It may not occur to a plant enthusiast that Marsilea is a fern. Unlike other ferns, spores are not borned on the underside of a frond but inside a bean like structure. The spore-bearing leaf has evolved into a special structure called sporocarp:
Just the day after i saw M. crenata in drying-up pool, i found another species M. drummonii growing in a plant enthusiast's collection. This triggered a recollection of Moran's essay**. He wrote of early Australian explorers sustenance on improperly prepared meal of nardoo (M. drummondii) sporocarp which resulted in poisoning and death. The photo below shows M. drummondii with silvery white hairs on its new leaves together with M. crenata. The hairs on top of having underground rhizomes may be an evolutionary adaption to protect young leaves from intense sunlight as M. dummondii is found on harsh seasonally climate that alternates between drought and floods. Interestingly, i found out that M drummondii is native to Thailand and absent from aseasonal Malaysia & Singapore.
*By the way, i. aquatica is 'kang-kong' a staple vegetable in found in south east asian cooking. And a popular dish is sambal kang-kong.
**The Natural History of Ferns, Timberpress by Robbin C. Moran is a wonderful read. It contains an unparalleled collection of highly readable essays on different aspects of fern life, evolution, cultural history, interesting facts and adaptations.