34 MORPHOLOGY OF THE CELL. 
In the epidermal cells, the cuticularisation either affects a shell of the outer wall, 
or it attacks the side-walls, as may be well seen, for instance, on the under-side of 
the leaf- veins of the holly. If a very thin transverse section (Fig. 37, A) is treated with 
. Schultz's solution, and submitted to a very high magnifying power (800), each cell-wall 
of the epidermis appears to be composed of two shells, of which the inner one, which 
is softer and more capable of swelling (c), becomes dark blue, while the outer shell 
does not. But this latter shows itself to be further composed of two chemically different 
layers, an inner [b), which assumes a yellow colour and penetrates laterally between 
the cells {b'), and an outer one which remains colourless [a), and extends continuously 
over the cells (the so-called true Cuticle). Between these two may be observed yet 
another boundary-zone, which, when the microscope is focussed to it, passes over the 
field of view like a shadow. The inner shell which assumes the blue colour, as well 
as the outer cuticularised substance, are each composed of a system of layers. In 
the latter moreover the radial stria- 
tion is more evident, as is shown 
in Fig. I'j , A, a, b \ these radial lines 
are not, as was formerly thought, 
pores, but are the transverse sec- 
tions of layers which, in a front 
view of the cuticle (Fig. 37, B, j), 
appear as stri8e, and, following the 
veins of the leaf lengthways, pass 
over the septa of the cells {q). 
An example of strongly lignified 
cell-walls split up into three shells 
occurs in the dark-brown-walled 
sclerenchymatous cells which com- 
pose the firm bands between the 
fibro-vascular bundles in the stem 
of Pteris aquilina (Fig. 38). The 
very thick wall between two cells 
contains a hard, dark-brown lamella 
{a) in the centre of the double cell- 
wall ; this is followed on each side 
by a light-brown, more horny shell 
{b) ; and this encloses a third shell 
likewise light-brown. By boiling in 
nitric acid with potassium chlorate 
the first {a) is dissolved, and the 
cells are thereby isolated (see Fig. 
28, p. 28); the two other shells of the cell-wall {b and c) remain unchanged by the 
maceration, except that they lose their colour; and hence the shell c is shown to 
be composed of different layers, some more and some less watery (Fig. 28, C, c). The 
three shells also show a diflerent behaviour on treatment with concentrated sulrhuric 
acid ; a becomes a dark reddish brown and does not swell, or only slightly ; b swells 
in the radial direction and becomes thicker ; while c swells in the radial, tangential, 
and longitudinal directions (see Fig, 38, C, c, and D, c) ; in transverse sections c breaks 
away from b and curves in a vermiform manner (C) ; in longitudinal sections it is bent 
in a wavy manner {D). 
In true wood-cells, e.g. in P'mus syl'vestris (Fig. 24, A, p. 25), three shells are likewise 
generally to be distinguished ; one in the centre of the double cell-wall {A, m), next 
a thicker one (x), and then an innermost (i) ; the two first turn yellow on treatment with 
solution of iodine or iodine and sulphuric acid, the innermost blue with the latter 
reagent ; % and i are dissolved by concentrated sulphuric acid, while the central lamella 
Fig. 36.— Pollen of Thnnbergia alata (X 550). / and // placed in 
concentrated sulphuric acid ; IV, V, VII after solution of the intine ; 
sometimes the fissures of the extine run so that isolated pieces of it fall 
off, corresponding to the lids of the extine of other pollen grains, e.g. of 
Cjtcurbita ; III in Schultz's solution, section; VI in strong solution of 
potash, e extine, t intine. The fissure? of the extine clearly arise from 
subsequent internal differentiation, in the same manner as the elaters are 
formed from the so-called 'special mother-cells' of the spores oi Equise- 
tii7?i. (See Book II., Equisetacere.) 
