327 - 11th Botany Textbook Volume 1

consists of 3 layers, the outer and inner
are fleshy (sarcotesta), the middle layer
is stony called sclerotesta. The inner
layer remains fused with the nucellus.
The nucellus grows out into a beak-like
structure and the upper part dissolves and
forms a cavity-like structure called pollen
chamber. A single megaspore mother cell
undergoes meiosis to form four haploid
megaspores. The lowermost becomes
functional and others get degenerated. The
nucellus gets reduced in the form of a thin
papery layer in mature seeds and encloses
the female gametophyte An enlarged
megaspore or the embryo-sac is present
within the nucellus. An archegonial
chamber with 3-6 archegonia are present
in the archegonial chamber below the
pollen chamber (Figure 2.46).
Micropyle
Pollen Chamber
Megaspore
Female Gametophyte
Outer Layer
Middle Layer
Inner Layer
Figure 2.46: L.S. of Ovule
Seed
(2n)
Embryo
(2n)
Oospore (2n)
CYCAS (Male)
(2n)
CYCAS(female)
(2n)
SPOROPHYTIC
GENERATION
(2n)
Male cone
(2n)
Female strobilus
(2n)
Fertilization
Male nuclei
(n)
(syngamy)
GAMETOPHYTIC
GENERATION
egg
(n)
(n)
Embryo sac
(n)
Antherozoids
(n)
Megaspores
(n)
Megasporangium
(2n)
Megaspore Mother cell
Meiosis
(R/D)
(2n)
Microsporangium
(2n)
Microspore Mother cell
(2n)
Microspores
(n)
Figure 2.49 2.47: Life cycle Cycle of Cycas of Cycas
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