=Pericarp.= The tomato is a typical berry, the ovary wall, free from the
calyx, forming the fleshy pericarp, which encloses chambers filled with
a clear matrix, containing the seeds. The pericarp consists of an outer
tough membrane, the epidermis, a more or less thick layer of parenchyma
tissue, the pulp, and an inner thin, delicate membrane, the lining layer
of the loculi or chambers in which are the seeds. The epidermis consists
of a single layer of cells which have a very thick continuous cuticle
about one-half of the diameter of the whole cell. The cuticle differs in
chemical composition from the rest of the cell walls, being impervious
to water, and resisting rotting longer than do the cellulose walls. As
it is continuous over the whole of the fruit, the skin can be readily
separated from the other tissues. Hot water facilitates the removal of
the skin, as it causes the cellulose of the walls to swell more than the
cuticle, producing an effect as of shrinkage of the outer wall and a
consequent curling of the skin. The radial walls of the epidermis are
short and irregularly thickened, leaving pits in the walls, and giving
them a beaded appearance. The skin constitutes about 1.3 per cent of the
tomato.
The layers of parenchyma just beneath the epidermis are closely united
and flattened, with their adjoining walls irregularly thickened. On
account of their position, they are called hypoderm. In the tomato the
hypoderm consists of two or three layers of cells, parts of which
usually separate with the epidermis. Below these cells are the
thin-walled parenchyma cells, which are approximately globular, vary
considerably in size, are very loosely held together, and have many
intercellular spaces. These cells constitute the mass of the pulp, and
with the juice constitute 96.2 per cent of the tomato.
The layer of cells which lines the chambers has the typical leaf
epidermal structure, the wavy outlines, the hollows and protuberances of
adjoining cells fitting one another so that they form a continuous
layer. They are also flattened laterally. The structure can be
understood readily when it is known that the pericarp is really a
metamorphosed leaf and that the outer side of the leaf forms the inner
wall of the ovary.
The chambers of the tomato are filled with a clear, slimy matrix in
which the seeds are embedded. The matrix consists of parenchyma cells of
various sizes and with delicate walls, and a small nucleus. The cells
are massed loosely, and can be separated readily. In those cells, as
well as in the wall cells, are starch grains which vary in size, being
round or approximately so, and having the hilum, when visible, a
straight line to one side of the center.
=Coloring Matters.= In the parenchyma cells are two coloring matters,
one yellow, which is amorphous in structure, and the other red and of
crystalline form. The sap contains a yellow color in solution which
differs in its reactions from those in the pulp.
=Red Color in Tomatoes.= The red coloring matter in tomatoes is in the
form of irregularly shaped crystal-like chromoplasts, which occur in
masses of various sizes. They are present in largest amounts usually in
the protoplasm which lies close to the ectoplasm and in that surrounding
the nucleus. They vary from sharp, bright-colored forms to those more or
less blunt in outline, and dull in color. They may be situated largely
in the periderm, the soft parenchyma beneath the periderm, or through
the whole mass of the parenchyma with the exception of the matrix
surrounding the seeds in the loculi. In tomatoes having the color in the
periderm a considerable amount is lost by adherence to the skin. The
chromoplasts are not affected by rotting to the same extent as are the
other constituents of the cell; they can be found floating free in the
debris from rotted cells, still retaining considerable color. They lose
their color gradually, in some varieties much more rapidly than in
others. In stored pulp which has fermented, the color may be faded to a
dull yellowish brown. In tomatoes intended for ketchup where a bright
red color is desirable, care should be used in the selection of a
variety having the chromoplasts bright, properly oriented, and in
sufficient quantity.
=Vascular Bundles.= In the pulp of the tomato are found strands of
vascular tissue, entering from the stem, and dividing and ramifying
through the soft pulp. These consist of long tubes with thin walls, some
of which have a strengthening band in spiral form on their interior
walls, the associated cells being without any special marking. The
strands vary in size from those having a few tubes to those having a
large number.
=Seeds.= The seeds of the tomato are small, flattened, yellow bodies
covered by a clear gelatinous membrane. Their peculiar characteristic is
the out-growth of hairs of varying lengths. The seeds constitute about
2.5 per cent of the weight of the tomato.