What does the plant represent in the play A Raisin in the Sun?

The plant in
represents Mama's dream of someday having her own house, with her own
little garden in the back.  The dream has been a long time coming, and at times she's been close
to giving up hope, but the fact that she keeps nurturing the plant and that it means so much to
her shows that she never gives up - on her dream, herself, or her children.

The plant is first mentioned moments after Mama appears in the play for the first time
in Act I, scene i.  The notes state that she "goes to the window, opens it, and brings in a
feeble little plant growing doggedly in a small pot on the windowsill.  She feels the dirt and
puts it back out" (39).  Just as the plant continues to "doggedly" grow despite
its poor environment, Mama continues to hold on to her dream of someday owning her own home.  In
the meantime, she nurtures and cares for her plant as best she can.

In a
household often filled with anger and arguments, however, Mama is frequently reminded of the
struggles the family continues to face.  She sees bothandslipping away from her, not holding on
to the values she has tried to instill or being the people she wants them to be.  This concern
shows itself in her concern for her plant:

"Lord, if
this little old plant don't get more sun than it's been getting, it ain't never going to see
spring again" (40).

Whenand Mama talk about possible
uses for the insurance money, Mama mentions that she is thinking of putting some money down on a
house so the family can move out of their cramped apartment.  This, she explains, has been her
dream for a long time, something she and her late husband had hoped to do many years before. 
They had even had a particular house in mind:

"Looks
right dumpy today.  But Lord, child, you should know all the dreams I had 'bout buying that
house and fixing it up and making me a little garden in back" (45).

They were never able to buy that house, and Mama's regret is
apparent as she looks at her plant.  When Ruth points out, "You sure loves that little old
thing, don't you?" Mama explains why her plants means so much to her:

"Well, I always wanted me a garden like I used to see sometimes
at the back of houses down home. This plant is close as I ever got to having one"
(53).

When Beneatha later criticizes Mama for wanted to
bring that old plant to the new house, Mama adamantly states: "It expresses me!"
(121). 

The news that the money has been lost leads Mama to give up
momentarily on her dream - she tells the others that they better call the moving men and tell
them not to come.  Then, as the notes state:

A sense of
waste overwhelms her gait; a measure of apology rides on her shoulder.  She goes to her plant,
which has remained on the table, looks at it, picks it up and takes it to the windowsill and
sits it outside, and she stands and looks at it a long moment (139).

In this scene, we see Mama resigning herself, it seems, to the idea
that this is the only garden she will ever have.  However, with the ultimate decision, after
Walter rediscovers his pride, to move into their new house, Mama readies her plant - it is one
of the final items to be packed, as she steps out to finally see her dream be
realized.