Annals of Botany 78 : 295–304, 1996
An Ultrastructural Study of The Embryo/Endosperm Interface in the Developing
Seeds of Solanum nigrum L. Zygote to Mid Torpedo Stage
C. L. B R I G G S
School of Biological Sciences, The UniŠersity of New South Wales, 2052, NSW, Australia
Received : 20 November 1995 Accepted
19 February 1996
The early developmental sequences in the formation of the Zone of Separation and Secretion in a hexaploid species
of Solanum nigrum L. are described. Ultrastructural changes which occurred during the development of the
embryo}endosperm interface could be related to the different stages in the embryo’s development. The first step was
the completion of the cell wall around the chalazal end of the zygote ; a thin wall was formed along the endosperm
cell(s) abutting the zygote. From the mature zygote stage to the quadrant stage, minute plasmalemma invaginations
occurred along the endosperm wall facing the zygote. These invaginations enlarged, and from the mid-globular stage
onwards became filled with a fine fibrillar material ; this material accumulated between the endosperm cell wall and
the plasmalemma before being released into the developing periembryonic and intercellular spaces to become the
extracellular matrix. Cell wall development in the endosperm cells abutting the embryo followed an unusual path.
During the quadrant stage, whilst the outer embryo wall increased in thickness due to vesicle fusion, the endosperm
cell wall facing the embryo showed a loosening of the wall fibrils as well as partial separation of these same endosperm
cells from each other. From the early-globular stage, the endosperm cell walls opposite the embryo became electrontranslucent, disappearing into the extracellular matrix. Enzymic secretions by the embryo may account for the
alteration in the abutting endosperm cell walls. Enzymic activity may also explain the development of a homogenous
electron-opaque layer over the outer embryo wall as well as the differences in the width of the fibrillar layer which
accumulated around the cotyledons as the embryo grew through the Zone of Separation and Secretion. The potential
roles of the extracellular matrix are briefly discussed.
# 1996 Annals of Botany Company
Key words : Solanum nigrum L., embryo}endosperm interface, Zone of Separation and Secretion, embryo
development, cellular endosperm.
INTRODUCTION
The interface which develops between the growing embryo
and the surrounding endosperm is an area of embryology
which has received little attention. This is surprising since it
is across this interface that solutes, metabolites, etc. must
pass from the endosperm to the growing embryo and
substances (e.g. hormones, enzymes) produced by the
embryo can be transferred to and act upon the endosperm.
Most electron microscope studies carried out to date have
concentrated on species which have an ab initio free nuclear
endosperm development (Schulz and Jensen, 1968 a, b ;
Newcomb, 1973 ; Smart and O’Brien, 1983 ; Schel, Kieft and
Van Lammeren, 1984 ; Gori, 1987 ; Jones and Rost, 1989 ;
Yan, Yang and Jensen, 1991 ; Johansson and Walles, 1993 ;
Chamberlin, Horner and Palmer, 1994) and the surface
features of the embryo have either not been, or incompletely
been, described. Ultrastructural studies on ab initio cellular
endosperm species are few (Van Went, 1970 a, b, c ;
Mogensen and Suthar, 1979 ; Briggs, 1990, 1992, 1993 a, b)
and again no details of the interface between the young
embryo and endosperm were given.
Early work on the Solanaceae described the occurrence of
a zone of degenerating cells which developed ahead of the
globular or early heart-shaped embryo (Beamish, 1955 ;
Dnyansagar and Cooper, 1960 ; Erdelska! , 1985). However,
a recent investigation into early seed development in
0305-7364}96}090295­10 $18.00}0
Solanum nigrum L. revealed that the zone developing ahead
of the embryo was not caused by the degeneration of the
endosperm cells but was a product of living endosperm cells
(Briggs, 1990). This zone was called the Zone of Separation
and Secretion (ZSS) and part of its development has been
described elsewhere (Briggs, 1993 b). What was not covered
in that paper were the earliest stages of the interaction
between the embryo and endosperm and the changes which
occurred to the outermost wall of the embryo as it undergoes
each morphological change. It is the aim of this paper,
therefore, to increase our understanding of endosperm}
embryo interactions by describing the development of the
interface in S. nigrum from the zygote to the mid-torpedo
stage, a period which encompasses the embryo’s growth
through pre-ZSS endosperm and through the ZSS itself.
