What are some good themes or main ideas developed in chapters 17 through 21 in the novel The Grapes of Wrath?

Here are the main ideas in Chapters 17-21 of The
Grapes of Wrath.

Chapter 17 describes how the
families traveling to find a better life camp along the road and become one large
family. While they are together, rules are established within the group. Those who will
not follow the rules are forced to leave, while those who conform are a part of this
larger "family," and visit as families might at the end of the day, sharing stories and
dreams, and then resting for the trials that will be faced the following
day.

In Chapter 18, Steinbeck describes what happens to the
Joads as they reach the Arizona border. A man they meet reports to Tom that migrant
workers (such as themselves) are called "Okies," which has become a derogatory term.
Noah decides to settle at a river he and Tom visit. In a delirium, Granma speaks to
Grampa.

Ivy Wilson reports that they will not be traveling
with the Joads any farther, as Mrs. Wilson is ill. (She has cancer, but has told no
one.) Casy refuses to pray over her, and the family leaves behind money and food for the
Wilsons.

As the family reaches the California border, Ma
reports to the border guard that they are rushing a sick woman to the doctor and he
allows them to pass, but when they arrive at a town along the way, she says Granma
doesn't need a doctor. When the group finally arrives at the final group of mountains
before them, Ma admits that Granma died before they reached the last check point, but
she lied in order to guarantee their passage through.

Ma's
comment that the "family" seems to be "falling apart" becomes more and more accurate as
time passes.

In Chapter 19, the reader learns of the
history of the landowners in California. These were people who settled in the area by
taking away land from the Mexicans. Ironically, as people pour into the area from the
dust bowl, the landowners are fearful that the migrants will take their land from
them.

Chapter 20 finds the Joads
leaving Granma with the coroner, as they cannot afford a funeral. Tom and the others are
warned by a man they meet to act "bull-simple" (mentally slow) in order not to get in
trouble with the police. Casy wants to do something to repay the Joan family. Rose of
Sharon tells Connie she is sick, he regrets leaving Oklahoma, and walks away. Later, Tom
goes looking for him and has to knock him unconscious to bring him
back.

Ma cooks for the family what little she has, and
leaves some for other hungry children there. A confrontation takes place between Floyd
and a contractor about work, and Floyd is arrested. Tom trips a deputy, but Casy takes
the blame and is arrested. Tom is trying to control his anger. As they travel on, one
town turns them away, so Tom drives around it.

Chapter 21
describes how the landowners feel threatened by the migrant workers. They arm
themselves. They keep wages low but raise the cost for the products they sell to control
or drive smaller growers out of business. They don't understand the hunger and fear they
see in the faces of the newly arriving migrant workers. They convince themselves that
they are in the right; they have lost sight of the needs of these segments of humanity
hit hardest by the Depression.