12 



ORGANOGRAPHT. 



these were called porous cells ; but, when carefully examined, it 

 may be readily discovered that this dotted appearance is caused 



Fig. 19. 



Fig. 20. 



Fig. 19. Pitted^ cells Fig. 20. Thick-walled cells from the fruit of a 



palm. a. Original cell-walls, b. Secondary layers, c. Pit canals, 

 d. Cavity of the cell. e. External pitted appearance. (From Unger.) 



by canals which run from the outer cell-membrane (Jig. 

 20, a) formed of cellulose, through the layers of thickening 

 (6), and open into the cavity of the cell (d), and thus 

 give a more transparent appearance, when viewed by trans- 

 mitted hght, to the cell-membrane in which they ai-e found, to 

 that of the surrounding thickened membrane. We arrive 

 therefore at the conclusion, that the successive deposits of 

 thickening layers take place, not as imperforate membranes, 

 as is the case with the primary cell-membrane, but as perforated 

 ones, which are deposited in succession from without inwards, 

 in such a manner that the openings in each of them shall exactly 

 correspond the one to the other, so as to form continuous canals 

 from the cavity of the cell towards, or to, the primary cell- 

 membrane which bounds those canals on the outside. Such cells 

 are therefore improperly called porous, and hence are now 

 commonly and correctly termed pitted or dotted cells. The pits 

 or canals of contiguous cells generally accurately correspond, so 

 that however the cell-walls may become thickened, their cavities are 

 only separated from eacli oilier at such spots by their primary 

 thin walls (Jig. 20, a), a contrivance especially designed to admit 

 of a free communication between the cells, notwithstanding the 

 thickening which their walls have undergone. It frequently 

 happens that two or more canals unite together at varying 

 distances from the walls of the cell, and thus form a common 

 opening into its cavity (fig. 18). 



Although we have thus shown that the dotted appearance 

 is not caused by external holes or perforations in the primary 

 walls of the cells, yet as the latter advance in age, and lose 

 their active vitality, they frequently become perforated, in con- 

 Bcqucnce of their thin primary membrane becoming absorbed or 

 breaking away. 



