Why Does Non-Photosynthetic Monotropastrum humile (Ericaceae) Have Scale Leaves?

Monotropastrum humile (D.Don) H.Hara (Ericaceae), a mycoheterotrophic plant, retains scale leaves of a certain size despite their lack of photosynthetic function. This study aimed to clarify the morphological basis for the persistence of these scale leaves by examining their relationship with floral organs through morphological and anatomical analyses. For the morphometric analysis, measurements were taken at seven locations. For the anatomical analysis, epidermal cells were photographed and analyzed in abaxial and adaxial views. The sizes of scale leaves and floral characters showed allometric growth. M. humile is pollinated by long-tongued bumblebees; it must maintain flower size for effective pollination. Therefore, its scale leaves cannot become allometrically smaller, and it is necessary to invest a large amount of resources into scale leaves. Our studies show that M. humile must constrainedly maintain scale leaves to form flowers, even if leaves lose the function of photosynthesis.