Leaves are critical to the survival of most plants. While they cost energy to build and are at risk of attack, these organs are vital to life, as through photosynthesis they capture the energy a plant needs to grow. But Monotropastrum humile, a ghost plant, does things differently. Monotropastrum humile gets its energy from fungi instead. Their roots tap into the local fungal network and pull everything they need from their hosts. That’s why they don’t need chlorophyll so, instead of a lush green, these plants are pallid white, like ghosts.
Even though they lack chlorophyll and don’t photosynthesise, these ghost plants still have leaves. The proportion of the aboveground plant that is leaf is comparable to photosynthetic species, even though the leaves have no obvious use. So why do they do it? Harada and colleagues took a close look at the plants, to see if the leaves were a similar size in all plants. Obviously some plants are bigger than others, but the key was to measure seven traits, to see if the leaves were the same size in proportion to the plant. This unlocked the puzzle.
The size of the leaves was in proportion to the size of the flower. The size of the flower is important to the plant, which is thought to attract long-tongued bumble bees. So it must have a flower of a certain size to attract the insects & get pollinated. But this leads to a genetic problem. The petals and sepals are specialised forms of leaf. Some of the genes that shape the petals and sepals also work on the ghost plant’s leaves. This connects the size of all three, so that shrinking the leaves also shrinks the flowers. Without the leaves there is no flower.
This research gets to a feature of genes. Often they don’t have a single function, but have an effect in collaboration with other genes. It explains why sometimes evolution gets ‘stuck’. It may not be able to eliminate one trait without mucking up another crucial function.
Harada and colleagues plan to test their ideas by testing the pattern with other ghost plants. They mention examining Pyrola aphylla and Cymbidium species. They believe that the leaves will have to be a minimum size, connected to attracting pollinators to their flowers.
Harada, S., Shiba, M., Kurosu, S., Izawa, H., Kurotaki, K., Yasuda, T. and Fukuda, T. (2025) “Why does non-photosynthetic Monotropastrum humile (Ericaceae) have scale leaves?,” Plant-Environment Interactions, 6(3), p. e70060. https://doi.org/10.1002/pei3.70060.
Cross-posted to Bluesky & Mastodon.
Image: Monotropastrum humile (Ginryo-so), by coniferconifer / Wikimedia Commons CC-BY.
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