Seed-coat development in Ranunculus sceleratus L. has been studied by
electron microscopy. Three layers developed from the single integument
. The outer epidermis consisted of elongated and flattened cells that
were always well attached to each other. The cells were characterized
by thin walls, the presence of chloroplasts and small vacuoles. The ce
lls of the middle layer were originally closely packed. Gradually, ext
ensive intercellular spaces were formed. The cells of the inner epider
mis elongated initially, until they became cubic and developed a thick
wall with numerous wall ingrowths at the side bordering the nucellus.
Thus, they give rise to a mechanical layer protecting the inner part
of the seed. The elongation of cells, the thickening of cell walls, th
e formation of wall ridges, and the formation of intercellular spaces
each coincided with characteristic configurations of microtubules. Pla
smodesmata were originally found between all cells of the integument b
ut their number decreased drastically during development, especially b
etween the three developing seed-coat layers. Well-differentiated chlo
ro-amyloplasts, present in all cells of the developing seed-coat, poin
ted to autotrophy during development. Maturation eventually led to the
disappearance of cytoplasm in all cells, the compression of the cells
of the outer epidermis and middle layers, and the formation of a mech
anical layer from the inner epidermis.