The Effect of Gamma Rays on Man-in-the-Moon Marigolds is a play by Paul Zindel, first produced in 1965.
It focuses on three members of the Hunsdorfer family: mother Beatrice and her daughters Tillie and Ruth. The Hunsdorfers live in what used to be Beatrice's father's vegetable shop, but the shop has been closed for years. Beatrice married young, a disastrous failed marriage that ended in divorce (later, her husband died by heart attack). Now the Hunsdorfers are scraping by in poverty, with apparently their only source of income being the $50 a week that Beatrice gets for boarding Nanny, a senile old woman. Beatrice is angry and bitter about her fate, hating the whole world, projecting that hate out onto her daughters. Ruth has epilepsy, and at some point in the past had a mental breakdown—a condition that runs in the family, apparently, given her mother's school nickname of "Betty the Loon". Younger sister Tillie is a bright high-school student with a talent for science, but her vicious mother, hating everyone who's better off in life than she is, seeks to crush Tillie's success.
- As You Know: Ruth got a peek at her permanent record at the high school, which she tells her mother about, which is how the audience learns that Beatrice is divorced and her husband died of a heart attack and Ruth had a mental breakdown.
- Bad People Abuse Animals: Discussed Trope. All of the Hunsdorfers are appalled to hear that Janice Vickery obtained her cat skeleton, which is in competition against Tillie's flower experiment, by boiling a cat alive. When Janice presents her skeleton at the science fair (in her only scene), she insists that she got a dead cat that was euthanized at the animal shelter.
- Then there's Beatrice herself who dumps beers on Tillie's pet rabbit while it's in its cage and finally makes good on her threats to kill it (leaving the body out in full view so her daughters are sure to see it.)
- Berserk Button: Beatrice, who seems on the verge of exploding with rage throughout the play, finally loses it completely when Ruth taunts her with the "Betty the Loon" nickname the other kids taunted her with in high school. Beatrice completely loses it, and kills Ruth and Tillie's pet rabbit.
- Dysfunctional Family: Bitter, drunken Beatrice, her angry, resentful daughter Ruth, and poor Tillie, just trying to get out.
- The Fantastic Trope of Wonderous Titles: Used with irony in this instance, as the primary mood in the play is not wonder but despair and anger. Although Tillie's wonder at the majesty of the atom does make this trope apply to a certain extent.
- The Ghost: Mr. Goodman the science teacher, Tillie's mentor. Often on the other end of phone conversations with Ruth or Beatrice, and spoken of by Tillie, but never seen or heard.
- Lady Drunk: Beatrice, bitter and hostile about her low station in life, jealous of anyone with anything better, self-medicates with alcohol. She's quite drunk by the last scene.
- Measuring the Marigolds: Doesn't actually have much to do with this trope (which is named for a song in the film Hans Christian Andersen (1952)). However, Tillie's appreciative knowledge of atomic radioactivity, and the derision of both her classmates and her own mother, is similar.
- Proud to Be a Geek: Tillie, who knows the half-life of radioactive elements cold, loves to talk about atoms, and enters in science fairs, despite the mockery from her schoolmates and the overt hostility of her mother.
- Minimalist Cast: There are only five characters—Beatrice, Tillie, Ruth, Tillie's science fair rival Janice Vickery, and Nanny the senile boarder. And of those five, Janice only appears in one scene while Nanny never talks, leaving the Hunsdorfers to carry the story.
- Significant Name: Probably not a coincidence that Tillie's science teacher and only positive role model is named "Goodman".
- The Speechless: Nanny.
- Title Drop: It's the title of Tillie's science fair experiment, which is studying marigolds grown from seeds that received varying amounts of exposure to radiation.