Parenchymatous cells that are thickened with cellulose at the corners are called Parenchyma and Sclerenchyma.

Explanation:
Parenchyma and Sclerenchyma are two types of plant tissues found in the ground tissue system of plants. They both consist of cells with thickened corners, but they have different functions and characteristics.

Parenchyma Cells:
- Parenchyma cells are simple plant cells that have thin cell walls and living protoplasts.
- They are the most common type of plant cells and are found in almost all parts of the plant.
- Parenchyma cells are responsible for various functions such as photosynthesis, storage of food and water, and secretion.
- They are usually isodiametric in shape, meaning that their length, width, and height are approximately equal.
- The corners of parenchyma cells are often thickened with cellulose, which provides additional support and strength to the cell.
- These thickened corners are known as sclerenchymatous cells, which are a modified form of parenchyma cells.

Sclerenchyma Cells:
- Sclerenchyma cells are specialized plant cells that have thick, lignified cell walls and dead protoplasts.
- They provide mechanical support and strength to the plant.
- Sclerenchyma cells are of two types: fibers and sclereids.
- Fibers are elongated cells with tapered ends that provide strength and flexibility to the plant.
- Sclereids are short, irregularly shaped cells that provide protection and support to the plant.
- The corners of sclerenchyma cells are thickened with cellulose, hemicellulose, and lignin, which make them rigid and strong.

Similarities and Differences:
- Both parenchyma and sclerenchyma cells have thickened corners, but parenchyma cells are living and have thin cell walls, while sclerenchyma cells are dead and have thick, lignified cell walls.
- Parenchyma cells are involved in various metabolic activities, while sclerenchyma cells provide mechanical support.
- Sclerenchyma cells have a higher concentration of lignin in their cell walls, which gives them more rigidity and strength compared to parenchyma cells.

In conclusion, the parenchymatous cells that are thickened with cellulose at the corners are called Parenchyma and Sclerenchyma. These cells play important roles in the structure and function of plants.