sexual reproduction in flowering plants male gametophyte pollen and female gametophyte embryo sac
Channel: ISHTA’s BIO ACADEMY
Date: 2021-01-28

a microscope. You will really be amazed at the variety of architecture –
sizes, shapes, colours, designs – seen on the pollen grains
from different species (Figure 2.4).
Pollen grains are generally spherical measuring about
25-50 micrometers in diameter. It has a prominent two-layered
wall. The hard outer layer called the exine is made up of
sporopollenin which is one of the most resistant organic material
known. It can withstand high temperatures and strong acids
and alkali. No enzyme that degrades sporopollenin is so far
known. Pollen grain exine has prominent apertures called germ
pores where sporopollenin is absent. Pollen grains are wellpreserved as fossils because of the presence of sporopollenin.
The exine exhibits a fascinating array of patterns and designs.
Why do you think the exine should be hard? What is the
function of germ pore? The inner wall of the pollen grain is
called the intine. It is a thin and continuous layer made up of
cellulose and pectin. The cytoplasm of pollen grain is
surrounded by a plasma membrane. When the pollen grain is
mature it contains two cells, the vegetative cell and generative
cell (Figure 2.5b). The vegetative cell is bigger, has abundant
food reserve and a large irregularly shaped nucleus. The
generative cell is small and floats in the cytoplasm of the
vegetative cell. It is spindle shaped with dense cytoplasm and
a nucleus. In over 60 per cent of angiosperms, pollen grains
are shed at this 2-celled stage. In the remaining species, the
generative cell divides mitotically to give rise to the two male
gametes before pollen grains are shed (3-celled stage).
Pollen grains of many species cause severe allergies and bronchial
afflictions in some people often leading to chronic respiratory
disorders– asthma, bronchitis, etc. It may be mentioned that Parthenium
or carrot grass that came into India as a contaminant with imported wheat,
has become ubiquitous in occurrence and causes pollen allergy