MATERIALS AND METHODS
Developing seeds were fixed, dehydrated, and infiltrated
with London Resin White (medium grade) according to the
method outlined in Briggs (1993 a, b). Specimens were
polymerised in gelatine capsules at 60 °C. One micrometre
thick sections were cut and stained with 1 % Toluidine Blue
in 1 % sodium bicarbonate to determine the stage of embryo
development. Over 90 seeds were examined.
# 1996 Annals of Botany Company
296
Briggs—Endosperm}Embryo Interface in S. nigrum
F. 1. Boundary between the apical end of the young zygote (Z) and the endosperm (EN) at the three-celled endosperm stage. The newly forming
zygote wall (arrow) consists of a thin, compact fibrillar layer whilst the adjacent endosperm wall (arrowheads) has a looser arrangement of fibrils.
Vesicles (v) can be seen fusing with the endosperm and zygote plasmalemmas. Islands of electron-opaque material (*) left over from fertilization
lie between the zygote and endosperm. Golgi body (g), mitochondrion (m), plasmalemma (pm). Bar ¯ 0±5 µm.
F. 2. Boundary between the apical end of the mature zygote (Z) and the endosperm (EN) 5 d after anthesis. Two distinct cell walls (arrowheads)
can be seen in some places. The endosperm plasmalemma (pm) undulates around the zygote ; smaller undulations occur along the adjacent zygote
plasmalemma. The plasmalemmasome (*) in the zygote may be a fixation artefact. Unbound ribosomes have increased in abundance within the
zygote but decreased in density in the endosperm. Mitochondrion (m). Bar ¯ 0±5 µm.
Briggs—Endosperm}Embryo Interface in S. nigrum
Electron microscopy
Ultrathin sections were cut with a diamond or glass knife.
Pale gold sections were collected upon copper slot grids
coated with 0±2 % Piloform. Sections were stained with
Reynolds’ lead citrate (Reynolds, 1963) for 5 min, 2 %
uranyl acetate (aq.) for 10 min then Reynolds’ lead citrate
again for 5 min. Grids were examined using a JEM-100CX
TEM at 60 kV, or a Phillips 400 TEM at 100 kV. Five to ten
seeds from each developmental stage were examined.
RESULTS
In S. nigrum, endosperm development is ab initio cellular
and becomes many celled before the first division of the
zygote 4–5 d after anthesis ; embryo development follows
the Solanad type (Saxena and Singh, 1969).
Egg, zygote and proembryo
The mature egg cell was approximately 60 µm in length and
the chalazal end was distinguished by an absence of any cell
wall. This resulted in the egg and central cell plasmalemmas
being either in close contact or slightly separated by an
electron-translucent space (Briggs, 1992). Following fertilization, fusion of vesicles with the plasmalemmas of both
the zygote and adjacent endosperm cells (Fig. 1) resulted in
the deposition of PAS-positive wall material around the
chalazal end of the zygote (Briggs, 1990). At the three-celled
endosperm stage (1–2 d after anthesis in summer, 3–4 d
after anthesis in spring), the new wall appeared as a very
thin, fibrillar layer consisting of a thin, compact, electronopaque layer on the zygote side and a less electron-opaque,
but more loosely arranged fibrillar layer on the endosperm
side (Fig. 1). By the late zygote stage, two very thin electronopaque lines were present (Fig. 2). One of these lines was
continuous with the wall surrounding the base of the zygote
whilst the other was continuous with the remaining portions
of the endosperm cell wall. Between the two cells were
islands of electron-opaque material which consisted of
degenerated cytoplasm released from the synergid following
penetration and discharge of the pollen tube (Figs 1, 4 and
5). Throughout maturation of the zygote, the endosperm
plasmalemma exhibited an undulated appearance (Figs 1
and 2). Infoldings of the zygote’s plasmalemma were
occasionally found in the apical region (Fig. 2). The
abundance of unbound ribosomes was initially low in the
early zygote stage, but later the population increased.
