THE CELL-WALL, 
33 
of which may again exhibit numerous layers and the striation already described. In the 
case of exposed cells which require protection (as pollen-grains or spores), or of those 
which themselves serve as a protection to other tissues (as cork), an outermost shell 
(of greater or less thickness) of each cell-wall is transformed into cork or cuticle. 
When the cells are destined to form a firm frame-work (as in wood-cells), the outer 
masses of layers become lignified ; in other cases, on the other hand, the outer layers, 
rarely the inner ones, are transformed into mucilage. Usually an inner layer of the cell-wall 
remains unchanged in all three cases, and gives the above-mentioned cellulose 
reactions, while the suberised and 
lignified shells of the cell-wall 
may, after previous treatment 
with alkalies or with nitric acid, 
also exhibit these reactions; the 
layers which are transformed into 
mucilage are the most refractory. 
Some of the morphological 
relations here treated of find 
their explanation only when we 
study the formation of tissues; 
but I cannot here discuss the 
chemical behaviour of the cell- 
wall ; the reactions must properly 
be regarded not as chemical tests, 
but only as the mean^ of recog- 
nising a morphological differen- 
tiation. The description of some 
examples will be sufficient to guide 
the beginner. 
The pollen of Thunbergia alata 
(Fig. 36) shows that the different 
development of two shells of .a 
cell-wall may go so far that the 
cuticularised shell, the Extine, be- 
comes actually separated from 
the non-cuticularised shell, the 
Intine, which still possesses the 
power of growth ; by this means 
it becomes broken up in most 
cases by fissures previously formed 
into one or two spiral bands. 
This can be artificially induced 
by laying these pollen-grains in 
concentrated sulphuric acid or potash solution ; the extine then assumes a very beautiful 
red colour, while the intine in the first case dissolves, in the second case swells a 
little and remains colourless. In the germination also of many spores {e.g. Spirogyra^ 
Mosses, &c.) the cuticularised exospore becomes completely separated and stripped 
from the endospore, which still continues to develop ; both shells, however — cor- 
responding to the extine and intine of the pollen-grain — consist, in their actual 
development, of systems of layers of a single cell-wall possessing different chemico- 
physical properties. 
Fig. 35. — A pollen-grain of Cticitrbita Pepo, wliich has emitted a pollen- 
tube (sp) into a papilla of the stigma. The cell-wall of the pollen-grain consists 
of a cuticularised extine (e), and an intine capable of growth (i) ; the latter is 
greatly thrckened at certain places (B i) ; on each thickening-mass the extine 
forms a roundish lid (d) ; when the pollen-grain is commencing to emit its 
pollen-tubes, the thick parts of the intine swell, and thus tilt up and lift off the 
lid-like piece of the extine ; one or two of these thickening-masses form pollen- 
tubes (X 550). 
as in the strise; another term must be employed for the structures now under consideration, and 
the expression ' shells' (Schalen) appears to answer the purpose. 
D 
