ELEMENTAEY STEUCTrRE. 



21 



discovered that these markings are caused Ijy canals which run 

 from the outer or primary cell-mem brans {fig. 37, a), through the 

 layers of thickening (h), into the cavity of the cell {d), and thus give 



Fig. 36. 



^^^mi^Fig.ZI. 



Fig. 36. Pitted cells. Fig. 37. Thick-wRlled cells from the fruit of a 



Palm. «, a. Original cell- walls, ft, 6. Secondarj- layers, c. Pit canals. 

 d. Cavity of the cell, e, e. External iMtted appearance. From Unger. 



to the parts of the cell-membrane in which they are foimd. 

 when viewed by transmitted light under the microscope, a more 

 transparent appearance than that possessed by the thickened 

 membrane surrounding them. We arrive therefore at the con- 

 clusion, 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 de- 

 posited 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 these canals on the outside. Such cells are therefore im- 

 properly called porous, and hence are now commonly and cor- 

 rectly termed pitted or dotted cells. The pits or canals of con- 

 tiguous cells generally acciu'ately correspond, so that however the 

 cell-walls may become thickened, their cavities are only separated 

 from each other at such spots by their primary thin walls (fig. 

 37, a), a contrivance especially designed to admit of a free com- 

 munication 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 (^%. 35). 



Although, as thus shown, the dotted appearance is not caused 

 by external holes or perforations in the primary wall of the cells, 

 yet as the latter advance in age, and lose their active vitality, 

 they frequently become perforated, in consequence of their thin 

 primary membrane becoming absorbed or breaking away. Such 