Although plasmodesmata were present in the micropylar
297
region of the mature egg cell wall (Briggs, 1992) they were
occluded by the mid-zygote stage (not shown) and no
plasmodesmata were found anywhere along the surface of
the mature zygote.
Proembryo to quadrant stages
The first division was transverse and resulted in the
formation of a two-celled proembryo. At this stage, the
developing cell walls of the proembryo and adjacent
endosperm cells were quite distinct (Fig. 3) and were
positively stained with toluidine blue and the PAS-reaction
(not shown). Around the chalazal end of the proembryo, the
plasmalemma of the apical proembryo cell was strongly
invaginated forming ‘ blebs ’ which contained dispersed
fibrillar material. Less numerous and less strongly developed
blebs (which contained electron-translucent material), occurred along the adjacent endosperm plasmalemma. Small
vesicles were again found close to the proembryo plasmalemma. Vesicles were also found outside the endosperm
plasmalemma (Fig. 3). These latter vesicles resemble the
plasmatubules described by Chaffey and Harris (1985).
The smaller apical cell divided horizontally to form two
cells, each of these divided longitudinally to form four cells
in two tiers (quadrant stage) ; the larger micropylar cell
divided transversely to start the filamentous suspensor.
(Note : the suspensor will not be considered further in this
paper, all subsequent references are to the cells which form
the embryo portion only.) By the three-celled embryo stage,
the large plasmalemma invaginations of the two-celled
proembryo stage had disappeared, the embryo wall became
more closely associated with its plasmalemma although
minor undulations of the plasmalemma could still be found
(Fig. 4). The adjacent endosperm cell wall was unusual. It
was less well defined compared with the two-celled proembryo stage. The fibrillar component was less compact
and it was either appressed to the surface of the embryo
(Fig. 4), so thin as to appear absent (Fig. 5) or thicker but
withdrawn from contact with the embryo (Fig. 6). During
the four-celled quadrant stage, some areas of the endosperm
wall became even more diffuse in appearance (Figs 7 and 8).
Fine fibrils extended from its surface towards the endosperm’s plasmalemma (Figs 6, 7 and 8) and also towards the
embryo (Figs 6 and 8). The endosperm’s plasmalemma was
slightly undulated. Microtubules were associated with the
endosperm plasmalemma (Figs 6 and 9) as well as the
embryo’s (Fig. 9). Plasmatubules were found along the
outer surface of the embryo (Fig. 5), within invaginations of
the embryo plasmalemma (Fig. 8) and as larger clusters
within invaginations of the endosperm plasmalemma (Fig.
F. 3. Interface between the apical cell wall of the two-celled proembryo (PE) and the adjacent endosperm. Two distinct cell walls are still present
(arrowheads). Plasmatubules (pt) occur outside the endosperm plasmalemma. Largish invaginations (*) of the proembryo plasmalemma occur
at this stage. Mitochondrion (m), vesicle (v). Bar ¯ 0±5 µm.
F. 4. Interface between the endosperm (E) and the apical end of the three-celled embryo (TE). The embryo’s plasmalemma (pm) exhibits
minute undulations. The endosperm cell wall appears to be appressed to the surface of the embryo and its fibrillar component is more loosely
arranged. Very small undulations of the endosperm plasmalemma occur. The islands of electron-opaque material (*) are still present. Rough
endoplasmic reticulum (rer), microtubules (mt), mitochondrion (m). Bar ¯ 0±5 µm.
F. 5. Same cell as for Fig. 4 but upper lateral side. In this section the endosperm cell wall appears to be absent. Plasmatubules (pt) can be seen
along the outer surface of the embryo. Apical cell of embryo (TE), endosperm (E), rough endoplasmic reticulum (rer), vesicle (v). Bar ¯ 0±5 µm.
298
Briggs—Endosperm}Embryo Interface in S. nigrum
F. 6. Same cell as for Figs 4 and 5 but a different part of the interface. The loose arrangement of the endosperm cell wall can be clearly seen
(arrowhead). Fibrils extend from the endosperm cell wall (arrowhead) to the embryo wall (w) and also to the endosperm plasmalemma (pm).
