In chapter 9, one of the interchapters, the narration asks: "  How can we live without our lives?  How will we know it's us without our past? No. Leave it. Burn it."  Then in the next chapter, before the family sets off on the trip to California, Ma goes through things one last time and goes through the old box of stationery that also held some possessions such as clippings, photos, earrings, etc.  She lovingly went through the box, choosing just a few tiny items, then burned the rest of it.  Steinbeck in those two chapters wants the reader to see and to feel the utter despair of the migrant people as they had to leave behind everything they'd ever known.  Not only did they have to leave their homes, families, and friends, they also had to leave their possessions.  Many of the possessions that were left behind were momentos like those in Ma's stationery box.  There simply was no room for sentimentality, figuratively and literally. The migrants could not afford to let themselves become overly saddened by their plight.  They had to move forward and remain as positive as possible rather then let themselves become wrapped up in the sadness of what they had to leave.  Literally, there was no room on the truck, or car, or wagon for anything that was not essential and momentos were not essential.  Old photos and letters and souvenirs would not feed, clothe, or shelter them so those things could not go with them.