Also titled "Drifting Spores". The fourth chapter in volume 14 of the Horror World of Junji Ito series, The Story of the Mysterious Tunnel.
Ryoichi is walking home late when he sees Masao, a boy who had recently stopped coming to school. Masao appears to be catching moths in a net, but when Ryoichi calls out to him, Masao acts scared and tells Ryoichi to go away.
Ryoichi tells his friend Anzu what happened. She is worried about Masao's absence, and Ryoichi suspects that she has feelings for Masao, but Anzu denied it. Suddenly, they spot a mysterious black spore that looks like a large dandelion clock. When Ryoichi touches it, it says in Masao's voice "I like you, Anzu-chan, I like you."
Later, Ryoichi is at home and notices the spore has followed him there. He reaches out to it and it again repeats in Masao's voice "I like you, Anzu." Ryoichi works out that it can speak when touched. His mother hears Masao's voice and thinks Masao is visiting, but he's not. They catch the spore in a jar. Ryoichi takes it over to Masao, but Masao gets embarrassed that the spore has told of his true feelings. He tells Ryoichi to ignore it.
Ryoichi's friend Kazuya finds another one of the spores: this time, saying in Masao's voice that it hates Kamamura (a classmate who idolizes Masao) and wants to kill him. Ryoichi says that Kazuya must return the spore to Masao. They go over to his house but when Masao doesn't open the door to them, Kazuya opens the window, accidentally letting out hundreds of the spores. Masao is humiliated as they speak his feelings, including his belief that Anzu would fall in love with him if Ryoichi weren't there.
Kazuya promises to collect the spores, but instead of returning them to Masao, brings them to school to show off to others. Masao, bereft and alone, recalls how he began coughing up mysterious particles when he was sick and they grew into the spores that spoke of his feelings. Anzu arrives to see Masao and hears them saying, in his voice, that he wants to die. Masao denies this and says it's a lie - but commits suicide after she leaves.
After this, the spores start to appear from other people and reveal all their innermost thoughts, including those of celebrities. People begin to catch them to hear others' secrets; and many arguments and even murders ensue. Ryoichi visits Anzu, who has started to stay home from school. He discovers from one of her spores that even though she knew of Ryoichi's feelings for her, she loved Masao.
Meanwhile, Kazuya has become obsessed with having the biggest and best collection of spores. His most treasured is one from a famous actress revealing that she plans to fire her manager. When his mother throws out his collection because she can't stand the noise, he beats her to death. The only one that survived is one of his own - created because Kazuya is getting sick. Kazuya tries to catch it, but is still ill, and loses energy quickly. He falls asleep and when he wakes, the room is full of the particles, saying in his voice that he killed his mother. They drift out of the window and escape.
Soon after this, the spores stop appearing and dissolve naturally into the ground. Ryoichi thinks it's all over, but then a giant tree appears in the school playground. It's Kazuya's spore, saying that he killed his mother. All the spores have grown into trees and begin to appear all over Japan, blaring and screeching. There's nothing anyone can do except to cut them down and hope they will become extinct.
Ryoichi takes Anzu to see another tree: a spore of Masao's, confessing of his real feelings for her.