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, /. 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 Vahnia 
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 differences of elevation ; sometimes the 
denser layers project a little on the inner side of the 
cell-wall (Fig. 3 2 5); or individual denser layers of one 
system of striation alone become prominent ; thus, for 
instance, a fine spiral band makes its appearance on 
the inner sides of the wood-cells of the Yew, which is 
not mifrequently 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-qjoall in surface and thickness. The 
Fig, 30.— Cells from the leaf of Hoya car- 
nosa (x8oo), showing striation. The strife 
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. 
^ See also Book II, Rhizocarpe£e. 
^ 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 Pi7i7es, bast-fibres, &c. ; one of the examples longest 
known are the bast-cells of Apocynacese possessed of alternate dilatations and constrictions. (Mohl, 
Veget. Zelle, Fig. 27.) 
