Zea mays

1.22
1.22 Group of rail Cordy/hIe al/stralis trees, which are
monocoryledons with anomalous secondary thickening. In
this and !>Ome other arborescem monocotyledonous genera
the secondary thickening meristem gives rise to discrete
bundles of xylem and phloem which are embedded in a
ground mass of fibres and parenchyma.
1.23 T$ through the seedlmg shoot apex of the
monocotyledon lea mays (maize). The shoot apex (I) is
invested by progressivel)' older leaf primordia (2--6) which
are arranged in twO ahernate rows. These leaves are
enclosed by the single coleoptile (7), a leaf·like cylindrical
structure which protects the plumule before its emergence
at germination. llM x 85.)
1 Shoot apex
2--6 Leaf primordia
7 Coleoptii(o
<
1.21 (Left).Diagrams showing the typical differences between monocotyledons (series A) and herbaceous dicotyledons
(series 8). Al to 81 arc longitudinal sections of seeds; note the single cotyledon (I) in Al but paired cotylt:dons (I) in B1.
A1 to 82 illustrate the following morphological differences: elongate leaves in monocotyledons which :Ire usually without
petioles and with parallel "eins; in dicotyledons relatively shorter, usually petiolate. leaves occur which show bmnched and
non'parallel veins. The floral parts are in thrtts in monocotyledons contrasting with fives (or fours or indefinite numbers)
in dicotyledons.
A3 to 83 and A4 to 84 show tht> v:lscular systems in T$ of rhe srem and root rcspecrively, with xylem coloured blue and
the phloem red. The monocotyledonous stem (A3) has many scanered vascular bundles consisting of inner xylem and
outer phloem, whereas in dicotyledons (B3) the stem bundles are usually arranged in a ring with vascular cambium present
between the phloem and xylem. The roOlS of monocotyledons (A4) are polyarch with numerous separate suands of
phloem separated b)' xylem and a central pith is usuaUy present. In dicotyledons (84) a pith is often lacking. and from the
xylem core several arms radiate outwards between the phloem strands. Plumule (2), radicle (31.
I CoT}'Il"don
2 Plumule
3 Radicle
19
1.23