seed is very unusual anatomically. The seed consists of an
by three well-defined tissue layers. The innermost tissue
layer surrounding the embryo is the endosperm. In many seeds,
the endosperm is a bulk type storage tissue that supplies
energy for germination. In lettuce seed, however, the endo-
sperm is a thin (2-4 cells) tissue layer that is associated
with the dormancy mechanism of the seed.
Lettuce seeds, with the
endosperm layer removed, germinate well in the dark and in the
heat. If only the pericarp and integument layers are removed,
dormancy still occurs. In lettuce, therefore, there is an
endosperm that acts like a seed coat, plus a seed coat (the
pericarp and integument layers) that is not as important a
barrier to germination as the endosperm.