the cells 

 near the inferior disk are more transparent, owing to 

 their contents being more fluid than those near the up- 

 per disk ; but in both we perceive a number of gra- 

 nules, which are more opaque and of a. deeper green, 

 as the cells containing them approach the upper disk. 

 In succulent leaves, and those which maintain a ver- 

 tical position, the opacity and green color of the gra- 

 nules, are the same towards every face of the leaf; 

 but they are generally colorless in its centre. In the 

 cells, also, of some leaves, regular crystallized salts 

 are found ; and in others the fluids are tinged of dif- 

 ferent hues besides green ; in which case the leaves 

 themselves display the same hues on one or both sur- 

 faces. 



The size of the cells varies in different leaves ; in 

 some, even when examined under the most powerful 

 glasses, they appear like the smallest vesicles ; while, 

 in others, they are so large as to be perceptible to the 

 unassisted eye. 



From these inquiries into the structure of the vas~ 