Embryo (TE), endosperm (EN), microtubule (mt). Bar ¯ 0±5 µm.
F. 7. Quadrant stage of embryo development. Interface between one lower tier embryo cell (QE) and endosperm (EN). Separation of the two
adjacent endosperm cells has occurred (*) to accommodate embryo enlargement. The endosperm cell wall remains a collection of loose electronopaque fibrils (arrowheads). Bar ¯ 0±5 µm.
F. 8. Quadrant stage and interface between one upper tier cell (QE) and the endosperm (EN). Small plasmatubules (arrows) line a plasmalemma
invagination in the embryo. The fibrils of the endosperm cell wall (arrowhead) are loosely arranged and fibrils extend from it to the embryo wall
(w). Bar ¯ 0±5 µm.
Briggs—Endosperm}Embryo Interface in S. nigrum
9). Also during the quadrant stage, separation of adjacent
endosperm cells commenced (Figs 7 and 8). Each of the cells
of the quadrant divided once to form the octant stage. There
was no noticeable difference between the quadrant and
octant stages.
Globular stage
Periclinal division by each of the octant cells resulted in the
formation of the protoderm. With the formation of the
protoderm the early globular stage could now be resolved ;
the mid-globular stage was recognized by the presence of
the procambial cells in the micropylar half of the embryo.
Between the early- and mid-globular stages, there was little
change to the embryo}endosperm interface. However, when
the embryo reached the mid-globular stage, numerous
distinct blebs were found along the plasmalemma of those
endosperm cells abutting the surface of the embryo. These
blebs contained a network of fine fibrils and were separated
from the adjacent embryo by a very thin electron-opaque
wall and a periembryonic space which contained very small,
electron-opaque particles (Fig. 10). Although dilated rough
endoplasmic reticulum cisternae were found to be continuous with the blebs in later stages of the development of
the Zone of Separation and Secretion (Briggs, 1993 b) no
such continuity was found during this early stage. Between
the mid- and late-globular stages, the fibrillar material
contained in the plasmalemma blebs began to accumulate
and as merging occurred, the distinctive blebbing disappeared. During this period the cell wall surrounding the
endosperm cells abutting the embryo became electrontranslucent (Note : thin, yet distinct cell walls were revealed
by staining with toluidine blue see Briggs, 1993 b.) An
incomplete electron-opaque layer now occurred along the
surface of the embryo (Figs 11 and 12). The accumulation
of the fibrillar material is part of the lipo-carbohydrate
matrix material described elsewhere (Briggs, 1993 b).
At the late-globular stage when the provascular cells had
differentiated in the upper part of the embryo, the outer wall
of the embryo had increased in thickness and consisted of a
dense, compact, fibrillar layer ; overlying this layer was a
thinner, non-fibrillar, homogeneous layer. On the outer
surface of this homogenous layer was found particulate
material which was continuous with the loose fibrils of the
lipo-carbohydrate matrix. Plasmastubules were surrounded
by the accumulating lipocarbohydrate matrix (Fig. 13).
Rupturing of cell walls of the endosperm cells ahead of the
embryo occurred, releasing the accumulated lipo-carbohydrate material (which now constitutes the lipo-carbohydrate matrix of the Zone of Separation and Secretion)
into the endospermal intercellular spaces and around the
embryo (but not the suspensor). The Zone of Separation
and Secretion now began to extend ahead of the embryo
(Briggs, 1993 b).
299
Heart-shaped and early torpedo stages
Increased cell division across the chalazal end of the
globular embryo resulted in the formation of a wedgedshaped embryo termed early heart-shaped stage. At the
early heart-shaped stage, the thickness of the outer embryo
wall and the overlying homogeneous layer was unchanged
but the thickness of the particulate material covering the
homogenous layer decreased (Fig. 14). The extracellular
lipo-carbohydrate matrix continued to accumulate around
the embryo, particularly around and ahead of the apex of
the embryo (Briggs, 1993 b). Whilst the embryo progressed
through the the mid- to late-heart shaped stages (indicated
by the enlargement of the cotyledonary domes and
hypocotyl), there was little change to the outer protodermal
wall. The fibrillar material in the lipo-carbohydrate matrix
was still dispersed regularly ahead of the embryo and the
adjacent walls of the endosperm cells were still electrontranslucent ; plasmatubules were no longer found within the
extracellular matrix surrounding the embryo (Fig. 15).
