The stem is elongated, springing from an irregular black tuber, called sclerotium.
"The Mushroom, Edible and Otherwise" by M. E. Hard
The latter differs in not springing from a sclerotium.
"European Fungus Flora: Agaricaceae" by George Massee
The grain is replaced by a dense fungoid tissue (sclerotium) largely charged with an oily fluid.
"The New Gresham Encyclopedia" by Various