THE CELL 



163 



268. It is seldom the case that the wall is thickened 

 uniformly. Often numerous round thin spots are left, so 

 that the cell has a dotted appearance (Fig. 222). When 



the thin spots in adjacent cells are contiguous, 

 as they commonly are, a ready means of inter- 

 communication is afforded. Sometimes the spots, 

 instead of being round, are oblong, so that the 

 cell under the microscope presents a ladder-like 

 appearance, and so is said to be scalariform. Then 

 again, the thickening may take the form of spiral 

 lands upon the inner surface ; or, instead of a 

 continuous spiral band, we may find a series of 

 Fig, 222. isolated rings, when the marking is said to be 



annular. Reticidated cells are also found, in which the 



markings, as the name implies, form a sort 



of network on the walls. Several of these 



forms are shown in Figs. 223 and 224. 



269. Sometimes round thin spots will 

 be left in the wall, and over each of these 

 a thick-walled dome with an opening at 

 the top will be formed. At the same time 

 a similar dome is raised at exactly the 

 same spot on the other side of the wall in 

 the next cell ; and, finally, the thin par- 

 tition between the opposite domes breaks 

 away, permitting free communication. 



Thus are formed what are called bordered Fig. 223. Fig. 224. 

 pits, which abound in the wood of Conifers. 



270. When cells stand end to end, and thin spots are 



Fig. 222. Dotted duct. (Gray.) 



Fig. 223. Spiral and annular markings on cell-wall. (Gray.) 



Fig. 224. Various markings on cell- wall. (Gray.) 



