20 MORPHOLOGY OF THE CELL. 



outwardly or inwardly. The former occurs commonly in the free-lying surface of 

 cell-wall, the latter in the partition-walls of adjoining cells. The thickenings which 

 project outwardly may appear in the form of knots, humps, spines, or ridges ; but 

 those which project on the inside are much more various. In this case peg-shaped 

 protuberances occur but seldom ; much more common are annular ridges or 

 spirally-curved bands ; these latter may be united in a reticulate manner, so that 

 thin polygonal interstices remain ; or the thicknesses may spread, and the thin parts 

 then appear in the thick wall as fissures or roundish pits. If the wall is very thick, 

 the latter become channels, which pass entirely or partially through the wall. Not 

 unfrequently the thin portion of the wall, which at first closes such a channel on 

 the outside, becomes absorbed, and the cell- wall is then perforated. But as, when 

 contiguous cells are united into a tissue, the partition-wall usually becomes thickened 

 in the same manner on both sides, the pits and pit-channels of both sides meet, 

 and the intermediate thin portion of membrane becomes absorbed ; a channel thus 

 arises uniting two cell-spaces (Bordered Pits, perforated septum of vessels). 



During the increase of the surface and of the thickness of the wall -by depo- 

 sition of new substance in both a tangential and radial direction between the 

 molecules already formed, a finer internal structure usually becomes visible, which 

 is termed Stratification and Striation. Both are the result of a different regularly 

 alternating distribution of water and solid substance in the cell- wall ; at every 

 visible point water is combined with cellulose, but in different proportions ; por- 

 tions less and more watery, denser and less dense, alternate. Thus, in every cell- 

 wall sufficiently thick, a system of concentric layers becomes visible, of which the 

 outermost and innermost are always denser, while between them alternate more 

 and less watery layers. The stratification is visible on the transverse and longi- 

 tudinal sections of the cell-wall, the striation also on the surface being usually 

 most evident there, but is in general less easily seen than the stratification ; it 

 consists in the presence of alternately more and less dense layers of cell- wall, cutting 

 its surface at an angle. Mostly two such systems of lamellae may be recognised 

 mutually intersecting one another. There are thus altogether three kinds of stra- 

 tification present in a cell-wall, one concentric with and two vertical or oblique to 

 the surface, cutting one another or mutually intersecting, like the cleavage-plain 

 of a crystal cleaving in three directions (Nageli); and as this cleavage takes place 

 in different directions, at one time the stratification, at another the striation is more 

 evident. 



Independently of this internal structure, chemical changes arise in the cell- wall 

 which never affect the whole mass uniformly, but usually divide the thickened 

 cell-wall into concentric layers which differ from one another chemically and phy- 

 sically. These chemical differentiations, which are always combined with an alter- 

 ation of physical properties, show a great variety, but can conveniently be reduced 

 to three categories; — Conversion into Cuticle or Cork (Verkorkung), Lignification 

 (Verholzung), and Conversion into Mucilage (Verschleimung). The first consists 

 in the change of the outer layers of the cell-wall into a plastic very elastic substance 

 which water cannot at all, or scarcely penetrate or cause to swell (as the outer 

 cell -wall -layer of the epidermis and of pollen-grains and spores and cork). 

 Lignification occasions an increase in the hardness of the cell-wall, a decrease of its 



