explain why the plant which grow …

The growth of plants depends largely on the phenomenon of osmosis. Plants are composed of a large number of microscopic units called cells. . Cells contain a fluid called cell sap.  and its wall is composed of a living cytoplasmic membrane which is semipermeable and is responsible for the phenomenon of osmosis  in the plants.
Osmosis : The net spontaneous flow of the solvent molecules from the solvent to the solution or from a less concentrated solution to a more concentrated solution through a semipermeable membrane is called osmosis.

When the plant cell comes in contact with water or some dilute solution , the osmotic pressure of which is less than that of cell sap present in the plant cell, there will be a tendency of water to enter into the cell through the cell wall. This causes rupture of cell and this process is called Hemolysis.
In marshy lands the soil is highly wet and most of its minerals are washed away. Thus plants decay due to Hemolysis in marshy areas because of the contact of cells with soil water which is a low concentration solution since most of the minerals are already washed away. Thus plant cells rupture and hence plants decay.