A microphotograph by Durham University biologist Phil Gates, originally published in the journal Chemical & Engineering News (C&EN), has captured the attention of science enthusiasts worldwide. The cross-section of marram grass (Ammophila arenaria) reveals what appear to be hundreds of tiny, smiling faces. Yet behind this charming optical illusion—a phenomenon known as pareidolia—lies a sophisticated feat of biochemical engineering.
What resemble the eyes and mouth of a smiley face are actually xylem vessels, reinforced with the sturdy polymer lignin to facilitate water transport. The "smile" itself is composed of phloem cells, which transport sugars and are built from flexible cellulose. To bring this pattern to light, Gates employed a double fluorescent staining technique. When exposed to ultraviolet and blue-violet light, the lignin and cellulose absorb energy and emit a brilliant, contrasting glow.
Perhaps the most fascinating aspect of this discovery is how these cells respond to rainfall. During dry periods, these smiles appear weary or "sad" as the vessel walls slightly contract from a lack of moisture. However, the onset of rain triggers the process of osmosis. Water rapidly enters the plant, generating high internal hydrostatic pressure, or turgor.
This turgor pressure causes the hollow xylem channels to expand into perfect circles while the cellulose phloem outlines stretch and broaden. This hydrodynamic cell expansion physically widens and sharpens the grass’s "smile." In this way, the lawn beneath our feet truly responds to every raindrop, assuming its most cheerful expression at the height of a summer rain.
