Newly-found ferns in an artificially illuminated deep karst cave have the most negative known carbon isotope values.

The carbon isotope ratios(δ 13C)of modern land plants have been widely studied,and they provide valuable information about modern and past atmospheric,ecological and climatic changes[1].According to their different photosynthetic pathways,land plants are mostly C3 and C4 plants.The δ13C values of C3 plants are mainly within the range of-20‰ to-34‰,with the most frequent values being around-27‰,which are always more negative than the range of-9‰ to-19‰ of C4 plants,with the most frequent values being around-13‰[2].For modern C3 plants,the reported most negative δ13C values are-36‰ to-37‰[3],which are generally observed in tropical rainforest,such as the Amazon[4],highlight-ing the impact of high atmospheric moisture on the δ13C values of C3 plants[1].Thus,the δ13C values of plants in distinctive environ-ments can help elucidate the impact of unusual environmental conditions on these plants.