30 



THE 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 whicii 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 tliat a slight strain suf- 

 fices to break it, releasing the rings 

 and coils. The number, the direc- 

 tion, and the stee[)ness 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 polj-gons. 



137. Tlie external sculpturing of 

 the cell-wall can be seen in many 

 pollen-grains, and in the hairs of 

 many plants, though in the latter 

 case the projections ma}' be partly 

 due to irregularities in the form of 

 the cell. 



13.8. Stratiflcation 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 whicli forms a 

 constituent of the wall obhterates every trace of sta'atification, 

 and this fact supports the hypothesis that the appearance of 

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

 tained in alternating layers of the wall. The less strongly 

 refractive la3-ers are supposed to contain more water than those 

 which are highly refractive. But there are cases of stratification 



in 



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



pilosa. (Jacobs.) 



