30 



THK VEGETABLE CELL IN GENERAL. 



133. Bordered pits are a very common modification of the 

 last. A comparatively large spot remains unthickened, but 

 becomes covered b}' a low dome which has at its top a small 

 aperture ; at a corresponding point of the wall of the neighbor- 

 ing cell another thickening produces a similar dome, so that the 

 two domes constitute a double convex body which appears as 

 a disc with a central perforation. These bodies are known as 

 discoid markings. 



134. Sometimes the spot covered by the arched projection or 

 dome is elliptical instead of round. When this kind of marking 

 becomes linear, or nearly so, it is termed scalariform. 



135. When annular and spiral thickenings occur the cell-wall 

 lying between them remains so thin that a slight strain suf- 

 fices to break it, releasing the rings 

 and coils. The number, the direc- 

 tion, and the steepness of the spi- 

 rals furnish in some cases diagnostic 

 features. 



136. Besides spirals and rings, 

 there are intermediate forms, which 

 pass easily over into netted or reticu- 

 lated thickenings. It happens some- 

 times that the reticulated markings 

 are so regular that their interspaces 

 appear as regular polygons. 



137. The external sculpturing of 

 the cell-wall can be seen in many 

 pollen-grains, and in the hairs of 

 man}' plants, though in the latter 

 case the projections may be partly 

 due to irreo-ularities in the form of 



-*. d? 



the cell. 



138. Stratification and striation. The cell-wall, even at an 

 early stage, frequently exhibits a distinctly stratified structure. 

 In some cases, at least, removal of all the water which forms a 

 constituent of the wall obliterates every trace of stratification, 

 and this fact supports the hypothesis that the appearance of 

 lamination is caused by differences in the amount of water con- 

 tained in alternating layers of the wall. The less strongly 

 refractive layers are supposed to contain more water than those 

 which are highly refractive. But there arc cases of stratification 



FIG. 4. Annular and spiral markings; vertical section through stem of Tradescantia 

 pilosa. (Jacobs.) 



