8ECT. I 



MORrHOLOGY 



75 



pits apparently act as valves. Seen from the surface a bordered pit 



appears as two concentric rings (Fig. 76, A). The smaller, inner 



ring represents the narrow opening of the pit into the cell cavity ; the 



larger, outer ring indicates the 



widest portion of the pit chamber 



when it abuts on the primary cell 



wall. 



The pit membrane of specially 

 wide pits between cells with thin 

 walls often shows thicker bands 

 which give it a lattice-work appear- 

 ance. The membranes of the 

 sieve - pits are perforated like a 

 sieve (Fig. 78). 



In cases where the greater 

 part of the cell wall remains un- 

 thickened, its character is determined by its thickened rather than 



Fig. 82.— Epidermal cell from the margin of a 

 radical leaf of Campanula persici/olia. The 

 lens-shaped thickening of the outer wall is 

 silicifled in this plant. (After Haberlandt. 

 X 515.) 



by its unthickened portions ; it 



is in this sense that the terms 

 annular, spiral, and reticulate are 

 used (Fig. 79). Just as in the 

 case of cells with bordered pits, 

 annular, spiral, and reticulate cell 

 walls are only acquired by cells 

 that soon lose their contents, and 

 act as water-carriers. Such wall 

 thickenings serve as mechanical 

 supports, to give rigidity to the 

 cells, and to enable the cell walls to 

 withstand the pressure of tlie sur- 

 rounding living cells. 



The thickened bands by which such 

 thickenings of the cell wall are effected 

 are attached by narrowed bases to the 

 primary membrane (™). AVhen tlie mem- 

 brane separates two water-conducting 

 elements its central portion is thickened 

 like the torus of the bordered pit. The 

 annular and spiral types of thickening 

 characterise the water-conducting ele- 

 ments of growing parts of the plant, since 

 they allow of extension of the wall during 

 growth. The thickening band can often 

 be removed as a continous spiral from 

 the lamella to which it is attached. 



OoLLENCHYMATOUS cells are living cells, the walls of which are 

 thickened principally at the corners (Fig. 81 c) ; such cells occur 

 commonly in the more highly organised plants and form a special kind 



Fig. 83. — Part of transverse section of a leaf of 

 Ficus elastica. c, Cystolith ; e, e, e, three- 

 layered epidermis ; p, palisade parenchyma ; 

 s spongy parenchyma, (x -240.) 



