THE CELL-WALL. 151 
proportion as those which have remained thin, the membrane 
presents, upon a transverse section, a necklace shaped (monili- 
form) appearance. Such cells are, for instance, highly charac- 
teristic of the coffee-bean.'| Where the thickened places do not 
attain great extension, and appear especially upon the inner 
Surface, they often assume the form of rings or spiral bands. 
Thus originate the spirals in many fibro-vascular bundles, as, 
for instance, in the sguiil (Fig. 66), as also the net-like and 
scalariform thickenings (Fig. 68) of the vessels and parenchyma 
cells (Figs. 67, 83, and 182). : 
Fra. 67.—Cells with net-shaped thickenings (Dippel). Compare also Fig. 182. 
When thickening of the cell-wall extends over the largest part 
of the inner surface, and exempts but a few expanded dot-shaped 
places, pores are produced (Fig. 69). With a considerable thick- 
ening of the cell-wall such places appear as dots, or by a still 
greater increase in the thickness of the wall as pore-canals 
(as in the stone cells, Fig. 70). Frequently a spiral-shaped 
arrangement of the dots may be observed (Fig. 71), and the 
course of the pore-canals also often approaches that of a 
spiral line. Many true bast-cells have cleft-shaped dots 
1 Berg’s ‘‘ Atlas,” Plate 49, Fig. 131. 
