Answer:
The plant represents hope and struggling life. Imagine how hard it is for that poor little plant to survive cooped up in that little tenement apartment. It has one little window that is shadowed most of the day by the larger buildings nearby. Mama gives it love and water, but it needs sunshine to thrive. She can't move the buildings to give it sunshine any more than she can change the world her son and daughter are encountering. She can give love and sustenance (water and in the case of her children food), but she can't move buildings or the power structures that built those tall buildings. Yet by moving to the house, she can provide her family--and the plant--room and possibly enough sunshine to thrive.