What is cell division ? Why do cell needs to to divide ?

Mitotic cell division enables sexually reproducing organisms to develop from the one-celled zygote, which itself was produced by meiotic cell division from gametes. After growth, cell division by mitosis allows for continual construction and repair of the organism.

Explanation:

Most of the plant's roots get submerged under water. In this condition, they may not get oxygen for respiration; so the roots are forced to respire anaerobically. This results into accumulation of lactic acid which kills the plant.

That is why there may be little plant life near a river or stream; especially in water-logged areas.