Zea mays

and the abaxial side forms the lower surface.
A simple leaf may be dissected or lobed, and a
compound leaf shows several leaflets (1.19); these,
however, do not subtend axillary buds. In the
lamina (leaf blade) a network of veins is present
(1.20) which links to the vascular system of the
stem. The axillary (Iatetal) buds may remain
dormant but normally develop into side shoots, or
form flowers. At the base of the main stem the
cotyledons (first leaves formed in the embryo)
demarcate it from the hypocotyl; the latter represents
a transition zone between stem and root.
In the angiosperms two groups have evolved
which show distinct morphological, anatomical
and floral characteristics (1.21). Thc dicotyledons
(crucifers, begonias, willows, oaks) constitute
about two-thirds of flowering plant species and
the great majority show some degree of secondary
(woody) thickening (1.2 to 1.4). Monocotyledons
(grasses, bananas, lilies, 1.1) do not undergo
secondary thickening in the same way as
dicotyledons, bur in some genera large trees may
develop (1.22).
Dicotyledonous leaves are commonly petiolate
(1.17) and normally show a narrow attachment to
the stem. In monocotyledons the leaf is frequently
sessile (without a petiole) and the leaf base often
encloses a large sector of the stem (1.23). Leaves
of dicotyledons are varied in shape and arrangement
of their major veins but normally show a
reticulatc pattern of the small interconnecting
veins (1.20). In monocotyledons the leaf is
typically elongate (1.22), with the main veins
paralleling its length. Their relatively rare lateral
connections 3re normally unbranched (1.24).
In dicotyledons, the radicle (seedling root) is
normally persistent and the older region often
increases in diameter by secondary thickening
(1.25). By contrast, in monocotyledons the radicle
is usually not persistent and an adventitious root
system develops from the base of the enlarging
shoot. In a number of larger monocotyledons their
heavy upright shoots are stabilised by adventitious
proproots (1.26).
Vascular anatomy of angiosperms
The primary vascular systems of mono- and dicotyledons
generally differ considerably (1.21). In
a transverse section of the monocotyledonous
stem (1.27) there arc many scattered vascular
bundles, while in dicotyledons a smaller number
of bundles is usually arranged in a cylinder outside
10
a wide pith (1.28). The roots of monocotyledons
frequemly show a central pith with a large number
of strands of alternating xylem and phloem on
its periphery (1.29). But in the dicotyledons a starshaped
core of x}'lem commonl}' occurs with strands
of phloem lying between its several anns (1.14).
In the majority of dicotyledons a fascicular
cambium separates the primary xylem and phloem
of both the stem and root (1.25, 1.28, 1.30). If
secondary thickening occurs the normally discrete
strands of cambium become linked, and the
continuous ring of vascular cambium produces
secondary xylem internally and secondary phloem
externally (1.4, 1.30). Thc vast majorit}, of
monocotyledons arc herbaceous; however, a
number of palms grow into tall trees as a resulr of
diffuse secondary growth. Others (Dracaena,
Cordylilze) produce new {secondary) vascular
bundles from a secondary thickening meristem
and may form large trees (1.22).
Floral and reproductive features
of angiosperms
In monocotyledons the floral parts (sepals, petals,
stamens and carpels) commonly develop in threes
(1.1, 1.21), whereas in dicotyledons these frequently
occur in fives or fours (1.21). However, a
large and indefinite number of floral parts occur
in many other dicotyledons (1.31). The mature
carpel (female pan of the flower, 1.31, 1.32)
consists of several pans: the terminal stigma
which receives the pollen (].l, J.31), an intermediate
style (1.1) and the basal ovary (1.32). In
most taxa the carpels are fused (syncarpy, 1.6)
rather than free from each orher (apocarpy, 1.31).
Within the ovary, one to numerous ovules are
present and each contains an egg cell at the micropylar
end of the ovule (1.32). The pollen grain
germinates on the stigma and the pollen tube
grows down the style to enter rhe ovule (1.32)
where it liberates two haploid sperm nuclei.
One of these fertilises the egg which forms the
diploid zygote, while rhe other nucleus fuses with
the twO centrally located polar nuclei (1.32) to
give rise to the nutritive endosperm for the
embryo. As the embryo develops from the zygote
it enlarges and the surrounding tissues of the ovule
expand to form the mature seed. The ovary
concomitantly increases in size to form the mature
fruit (1.6). In dicotyledons tWO cotyledons are
present on the embryo (l.21, 1.33), bur in monocotyledons
only a single one occurs (1.21).