The leaves of young lotus plants sit on the water as a flat circle. Full-grown lotus plants push their leaves above the water’s surface, with leaves that are wavy and warped. The difference lies in vitality and biophysics.
Plants in a community commonly compete for light. Lotus leaves on a pond are no different.
The most that a young lotus can muster is getting its leaves to the surface. As water supports the leaves from underneath, the leaves optimize their exposure to the Sun lying flat.
Mature lotus plants go one better. With other plants crowding the watery landscape, adults push their leaves above the water via long stems. This puts these leaves in better position to catch more of the Sun’s rays than those of youngsters lying flat on the pond.
But something more is needed in leaves standing tall, which must be self-supporting. To keep themselves spread out, the cells in adult leaves organize themselves with structural ruffles. This ribbed structure is optimized so that a leaf is as rounded as possible while being stiff enough to retain its circular shape when butting up against other leaves and bearing up against a breeze. Lotus plants do what they can to let the Sun shine on their leaves.
Fan Xu et al, “Water affects morphogenesis of growing aquatic plant leaves,” arXiv (7 January 2020).
“Rubber ‘leaves’ reveal the physics of the floating lotus,” Nature (29 January 2020).