Chapter 8 of Steinbeck's The Grapes of Wrath reunites Tom with his family and the reader is given a perspective on the dynamics of the Joad family.  Clearly, Ma is the emotional anchor for the others: 

Her full face was not soft; it was controlled, kindly.  Her hazel eyes seemed to hae experienced all possible tragedy and to have mounted pain and suffering like steps into a high calm and a superhuman understanding.

The others look to Ma as a gauge of their own emotions; if she does not display hurt, they are not hurt.  When a happy occasion comes, she controls the joy by building her laughter for them to nourish their own.  Her generosity is suggested when Old Tom tells her there are men outside who are hungry and she eagerly invites "them" in.  Of course, her husband has tricked her so she will not know Tom has returned.  At the sight of him, she is taken aback, then grateful, uttering, "Thank God," but suddenly she worries that he has escaped from prison, "Tommy, you ain't wanted?  You didn't bust loose?" 

As Tom, like the others derives his emotion from hers, Ma controls her tears of happiness as "her joy was nearly like sorrow."  When she sees that Tom has bitten his lips to control himself from crying, she gains control.  Anxious yet about her son, she asks if he has become "mad"; she worries that prison has made him into a true criminal.  Tom denies that he has stayed away from things that would have made him hate and be "mad."   Ma again thanks God, and she cautions him not to try to fight alone against the men who are tearing down the houses.