THE CELL- WALL, 



29 



mediate density are found. The intersecting systems of striation must form prisms 



which stand vertically or obliquely upon the surface of the cell-wall. If the concentric 



stratification is very strongly developed, every one of these prisms must be cut up into 



more and less dense sections parallel with its base ; if the concentric stratification is 



feebly developed, the prismatic structure may predominate. The peculiar internal 



structure of the epispore of Rhizocarpeae (Fig. 33, p. 31)^, and the yet more various 



structure of the extine of many pollen-grains, may perhaps be resolved into a further 



development of this kind of process ; but our space 



does not permit us to pursue this question in detail. 



The layers which produce the external appearance 



of striation may possess the form of closed rings, i. e. 



may be similar to thin sections of the cell, or may 



run in a spiral manner round the axis of the cell. A 



distinction must accordingly be drawn between annular 



and spiral striation ; it is often, however, very difficult 



to decide which of the two is present ; sometimes both 



are developed at different parts of the same cell-wall. 



Sometimes one system of striation is very obscure, the 



other all the more strongly marked ; or one system may 



be the better developed in one layer of the cell-wall, 



the other system in another layer; and this is genetically 



connected with the above-mentioned twisting of the 



pit-fissures. The striation is mostly clearest in cells 



with broad uniform thickening-surfaces, as Falonia 



utricularis, hairs of Opuntia, pith-cells of the root-tubers 



of the Dahlia (in the latter case remarkably plain) ; but 

 [it may also be recognised when the sculpture of the 

 cell-wall is complicated ; e. g. in the walls of very wide 

 [vessels of Cucurbita Pepo provided with densely crowded 

 small bordered pits (after Schulze's maceration, espe- 

 .cially in vessels of the root, where the crossed spiral 

 [striation is very clear). The striation may itself give 

 ^occasion to diflferences of elevation ; sometimes the 

 denser layers project a little on the inner side of the 

 cell- wall (Fig. 32 jB); or individual denser layers of one 

 system of striation alone become prominent ; thus, for 

 instance, a fine spiral band makes its appearance on 



not unfrequently crossed by one running in the opposite 

 ^direction. When elongated fissure-like pits are ar- 

 ranged in spiral lines on the cell-wall, a system of stria- 

 tion is generally found in a corresponding direction. 



This slight sketch must suffice to explain the nature 

 of stratification and striation, and their relation to the 



sculpture of the cell-wall; further detail would exceed the limits of this work^. 

 (d) Intussusception the cause of growth of the Cell-quail in surface and thickness, 



Fig. 30.— Cells from the leeit of //oya car- 

 nosa {X800), showing- striation. The striae 

 are not nearly so strongly marked in nature, 

 but are quite as evident ; a optical longitu- 

 dinal section of the crossed annular stria- 

 tion ; b front view where the annular 

 striations cross ; c, d front view where they 

 do not cross ; e a piece of cell-wall, where 

 only a few annular striations are to be 

 seen. 



The 



* See also Book II, Rhizocarpeae. 



"^ The striation may easily be seen, even with slight magnifying power, on the large pith-cells of 

 the root-tubers of Dahlia, on the hairs of Opuntia, and on Valonia utricularis, but only with very high 

 magnifying power on isolated wood-cells of Firms, bast-fibres, &c. ; one of the examples longest 

 known are the bast-cells of Apocynaceae possessed of alternate dilatations and constrictions. (Mohl, 

 Veget. Zelle, Fig. 27.) 



