Parenchyma cells have secondary walls that them retain their structure.

The parenchyma cells have secondary walls that help them retain their structure is called as sclerenchyma.

Explanation:

This secondary wall known as sclerenchyma is form just in the inner region of the primary wall. They provide support in the structure of the plant cell. They are quite strong and thick cell wall. They are not also involve in other function of the plants like photosynthesis since its main function is for support and structure.

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There are 2 seismic waves that travels below the Earth's Surface or through the Earth's interior. They are called the Primary (P) and Secondary (S) Waves. These waves are also called the Body Waves.