Zea mays

times becomes highly lobed (2.15, 2.23) and in
elongate, narrow cells may be spindle-shaped.
The chromatin (DNA complexed with histones)
is not organised into chromosomes in the interphase
cell of higher plants but regions of denselystaining
heterochromatin and lighter euchromatin
arc often visible (2.14, 2.23, 2.24). One to several
nucleoli (2.5, 2.25) occur within the nucleoplasm;
these contain stores of ribosome precursors (2.26)
which apparenrly migrate into the cytoplasm via
the nuclear pores. Prominent vacuoles arc sometimes
evident within the nucleoli (2.15, 2.25,
2.26). The nuclei of meristematic cells are usually
diploid; bU( D:-.JA replication in interphase is nor
necessarily followed b)' nuclear division and in
actively metabolising tissues the cells are frequently
polyploid.
From the Onset of mitosis (prophase) the
chromatin is organised into discrete chromosomes
which become aligned at metaphase (2.27) on the
equator of the mitotic spindle. Meanwhile the
nucleolus has disappeared and the nuclear
envelope fragmented. The fibres of the mitotic
spindle. which are just visible at LM level (2.28).
actually consist of aggregated bundles of microtubules.
These are generated at the poles of tbe
spindle and attach to chromosomes and the fibtes
run between the twO poles.
At metaphase each chromosome consists of n'Vo
chromatids joined at their kinctochores, while
mictotubules arc linked to each kinetochore
(2.27). At anaphase the sister chromatids separate
along the fibres to opposite poles of the mitotic
spindle (2.29). Finally, at telophase, the chromatin
becomes dispersed so that discrete chromosomes
are no longer visible. Each nucleus becomes
invested by an envelope, the nucleoli reappear
whilst a cdl plate (new cell wall) separates the two
progeny nuclei (2.25).
Plastids
A variety of plastids with differing metabolic significance
occurs in plants (2.30) but, within a
particular cell, generally only one form is present
(2.24). However, in dedifferentiating cells both the
mature and juvenile types may occur (2.15,2.32).
All plastids arc bounded by a membranous
envelope (2.30, 2.33); the outer membrane is
continuous bur the inner membrane sometimes
shows invaginarions into the matrix (stroma). The
latter contains ribosomes (70s) and circular DKA,
which are both chemically distinct from those of
the nucleus, while starch and lipid droplets
(plastoglobuli) are frequendy present (2.12, 2.30,
2.33). Imernal membranes usually occur in the
stroma and these sometimes form complex con-
figurations (2.30, 2.33). Plastid interconversions
arc common (2.30), for instance amyloplasts can
turn green (2.34) and form chloroplasts, while the
latter may divide to fotm young chloroplasts
(2.30, 2.35) or senesee and give rise to chromoplasts
(2.36).
Proplastids
These precursors of othet plastids are usually 1-2
]lm in maximal width and may be rounded or
amoeboid and they contain few internal membranes
(2.37). Proplastids occur in meristematic
cells (2.2, 2.37) and in the root apex up ro 40 are
ptesent per cell. Proplastids apparently divide and
their population remains more or less stable in the
cells formed during the numetous divisions within
the apical meristcms. However, as the derivatives
of the apical cells grow and differentiate (1.I8)
their plastid populations generally increase, and
the plastids develop into the various types
characteristic of different tissues and organs of the
mature plant (2.30).
Chloroplasts
At maturity these are usually oval ro lenticular
and 5-10 ]lm in length (2.24, 2.33. 2.35). In the
leaf mesophyll (2.24) up to 50 chloroplasts per
cell are commonly present. but in some species
they arc even marc abundant. The membranes of
the envelope are separated by a space 10-20 nm
wide (2.33) and the inner member sometimes
shows connections with the thylakoid membranes
in the stroma. These membranes are predominantly
orientated parallel to the long axis of the
plastid (2.33, 2.35) and are normally elaborated
into a complex three-dimensional, photosynthetic
system. This consists of grana interconnected by
stromal lamellae or frets (2.33, 2.35, 2.38).
Each granum consists of 2-100 flattened and
stacked membranous discs and each granal
membrane encloses an intrathylakoidal space
(2.33, 2.39). The sub-structure of the granal
membranes is highly complex, with the chlorophyll
molecules being integral components, and
theit functioning in the light reactions of
photosynthesis is under intensive research. The
major proteinaceous component in the stroma is
the enzyme ribulose diphosphatate carboxylase.
Starch is commonly present in the stroma of the
chloroplasts (2.24, 2.35) but represents a temporary
store of excess carbohydrate. Plastoglobuli
(233, 2.35) are frequent and comain pigments
concerned in electron transport while phytoferritin
(2.40) deposits are sometimes observed. During
leaf development the chloroplasts increase III
number per cell by division (2.30, 2.35).
27