Question

Draw the given diagram and label the following parts.
i) Exine
ii) Tube nucleus

Answer 1

Exine and tube nucleus are the distinct parts of pollen grains. Pollen grains are important as they carry male gametes from anther of one plant to the stigma of another cross-pollinating plant. Exine is the hard protective layer which prevents the pollen from high temperature and other chemicals. Tube nucleus controls the growth of the pollen tube.

Complete answer:
To answer this question, we must know about the structure of pollen grain.
The pollen grain has three parts-
- Central cytoplasm- It contains nuclei which takes part in fertilisation.
- Exine or the outer layer which is the most durable layer of the pollen grain. It is very resistant to disintegration. Intense heat, strong bases or acids do not affect the exine layer. The exine is made up of sporopollenin.
- The inner layer is called the intine which contains cellulose or hemicellulose. This layer can be easily broken.

ii. Tube nucleus- The tube nucleus controls the growth of the pollen tube. It grows down the style into the ovary of a flower. It enters into the micropyle of the ovule and releases its two sperm into the 8 nucleate embryo sac. One sperm fuse with the egg nucleus to form zygote.

Note: The exine prevents the pollen grains from damage. This layer is very useful for the plants which cross pollinate by the help of vectors like wind, water, animals etc. The exine helps in the attachment of the pollen to the insects or helps in adhering to the stigmatic surfaces.