The novel's climactic section, "The White Mulberry Tree" opens on a June afternoon, with Emil Bergson's return from a year-long stay in Mexico City. He accompanies his sister Alexandra to a supper and fair at the local Catholic church. Marie Shabata is there, as well. The two discover that their love has only grown during the year and, during an opportune blackout, they kiss for the first time.
Following the wedding of Signa, Alexandra's maid, and Nelse Jensen, Emil walks Marie back to her farm, and they finally discuss their love openly. Marie recognizes, however, that she cannot run away with him; Emil resolves to leave the Divide soon.
Just over a week later, Emil rides to the farm of his best friend Amedee Chevalier, the prosperous and happy father of a baby boy. Amedee collapses in a field, and it is discovered that he has a ruptured appendix. The operation comes too late, and Amedee dies. Marie Shabata considers the fact that Emil did not come to tell her about Amedee as a sign that he has cut her loose. Overcome by bittersweet emotion in the moonlit beauty of the prairie, Marie determines to embrace a new, ecstatic freedom: "a new life of perfect love."
The following Saturday, the local Catholic bishop is to confirm a hundred children. Emil attends the church ceremony, the joy of which is tempered by the sadness of Amedee's death. Overcome by emotion and the choir music, Emil experiences a rapture that gives him a vivid awareness of life and transcends fears of death. Still in this ecstasy, he goes to say his farewells to Marie. He finds her lying in her orchard, still drifting in her own reverie, and lies down with her. Frank Shabata comes down to the orchard with his gun when he sees Emil's horse in the stable. Shocked to see his jealousy justified, Frank reacts mechanically, shooting blindly through the bushes at the two lovers. Horrified by his actions, Frank mounts Emil's horse and rides wildly away into the countryside.
The next morning, Ivar finds Emil's horse, which has returned to the stable. Fearing something wrong, Ivar goes in search of Emil, and finds both Emil and Marie Shabata dead in her orchard. Aghast, Ivar runs to tell Alexandra.
Some of the most vivid scenes in the novel, such as the fairs at the French Church and the conversations between Alexandra and Marie, do not play particularly important roles in advancing the plot of the novel. But these brief scenes of prairie existence help paint a picture of both the beauty and the difficulty of pioneer life. The novel takes as one of its central themes the role of the prototypical pioneer individual in American society. The fact that the protagonist is female demonstrates Cather's interest in the relationships and communal functions that made prairie life bearable for women; Alexandra's friendships with Marie and Mrs. Lee, and even with her maid, Signa, are integral parts of her life. The great historian Frederick Jackson Turner famously observed that it was in the small, isolated pioneer settlements where raw necessity was the rule that the authentic stuff of American democracy, that sense of coherence among men and women of different ethnicities and classes, was shaped.
It is after Signa's wedding to Nelse Jensen, a marriage seemingly destined for unhappiness, that the romance between Emil and Marie takes a turn that steers the plot towards its eventual tragedy. In his conversation with Marie, Emil resembles Carl Linstrum in his striking undesirability. In contrast to the now composed, sensitive, and mature Marie, Emil shows himself to be pathetic and self-pitying. He is capable neither of resigning himself to reality nor of empathizing with Marie's pain.
Marie's tendency toward infidelity again puts her on the brink of sinfulness, though it is not clear whether she actually commits adultery with Emil. Crazy Ivar, who at times seems a repository of wisdom, has no doubt about Marie's actions, exclaiming, "Sin and death for the young ones!" Alexandra vacillates between blaming the young lovers and absolving them of responsibility; her willingness to consider that perhaps they could not help loving each other stresses her understanding of the role of uncontrollable forces in pioneer life. Once again, the novel throws a veil of ambiguity over human agency and responsibility. Further, the deaths of Emil and Marie allude to the myth of Pyramus and Thisbe, whose tragedy the Latin poet Ovid told at the turn of the first millennium. Pyramus and Thisbe are two teenage lovers whose dying blood courses over white mulberries, turning them crimson. Emil and Marie similarly darken the mulberries in her orchard. One of the functions of this mythic allusion is to universalize the experience of Emil and Marie; as Carl Linstrum remarks, "There are only two or three stories, and they go on repeating themselves."
As has been observed repeatedly, the novel does not really allow for successful romantic relationships. As the critic Blanche Gelfant observes in her introduction to O Pioneers!, there is here "a fatal coupling of love and death, thematic in Cather's fiction." Some critics, Gelfant notes, see this coupling as "reflecting an aversion to heterosexual love." The novel itself, however, presents another possibility: that death is not tragic, but rather transcendent. Death is perhaps the apex of ecstasy; rather than eradicating love, death makes it immortal.
That Emil's revelation comes to him in a Catholic church is no accident: Catholicism is the denomination of Christianity that specifies most clearly the mortification of the flesh--bodily death--as indispensable for the attainment of eternal life. This point is reinforced as Emil leaves the churchyard: "The heart, when it is too much alive, aches for that brown earth, and ecstasy has no fear of death." Death frees Emil from worldly considerations; over the corpses of the two lovers, Ivar sees "two white butterflies... fluttering in and out among the interlacing shadows; diving and soaring, now close together, now far apart." Emil and Marie have attained a blissful and eternal life.