However, throughout the torpedo stage, as the cotyledons
elongated and were pushed into the lipo-carbohydrate
matrix by the lengthening hypocotyl, a fibrillar layer
accumulated around the apex (Fig. 16) and sides of the
cotyledons. This fibrillar layer decreased in thickness and
degree of compaction the further it was from the cotyledon
tips (Fig. 17) ; it was absent across the protodermal cells
covering the apical meristem (Fig. 18) and along the
hypocotyl (Fig. 19). From the mid-torpedo stage onwards,
endosperm cells adjacent to the lower portion of the
hypocotyl developed a thick electron-opaque cell wall facing
the embryo (Figs 19 and 20).
DISCUSSION
In Solanum nigrum, each morphological stage in the
embryo’s development was associated with important
changes in the embryo}endosperm interface. The first
change was the sealing off of the zygote from the developing
endosperm. A similar sealing off process was found in
Capsella bursa-pastoris (L.) Medic. (Schulz and Jensen,
1968 a), Quercus gambelii Nutt. (Singh and Mogensen,
1975) and Helianthus annuus L. ‘ Dang Yang ’ (Yan et al.,
1991). However, in H. annuus only a single wall is formed
between the zygote and endosperm and this is not completed
until the two-celled stage (Yan et al., 1991) ; no data was
given as to whether an endosperm cell wall was produced in
Capsella or Quercus. The second change occurred between
the two-celled and quadrant stages, when the endosperm
cell walls abutting the embryo became altered and the cells
became partly separated from each other. Such a loosening
of the wall fibrils and the separation of the cells implies
enzymic activity, possibly induced by signals from the
dividing embryo, and would enable the endosperm cells to
F. 9. Quadrant stage and interface between the endosperm (EN) and junction between the two embryo tiers (QE). An invagination of the
endosperm plasmalemma contains many plasmatubules (arrow). Opposite, a small invagination of the embryo plasmalemma contains electronopaque material (*). Microtubules (mt) run along the embryo and endosperm plasmalemma. Bar ¯ 0±5 µm.
300
Briggs—Endosperm}Embryo Interface in S. nigrum
F. 10. Mid-globular stage interface at the lateral side of the embryo (G) and the endosperm (EN). The endosperm’s plasmalemma has formed
many medium-sized blebs which are filled with matrix material (*). A thin electron-opaque line (wall ?) contains the matrix material within the
endosperm cell. Note : the diffuse type of endosperm cell wall is now absent whilst the embryo cell wall (w) is thicker. Bar ¯ 0±5 µm.
F. 11. Mid-globular stage. The distinct blebbing in the endosperm cells has disappeared as the matrix material (*) builds up outside the
plasmalemma. Electron-opaque material occurs over portions of the embryo’s surface (arrowhead). The endosperm cell wall has also become
electron-translucent. Bar ¯ 0±5 µm.
F. 12. Mid-late globular stage. Accumulation of matrix material (*) around the embryo. The electron-opaque material, present over the surface
of the embryo, is not yet continuous (arrowhead). Bar ¯ 0±5 µm.
Briggs—Endosperm}Embryo Interface in S. nigrum
be deformed to accommodate the enlarging embryo without
damage or death to either tissue. These features signalled
the commencement of the Zone of Separation and Secretion.
The third major change was a sequence of events which
started with the production of the lipo-carbohydrate matrix
during the mid-globular stage, the thickening of the outer
protodermal cell wall, and ended with the release of the lipocarbohydrate matrix into the periembryonic and intercellular spaces of the Zone of Separation and Secretion
during the late-globular stage.
A notable feature of the early developmental stages of the
embryo}endosperm interface was the presence of the
plasmalemma invaginations along the embryo and endosperm cell walls which gradually increased in size when
production of the lipo-carbohydrate matrix commenced.
