Memoirs of an Anthophile

On his way home from a coworker's wedding, Hasumi buys a seedling in hopes of leading a lush life after witnessing the happy bride and groom. While growing the overpriced and labor-intensive plant, he overhears talk of the smitten newlywed at the office. He drops the plant in the bathtub one day, and when he returns home the next, he finds that the plant has soaked up a considerable amount of water and grown twice as large. After continually adding more water over time, the plant finally reaches the point where it spills forth from the tub and a humanoid man emerges from its leaves. Hasumi keeps his distance at first, but after becoming accustomed to their life together and catching a glimpse of the plant man's cock, he takes action.

(Source: Kaiohsha, translated)