The haustorium is the cotyledonary hyperphyll
or the distal part of it that is inside the seed in contact with
the nutritive tissue. When the haustorium is embedded in
the endosperm, its shape is cylindrical, globular or flattened
to some extent, depending on the shape of the seed. When it
is laterally attached to the endosperm, its shape resembles a
lens or shield. Often the haustorium increases in size during
the germination process to keep contact with the dissolving
endosperm. The spatial shape of the haustorium is predetermined
by the available space inside the seed. Since
the haustorium is part of a unifacial hyperphyll, it cannot
bear a margin in a strict morphological sense, and any
ribs, keels or other surface structures have no particular
morphological significance. It thus cannot be considered a
lamina, nor can it have a margin as has sometimes been
suggested for the scutellum of Poaceae.