Similar plasmalemma invaginations were found along the
wall of the endosperm cells abutting the young multicellular
embryo in Zea mays L. strain A-188 but electron-opaque
rather than fibrillar material was present (Schel et al., 1984).
Caution must be exercised in determining the reality of the
initial small invaginations since in many cases it has been
conclusively proven that the undulation of the plasmalemma
is an artifact of chemical fixation (Browning and Gunning,
1977 ; Chaffey and Harris, 1985). However, the larger
invaginations from the globular stage onwards were only
found in endosperm cells producing the lipo-carbohydrate
matrix and may represent adjustment of the endosperm
plasmalemma to the build up of the matrix (they are absent
from areas where the matrix has been released and are not
found in any other part of the endosperm (see Briggs,
1993 b). The question as to the reality of both types of
plasmalemma invaginations along the embryo}endosperm
interface will only be resolved when the same tissues are
preserved by freeze-substitution techniques.
Plasmatubules are fine tubular evaginations of the
plasmalemma which have been found in a variety of tissues
from various plant species (see Harris and Chaffey, 1986).
Because they are found in freeze-substituted as well as
chemically fixed material, they are considered to be real
structures with a postulated role in the short term, high flux
uptake of solutes from the apoplast (Harris, 1981 cited by
Chaffey and Harris, 1985 ; Harris and Chaffey, 1985). No
specific role can as yet be assigned to the plasmatubules
found in S. nigrum. Their presence from the two-celled to
quadrant proembryo stages coincides with the gradual
removal of synergid cytoplasm forced between the egg and
central cell during fertilization. This is also the period when
modifications to the endosperm cell wall facing the
proembryo take place. It is possible that the plasmatubules
are involved in the retrieval of mobilized products released
from the modification of the cell wall, as has been proposed
for cells in developing minor leaf veins of Pisum satiŠum L.
301
(Harris and Chaffey, 1985), as well as those released from
removal of the degenerated synergid cytoplasm.
There are only a few reports on enzymic activities around
and within the developing embryo. Erdelska! (1985) found
the hydrolytic enzymes, acid phosphatase and non-specific
esterase, over the surface of the late-globular embryo as well
as within the adjacent endosperm cells in PapaŠer
somniferum cv. Amarin and Nicotiana tabacum cv. Tekne.
Acid phosphatase activity has also been found within the
protodermal cells of the heart-shaped embryo of Stellaria
media (Pritchard and Bergstresser, 1969) whilst in Nicotiana
tabaccum, starting at the quadrant stage, there was
considerable plasma-associated phosphatase activity along
that portion of the plasma membrane in contact with the
outer wall of the embryo and suspensor (Mogensen, 1985).
In S. nigrum, the origin and function of the particulate,
electron-opaque material present on the surface of the
homogeneous layer is unknown. However, the homogeneous
layer may represent hydrolysis products derived from
digesting the adjacent extracellular matrix rather than a
cuticle as was reported in Glycine max (L.) Merr. cv.
Harosoy (Chamberlin et al., 1994). The gradual removal
from the surface of the apical meristem and hypocotyl of the
compacted layer of matrix fibrils, which had collected
around the elongating cotyledons, also implies hydrolysis of
the lipo-carbohydrate matrix by the protodermal cells of the
embryo.
The effect of the endosperm on embryo morphogenesis
has been implied by hybridization studies in Solanum where,
although the embryo continued to grow for a short period
after endosperm failure had occurred, it nonetheless failed
to differentiate (Beamish, 1955). Growth and morphogenesis
in plants has also been shown to be regulated by
oligosaccharides, which are derived from the hydrolysis of
complex wall polysaccharides (Fry, 1986). Is it just
coincidence that in S. nigrum the increased cell divisions
which lead to the formation of the heart-shaped and
torpedo stages occur after the start of matrix accumulation
around the apical end of the embryo and the hydrolysis of
the extracellular lipo-carbohydrate matrix ? The formation
of the Zone of Separation and Secretion, apart from
nourishing the embryo and facilitating its passage through
the endosperm, may also be involved the process of
morphogenesis.
