THE CELL- WALL. 
27 
viewed one over 
cell, the corresponding fissure may incline to the right in the other 
the other they then appear crossed ^. 
In cells which form a tissue, the partition-wall is always at first a very thin lamella; 
as the thickness increases, the thickening-masses project into the adjoining cell-cavities. 
Generally, as we have already seen, the thickenings on either side of a partition-wall 
correspond; and this is very evident in the formation of pits, inasmuch as the pit- 
canals of adjoining cells meet one another. But since a cell often adjoins cells of 
a very different character, different sides of the same cell may show different forms 
of thickening and different descriptions of pits. The 
total growth in thickness may also be very different 
on different sides ; thus, for instance, epidermal cells 
are mostly strongly thickened on the outer exposed 
wall (cuticle) ; the inner wall, where they adjoin 
parenchymatous cells, being either very thin or cor- 
responding in form to that of the adjoining cells. 
The correspondence in the growth of the thick- 
enings is less evident when they have a distinctly 
spiral structure, or when they occur in the form 
of strong spiral bands, as in spiral vessels; if, in 
this case, in each adjoining cell one or more spiral 
bands wind in the same direction, they must neces- 
sarily cross on the common partition-wall. 
(c) Stratification and Striation of the Cell-^aW^. 
When the cell-walls have attained a certain thick- 
ness and extent of surface, stratification and stria- 
tion become more or less evident. In consequence 
of stratification the cell-wall appears as if composed 
of very thin membranes enclosed one within another 
and fitting very closely together ; the stratification 
is seen both in the transverse and the longitudinal 
section of the cell-wall. The striation is generally 
to be seen most plainly from the front ; it may be 
observed in the form of two systems of lines (some- 
times apparently more) marked on the surface. The 
one system, consisting of parallel striae, is always 
intersected by the other system which also consists 
of parallel striae. A closer investigation shows that 
the appearance of striation does not belong to the 
surface only or to one layer of the cell-wall, but 
that the striation rather penetrates the whole thick- 
ness of the cell-wall, and that the striae are due to 
layers which intersect the surface obliquely, and all 
other layers concentric with it. If the striation is 
very strongly marked, and nearly parallel to the 
longer axis of the cell, it may be recognised in a 
transverse section in the form of lines crossing the 
concentric layers ; in a longitudinal section only those systems of striation are easily 
seen which, viewed on the surface, cut the longer axis nearly at right angles. 
Fig. 27.— Vessel from the underground stem 
of Pteris aqicilina tliickened in a scalariforni 
manner; A half of a vessel, isolated by Schulse's 
maceration; /^—/? sections obtained from pieces 
of the stem hardened in absolute alcohol ; B after 
a very clean section, represented in a partly dia- 
grammatic manner; to the right, front view of 
the wall of the vessels from within ; c c vertical 
section of the same ; C front view of the young 
wall of a vessel ; D its vertical section ; E place 
where a vessel adjoins a parenchymatous cell, in 
section vertical to the thickening-ridges of the 
vessel (X 800). 
^ A representation of a twisted pit-canal, whose outer and inner fissure (within the same cell- 
wall) cross, may be seen in Nägeli, Berichte der Münchener Akademie, 1867 (July 9), t. v. fig. 45. 
^ H. von Mohl, Bot. Zeitg. 1858, pp. i, 9.— Nägeli, lieber den inneren Bau der vegetabilischen 
Zellenmembran, in the Sitzungsberichte der Münchener Akad. der Wissenschaften, 1864, May and 
July. — Hofmeister, Lehre von der PHanzenzelle, p, 197. 
