Video Transcript
The diagram shows pollen landing on
the stigma of a flower. What has to happen next in order
for fertilization to occur? (A) This pollen cannot fertilize
the egg cell. Fertilization can only occur once
the stigma has broken off, allowing the pollen to land directly on the now-open
ovule. (B) The pollen releases its nucleus
into the stigma. Transport proteins allow the
nucleus to travel from cell to cell down the style. Eventually, the nucleus enters the
ovary and then the ovule. (C) The pollen germinates and forms
a pollen tube through which two sperms can travel down to enter the ovule. Or (D) the pollen swims down the
style, digests the ovary wall, and with that gains access to the ovule.
Fertilization is a process that
involves a male sex cell fusing with a female sex cell. Let’s start by learning more about
the structures that have been mentioned in the question, pollen and the stigma,
before we take a closer look at what needs to happen for fertilization to occur.
Pollen contains the male sex cells
of flowering plants, sperm cells. The stigma is a part of the female
reproductive organ of a flower. To access and fertilize the female
sex cells, the male sex cells within pollen first need to land on the stigma. Each female sex cell, which is
called an egg cell, is located in a structure called an ovule. Ovules are found in a flower
structure called the ovary, located below the stigma and separated from it by a
structure called the style. In this diagram, there is only one
ovule within the ovary. So there is only one egg cell. So what happens after the pollen
grain that contains the male sex cell lands on the stigma?
First, the pollen grain is said to
germinate. This results in a structure called
a pollen tube growing out of the pollen grain down through the style toward the
ovary and the ovule within it. The pollen tube allows the passage
of two sperm cells away from the pollen grain and into the flower’s ovary. From there, the sperm cells can
enter the ovule, where one of them can fertilize the egg cell.
Therefore, after pollen has landed
on the stigma of a flower, what needs to happen next for fertilization to occur is
best described in option (C). The pollen germinates and forms a
pollen tube through which two sperms can travel down to enter the ovule.