A C K N O W L E D G E M E N TS
This work was supported by a Sydney University Research
Grant and the work was carried out at the Sydney University
EM Unit. Support was also received from Professor A. E.
Ashford, School of Biological Sciences, The University of
New South Wales.
F. 13. Late-globular stage. The thick, fibrillar outer cell wall of the embryo has become completely overlain by a homogeneous, less electronopaque outer layer (arrow). Particulate material (large arrow) of unknown origin, occurs between the surface of the embryo and the accumulating
matrix (*). The electron-translucent endosperm cell wall is indistinguishable from the matrix. Plasmatubules (pt) can be seen within the matrix.
Plasmalemma (arrowhead). Bar ¯ 0±5 µm.
F. 14. Mid-heart-shaped stage, tip of one cotyledon. The particulate material seen over the surface of the late globular embryo has diminished.
The outer embryo wall is still overlain by the homogenous layer. The protodermal cells are full of unbound ribosomes. Microtubules (mt), embryo
(H), extracellular matrix (*). Bar ¯ 0±5 µm.
302
Briggs—Endosperm}Embryo Interface in S. nigrum
15
16
17
18
F. 15. Mid-heart-shaped stage, lateral region of the embryo showing the extracellular matrix (*) filling the space between the embryo (H) and
the adjacent endosperm cell (EN). Whilst the embryo grows through the endosperm, the adjacent endosperm cell walls remain electron-translucent.
Particulate material (arrowheads) occurs in places along the outer surface of the embryo. Lipid body (l). Bar ¯ 0±5 µm.
F. 16. Curved torpedo stage, apical region of the inner cotyledon. As the embryo grows through the Zone of Separation and Secretion, a layer
of compacted fibrils (f) accumulates around the tips of the cotyledons. The embryo wall (w) has increased in thickness but the homogeneous layer
has disappeared. Vesicles (v) are found close to the plasmalemma. Bar ¯ 0±5 µm.
F. 17. Same embryo as Fig. 16. Glancing section through an inner protodermal cell of the outer cotyledon which is close to the meristem. The
layer of fibrils is wider yet less compact. The homogeneous layer over the embryo wall is less distinct. Microtubules (mt). Bar ¯ 0±5 µm.
Briggs—Endosperm}Embryo Interface in S. nigrum
303
F. 19. Curved torpedo stage, radicle end of the hypocotyl. The endosperm cell lying opposite the hypocotyl’s protodermal cell has developed
a thick cell wall (w) consisting of loosely arranged electron-opaque material. The outer wall of the embryo (H) is again overlain by the
homogeneous layer (arrowhead). Glyoxysome (g). Bar ¯ 1±0 µm.
F. 20. Enlargement of the endosperm cell wall (W) from Fig. 19. Bar ¯ 0±25 µm.
LITERATURE CITED
Beamish KI. 1955. Seed failure following hybridisation between the
hexaploid Solanum demissum and four diploid Solanum species.
American Journal of Botany 42 : 297–304.
Briggs CL. 1990. Embryo sac nutrition in Solanum nigrum and
Tephrosia grandiflora. PhD. Thesis, Macquarie University, Sydney
Australia.
Briggs CL. 1992. A light and electron microscope study of the mature
central cell and egg apparatus of Solanum nigrum L. (Solanaceae).
International Journal of Plant Sciences 153 : 40–48.
Briggs CL. 1993 a. Endosperm development in Solanum nigrum L.
Formation and distribution of lipid bodies. Annals of Botany 72 :
295–301.
Briggs CL. 1993 b. Endosperm development in Solanum nigrum L.
Formation of the Zone of Separation and Secretion. Annals of
Botany 72 : 303–313.
Browning AJ, Gunning BES. 1977. An ultrastructural and cytochemical
study of the wall-membrane apparatus of transfer cells using
freeze-substitution. Protoplasma 93 : 7–26.
Chaffey NJ, Harris N. 1985. Plasmatubules : fact or artefact ? Planta
165 : 185–190.
Chamberlin MA, Horner HT, Palmer RG. 1994. Early endosperm,
embryo, and ovule development in Glycine max (L.) Merr.
International Journal of Plant Science 155 : 421–436.
