IC2 
Peirce , — The Dissemination and 
tinous seed-coat, (5) then three or four layers of tannin cells, (6) the 
endosperm, (7) and finally the embryo. Thus we have seven concentric 
layers of differentiated tissues. As A. occidentals is slightly different I may 
go into some detail as to the structure of the fruit. Figure 3 is a diagram 
of a longitudinal section of a fruit with the ‘ seed ’ still in it, and with a part 
of the stalk still attached. The diagram falls into three parts from top to 
bottom. The uppermost part, cut off from the rest by the oblique line 
a-b , is covered by heavily cutinized epidermis ( c . ep), one-layered, with the 
outer cell-walls much thickened, and many stomata, the guard-cells of 
which are depressed (see Fig. 5). Underlying this are several layers of 
parenchymatous cells (< c,p ) containing chlorophyll grains in abundance, 
and with cellulose walls. Under this are two or three layers of somewhat 
elongated cells (/.) with lignified walls more or less spirally thickened. This 
conical layer is continuous nearly or quite to the top. Within this, again, 
are parenchymatous cells in which part of the gelatinous f seed 5 is em- 
bedded. Below the line a-b the structure is surprisingly different. The 
epidermis (ep) is scarcely cutinized, and has no stomata. Underlying this 
is a very gelatinous collenchyma (g. c), the thin places in the cell-walls of 
which are still cellulose. This gelatinous collenchyma is several layers 
thick, and abuts within upon the gelatinous ‘ seed ’ coat (g.f. c). This coat 
extends nearly around the ‘seed/ is absent on the end which is to be 
forward when it is thrown out, and is thickest at the top, which will be 
the back end in flight. The ‘ seed * is covered all around with a sclerotic 
coat (s.f. c.) from one to three cell-layers thick. Enclosed within this is the 
endosperm (end), in one end of which the embryo (emb) lies. At the line 
c-d is the so-called abscission layer, a single row of very thin-walled cells 
(Fig. 6) lying between masses, above and below, of thick gelatinous- 
walled cells. 
A cross-section of a fruit from which the seed has been expelled is 
shown in Fig. 4. 
Passing now to the mechanics of discharge one sees at once that the 
top of the fruit is so made as to be able to resist considerable pressure from 
within the fruit, that the middle and lower parts will develop pressure when- 
ever they can absorb enough water, and that the line of mechanical 
weakness is the so-called abscission layer, where the fruit is attached to the 
stalk. Given, then, the conditions such as I have described above — 
abundance of water in the soil and in the host, from which the parasite 
absorbs water and the gelatinous parts of the fruit take it up, and air so 
moist that little water is lost from the fruit by evaporation — it is inevitable 
that a very considerable pressure must develop in the fruit. MacDougal 
(1899), describing another species, says: ‘During the ripening period the 
contents of the expulsory layer undergo such chemical changes as to give 
the contents a very high isotonic coefficient. The consequent osmotic 
