Study on seed morphometry of seven threatened Himalayan orchids exhibiting varied life modes

A study was conducted on seed morphometry of seven threatened Himalayan orchids (listed in Appendix II of CITES) belonging to three different subfamilies, and exhibiting terrestrial (Arundina graminifolia, Cypripedium cordigerum, Habenaria marginata, Herminium monorchis), mycoheterotrophic (Gastrodia falconeri) or epiphytic (Aerides multiflora, Pholidota articulata) life modes. Seeds were fusiform, filiform or ovoid in shape and their size showed direct correlation with plant habit. They were relatively smaller in epiphytes than their terrestrial or mycoheterotrophic counterparts. Seeds were truncated except in Arundina graminifolia (l/w > 6.0). Scanning electron micrographs revealed that the testa cells were longitudinally oriented, arranged either straight or in a spiral, and their periclinal walls were comparatively irregularly placed in epiphytic species. The wall thickenings were also more pronounced in epiphytes. Embryos were tiny and most of the seed space (even > 90% in Pholidota articulata and Gastrodia falconeri) was occupied by air. Highest seed to embryo (s/e) volume ratio (17.94) was recorded in Pholidota articulata, and lowest (2.14) was in Arundina graminifolia. Though the orchids producing seeds with higher s/e volume ratio and higher percentage of air space find wider distribution, the localized distribution of Gastrodia falconeri (mycoheterotroph) seems to be related to its complex nutritional requirements.