Dnyansagar VR, Cooper DC. 1960. Development of the seed of
Solanum phureja. American Journal of Botany 47 : 176–186.
Erdelska! O. 1985. Dynamics of the development of embryo and
endosperm I. PapaŠer somniferum, Nicotiana tabacum and Jasione
montana. Biologia (BratislaŠa) 40 : 17–30.
Fry SC. 1986. In ŠiŠo formation of xyloglucan nonasaccharide : a
possible biologically active cell-wall fragment. Planta 169 :
443–453.
Gori P. 1987. The fine structure of the developing Euphorbia dulcis
endosperm. Annals of Botany 60 : 563–569.
Harris N, Chaffey NJ. 1985. Plasmatubules in transfer cells of pea
(Pisum satiŠum L.). Planta 165 : 191–196.
Harris N, Chaffey NJ. 1986. Plasmatubules—real modifications of the
plasmalemma. Nordic Journal of Botany 6 : 599–607.
Johansson M, Walles B. 1993. Functional anatomy of the ovule in
broad bean (Vicia faba L.) I. Histogenesis prior to and after
pollination. International Journal of Plant Science 154 : 80–89.
Jones T, Rost TL. 1989. Histochemistry and ultrastructure of rice
(Oryza satiŠa) zygotic embryogenesis. American Journal of Botany
76 : 504–520.
Mogensen HL. 1985. Ultracytochemical localization of plasmamembrane-associated phosphatase activity in developing tobacco
seeds. American Journal of Botany 72 : 741–754.
Mogensen HL, Suthar HK. 1979. Ultrastructure of the egg apparatus of
Nicotiana tabacum (Solanaceae) before and after fertilization.
Botanical Gazette 140 : 168–179.
Newcomb W. 1973. The development of the embryo sac of sunflower
Helianthus annuus after fertilization. Canadian Journal of Botany
51 : 879–890.
Pritchard HN, Bergstrasser KA. 1969. The cytochemistry of some
enzyme activities in Stellaria media embryos. Experientia 25 :
1116–1117.
Reynolds ES. 1963. The use of lead citrate at high pH as an electron
opaque stain in electron microscopy. Journal of Cell Biology 17 :
208–212.
Saxena T, Singh D. 1969. Embryology and seed development of
tetraploid form of Solanum nigrum L. Journal of the Indian
Botanical Society 48 : 148–157.
F. 18. Protodermal cell from the apical meristematic zone of the embryo in Fig. 16. Note that the compact fibrillar layer is absent.
Mitochondrion (m). Bar ¯ 0±5 µm.
304
Briggs—Endosperm}Embryo Interface in S. nigrum
Schel JHN, Kieft H, Van Lammeren AAM. 1984. Interactions between
embryo and endosperm during early developmental stages of
maize caryopses (Zea mays). Canadian Journal of Botany 62 :
2842–2853.
Schulz R, Jensen W. 1968 a. Capsella embryogenesis : the early embryo.
Journal of Ultrastructural Research 22 : 376–392.
Schulz R, Jensen WA. 1968 b. Capsella embryogenesis : the egg, zygote
and young embryo. American Journal of Botany 55 : 807–819.
Singh AP, Mogensen HL. 1975. Fine structure of the zygote and early
embryo in Quercus gambelii. American Journal of Botany 62 :
105–115.
Smart MG, O’Brien TP. 1983. The development of the wheat embryo
in relation to the neighbouring tissues. Protoplasma 114 : 1–13.
Van Went JL. 1970a. The ultrastructure of the synergids of Petunia.
Acta Botanica Neerlandica 19 : 121–132.
Van Went JL. 1970b. The ultrastructure of the egg and central cell of
Petunia. Acta Botanica Neerlandica 19 : 313–322.
Van Went JL. 1970c. The ultrastructure of the fertilized embryo sac of
Petunia. Acta Botanica Neerlandica 19 : 468–480.
Yan H, Yang H-Y, Jensen WA. 1991. Ultrastructure of the developing
embryo sac of sunflower (Helianthus annuus) before and after
fertilization. Canadian Journal of Botany 69 : 191–202.