CELL-WALLS 



267 



441. Markings in cell- 

 walls, sp, spiral; an, 

 annular; sc, scalari- 

 form. 



thick walls, and are called sclerenchymatous cells. In many 

 cases the cell-walls are intermediate between these extremes. 

 446. Cell-walls often thicken by additions to their 

 inner surface. This increase in thickness seldom takes 

 place uniformly in all parts. Many times the wall remains 

 thin at certain places, while the most of the wall becomes 

 very thick. Again the walls may thicken 

 very much in angles or along certain 

 lines, while most of the wall remains 

 thin. As a result of this uneven thicken- 

 ing, the walls of cells take on certain 

 definite markings. Some of the names 

 applied to these markings are: 



Pitted, with little holes or depres- 

 sions, forming very thin places, as 

 seen in seeds of sunflower, and in the large vessels in 

 the stem of the cucumber. 

 Bordered pits, when the pits are inclosed in the cell-wall, 



as in wood of pines and other conifers. (Fig. 440.) 

 Spiral, with the thickening in a spiral band, as in the 

 primary wood of most woody plants and in the veins of 

 leaves. (Fig. 441.) 

 Annular, with thickening in the form of rings; seen 



in the small vessels 



of the bundles in 



stem of Indian corn. 



(Fig. 441.) 



Scalariform, with 



elongated thin 



places in the wall, 



alternating with the 



thick ridges which 



appear like the rounds of a ladder. (Fig. 441.) These 



are well shown in a longitudinal section of the root of 



the brake fern (Pteris). 



442. Four steps in process of cell-*li 

 cell at left, fai advanced in div 

 cells at right. 



