Kumagusu collected all through his life. He copied from dictionaries and encyclopedias in childhood. In adulthood, he sought to comprehend the world through the collection of fungal, plant, animal, mineral and other samples. Witnessing the diversity of Kumagusu's collection, Misawa is amazed at “ his tremendous curiosity and inquisitiveness about things that are a bit rare and things he thinks he will never encounter again.”
In the bird catalogue, which is said to have been drawn in his teens, each illustration of bird and name are inserted in a uniform grid which treats every entry as being of equal worth.

Kumagusu’s fungus collection was particularly outstanding. This became his lifework from around 1900, continuing on through his final years. He authored a number of comprehensive guides on the subject, including “The Illustrated Book of Bionomics of Japanese Fungi”. The book combines as many as 100 specimens with hand-drawn illustrations. Each page contains an actual dried specimen, a colorful sketch by Kumagusu, detailed record of his observations in English, and a spore specimen. “Every page powerfully reveals
the fungi in their 3D glory. I can almost smell them. We sense the closeness of Kumagusu’s gaze manifested in these things,” says Misawa. She describes Kumagusu's unique way of looking at things as “endlessly long, humorous, and perhaps oddly shaped.”

Kumagusu had his mineral chests custom-made. He specified the details of the partitions and boxes in the drawers. Each item is accompanied by a small piece of paper with information about where it was collected. Misawa, who saw how the items were organized, commented, "From things brought from the far side of the world to others he discovered in his garden, he didn’t look for the amazingly meaningful or insignificant. He observed everything with the same all-encompassing gaze," she said.