Page 159 - Biology_F5

Biology for Advanced Level Secondary Schools

reduced into three nuclei, namely two Members of class Monocotyledoneae,
male nuclei and a pollen tube nucleus. The include maize, grass, millet, and sugar
female gametophyte is found in the ovule’s cane plants; and members of class
embryo sac; the ovary is reduced into eight Dicotyledoneae, include bean plant,
nuclei, namely three antipodal nuclei, two mango tree, and orange tree.
polar nuclei, and an egg contained between A recent estimate of the number of
FOR ONLINE READING ONLY
two nuclei called synergids (Figure 3.29). flowering plant species globally is
Following pollination, the pollen grain on approximately 400,000, distributed into
the stigma develops a pollen tube, carrying about 393 families. When partitioned
two sperm nuclei that grow down to the into their old two broad classes, a larger
number of species are dicots, represented
embryo sac via micropyle. One of the by 336 families. In contrast, monocots are
sperms fertilises a haploid egg to form a represented by about 57 families.
zygote, which later develops to form an
embryo. The second sperm fertilises the two Distinctive features of class
polar nuclei forming triploid nutritive tissue Monocotyledoneae
called endosperm. This process is called The following features possessed by
double fertilisation, because fertilisation members of the class Monocotyledoneae
occurs twice. A fertilized ovule becomes a (Figure 3.30) differentiate them from
seed surrounded by two integuments (seed those of class Dicotyledoneae.
coat) and the ovary develops into a fruit. (a) Leaf venation is parallel in monocots
and the leaf blade or lamina is elongated
Germinating pollen grain with dorsal and ventral surfaces more
Stigma or less identical.
Style
Pollen tube (b) The stem anatomy shows vascular

bundles, scattered in the ground tissue
Ovary wall
Nucellus and closed as they lack vascular
Embryo sac Antipodal nuclei cambium, hence no secondary growth.
Polar nuclei
Synergids (c) They have a fibrous root system, which
Ovum
Micropyle arises at the base of the stem, replacing
Tube nucleus Sperm nuclei the primary root.
(d) The monocot seed’s embryo bears one
Figure 3.29: An ovary
cotyledon.

Classes of division Angiospermophyta (e) Flower parts are usually trimerous (in
The classification system recognised two threes or multiples of three).
classes of Angiospermophyta, based on (f) Pollen grains in the monocots mostly
the number of cotyledons. The two classes have one aperture.
are class Monocotyledoneae and class
Dicotyledoneae.

Form Five Student’s Book
152