Zea mays

pappus (calyx, 8.9) of each floret assists in the
wind dispersal of the numerous small fcuits (8.10).
In mOSt flowers the corolla consists of a
number of petals (8.11) which arc variously modified
to attract pollinators (5.7, 8,6, 8.7) but the
corolla is greatly reduced or absent in flowers
where the pollen is wind-distribured (8.8). In
animal-pollinated flowers nectaries are often
present (3.30, 8.6) and their sugary secretion
attracts pollinators; in many bee-pollinated
flowers the epidermal cells of the petals contain
ultraviolet-absorbing flavonoid pigments which
arc visible to the bee and act as guides to the
nectaries (8.ll). Beracyanins in the epidermal
vacuoles of the petals and carotenoids in the
chromoplasts onen cause bright colouring (8.6,
8.7) which ;)trracrs pollinators.
Petals are usually ephemeral structures with a
thin cuticle and few stomata, while the mesophyll
is usually non-photosynthetic. In actinomorphic
(radially symetrical) flowers perianrh members are
all of similar size and distribured regularly around
the receptacle {1.1, 8.6, 8.11, 8.12}. However,
many flowers are bilaterally symmetrical (zygomorphic,
5.7, 8.7). As with the other floral
organs, the perals are often fused (8.5, 8.13)
although sometimes their tips remain free (5.7).
Androecium
E.1ch stamen is terminated by an anther (1.1, 1.31)
which contains the po11en grains (8.14, 8.16). The
anther is borne on a filament which is eirher
inserted directly onto the receptacle or the corolla.
The filaments are sometimes of unequal length
(8.15), and may be very shaft (1.31) whilst in
orchids filaments arc absent. In some flowers the
filaments arc fused to form a rube round rhe gynoecium
and in the Composirae the amhers are fused
(8.13). A vascular bundle runs along the filament
(8.17) and supplies water and nutrients to rhe
anther. The anther is commonly bilobed (8.15l
with two cylindrical pollen sacs present in each
lobe (8.13, 8.18). The ripe anthers usually split
longitudinally to release their pollen (8.14, 8.18),
but in some plants (e.g. Rhodode'ldron, heathers)
the pollen is released from apical pores (8.16).
In the young anther (8.5) several layers of
hypodermal cells divide periclinally within each
pollen sac to give rise to a central mass of sporogenous
cells and the investing parietal tissue
(8.l3). The larrer differentiates into the hypodermal
endothecium, an intermediate parenchymatous
layer and the tapetum which surrounds
the sporogenous rissue (8.19). The endorhecial
walls normally develop numerous bands of
thickenings on the amiclinal and inner periclinal
walls (S.20) which may be lignified. Most anthers
146
dehisce along specialised longitudinal tracts of
epidermal cells (sromia, 8.18, 8.20) which arc
underlain by unrhickened endothecial cells. In
species such as Phaseo/lts vulgaris (bean), self
pollination occurs in the unopened flower (cleistogamy).
The grains germinate in situ and the pollcn
tubes penetratc the anther wall to reach the
stlgma.
The tapetum plays a vital part in the nutrition
of the developing sporogenous tissue, and consists
of a layer of densely cytoplasmic cells (8.19)
which are commonly bi- or multinucleate (8.21).
In many amhers these cells remain intact but in
species with an invasive (amoeboid) tapetum their
walls degenerare (8.22). As a result a coenocytic
periplasmodium is formed from rheir combined
protoplasts, which direcrly invests the developing
sporogenous cells. Within each anther the pollen
mother cells typically undergo synchronous
meiotic divisions (8.21, 8.22).
In many dicotyledons the four immature
haploid pollen grains lmierospores) formed by
meiosis of the pollen mother cell (microsporogenesis)
lie in a tetrahedron (8.22). Hence each
grain has rhree inner faces in contact wirh thc
orher grains. However, an isobilateral segmentation
pattern is more frequent in monOCOtyledons.
The pollen grain is initially uninucleate
(8.23) bur the first mirotic division, thar often
occurs in rhe anther, divides the maturing grain
into a large vegetative and a smaller generative
cell. In Drosera the pollen grains remain united as
tetrads, but in roe majority of flowering plants toe
four grains separate as roc im'esting callosic wall
of toe pollen mother cell dissolves. In many
orchids and Rhododendron the pollen adheres
together in a sticky mass termed a pollinium (8.16).
The outer wall of the mature pollen grain
(exine) is extremely complex (8.23 to 8.25) and
comprises an inner layer of sporopollenin (nexine,
which is very resistant to decay), and the outer
sexine. The larrer is permeated by sporopollcnin
and is composed of fused rods (baculac). The
exine parrern of different species is very variable
(8.24, 8.25) and of great taxonomic significance.
The inner region of the pollen grain wall is
cellulosic and is termed the inrine.
Gynoecium
In many flowers (8.11, 8.13, 8.14) the carpels
show fusion (syncarpy). In such flowers the
ovaries are united but sometimes the styles remain
separate (8.14) or rhe stigmas are lobed (8.26).
While most floral parts arc generally ephemeral
the ovary, containing the ovules, continues growth
after fertilisarion and develops into the fruit (1.6,
8.27).