Zea mays

stack (2.14, 2.52) of smooth cisternae 1-2 I-lm in
diameter. Their margins arc frequemly branched
imo a tubular network (2.51) which proliferates a
number of small vesicles. The cisternae are
separated from each other by about 10 nm, but
individual dictyosomes remain intact when
isolated from the cell.
In the longitudinal view of a dictyosome a
polarity is sometimes evident (2.52). At its
forming face the cisternae are thought to be
reconstituted from vesicles budded off from
adjacent membranes of the ER (2.14,2.52). The
cisternal membranes progressively increase in
thickness across the dictyosome and at irs maturing
face the cisternae frequently become concave,
witb vesicles budding off from their margins
(2.14,2.51,2.52).
The vesicles apparently migrate and fuse with
the plasmalemma where the contents are voided
into the apoplasr. Within the dict)'osome, proteins
derived from the RER combine with various
sugars; the secreted vesicles contain carbohydrates
and glycoproteins concerned with cell wall synthesis
(2.8), mucilage (2.19), or nectar secretion.
Dicryosomes are particularly abundant in cells
actively undergoing extensive wall thickening or
forming new dividing walls (2.8, 2.53); in root cap
cells, where cisternae often become inflated with
mucilage and sloughed off whole (2.19); and in
glandular cells. In actively secreting cells a vast
pOtential excess of membranous components
(derived from the dicryosomes) arrives at the plasmalemma,
but some of this material is apparently
returned to the cytoplasm in the 'coated' vesicles
(2.14).
Vacuole
1n the meristematic cell a number of small
vacuoles occur, each bounded by the membranous
tonoplast (2.2, 2.4, 2.13). However, during cell
growth these vacuoles massively enlarge and fuse
so that up to 90% of the volume in a parenchyma
cell is occupied by the vacuole (1.18), "while the
cytoplasm and other organelles are mainly
peripheral (2.24, 2.42). The vacuole contains
various solutes (normally at aboUT O.S M concentration)
and its consequent turgidity greatly contributes
to the [Urgor of the whole protoplast.
The vacuole also contains a number of hydrolytic
enzymes and the tonoplast breaks dmvn during
differentiation of sclercnchyma and tracheary
cells (2.7 to 2.9). The enzymes which are liberated
digest the proroplasts so that only the walls
remain intact (lA, 1.11). Vacuoles may contain
anthocyanins and other pigments and also become
modified as protein bodies in a number of seeds
(2.43,2.54).
Microbodies
These small, membrane-bounded bodies are of
two types. Pcroxysomes occur in close proximity
to chloroplasts (2.38, 2.39); they contain a variety
of enzymes which oxidise the glycolic acid
resulting from photorespiration and then return
glyoxylic acid to the chloroplasts. Glyoxysomes
occur in fatty seeds and contain enzymes catalysing
fatty acid breakdown to release energy
during germination, while the hydrogen peroxide
produced is broken down by peroxidase.
Ribosomes
The cytoplasmic ribosomes are 17-20 nm wide
and occur both floating free within the cytosol
(cytoplasmic ground substance) as well as
attached to the outer surfaces of the RER (2.48,
2.50 and 2.51). Ribosomes contain RNA and
protein and these are composed of two sub-units
which are synthesised in the nucleolus (2.26) bur
subsequentiy combine in the cytoplasm. Ribosomes
arc especially dense in cells which are
rapidly synthesising protein where they frequently
occur in clusters termed polysomes (2.48). The
cytoplasmic ribosomes (80S) are slightly larger
and biochemically distinct from those located in
the plastids and mitochondria (70S).
Microtubules and microfilaments
The microtubules are proteinaceous structures
about 25 nm wide, with a hollow core 12 nm
wide, and may be up to several micrometers long
(2.55). In the non-dividing cell they are normally
located adjacent to the plasmalemma (2.2, 2.55
and 2.56), bur at nuclear division these become
reassembled and aggregate into the fibres of the
mitotic spindle (2.28, 2.56), When mitosis is
completed, the microtubules apparently guide
dictyosome vesicles to the equatorial region of the
spindle where the vesicles fuse to form the cell
plate (2.53. 2.57, 2.58). It has been suggested that
the peripheral microtubules arc concerned with
the orientation of the cellulose microfibrils which
are being formed in the young wall on rhe outside
of the plasmalemma (2.16, 2.17); however, the
evidence for [his is equivocal.
Another smaller type of proteinaceous element
has sometimes been observed in [he plant cell.
This is termed a microfilament and is abom 7 run
in width. In the green alga Nite/la, the micro·
filaments a.re concerned with cytoplasmic streaming;
in pollen tubes of flowering plants they
apparentl)' guide vesicles concerned with wall
synthesis through the cytosol to the growing tip of
the tube.
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