I believe that organ is the brain...
Because, it is the process by which they make or produce their food with the help of sunlight, water and carbon dioxide.
If photsynthesis is not there then there would be no production of food and ultimatley plants will die.
C I know this is hard but it’s easy
Answer:
In the case of the algae, each individual cell is responsible for absorbing its own water. This makes the algae nonvascular compared to the highly vascular plant species. In this connection, algae also lack several key structures that are normally present in ordinary plants like the leaves, roots and stem.