Autecological studies of three species of Callitriche native in California

Study of three species of Callitriche from California revealed different degrees of tolerance of varying environmental conditions which help explain the plants' occurrence. Plants were studied in nature and experimentally under sterile conditions. Callitriche palustris occurs over a wide geographical range in highly divergent habitats, but C. longipedunculata and C. hermaphroditica grow in more restricted conditions, the former being confined to low altitude vernal pools in central and southern California. Callitriche palustris tolerated wider temperature ranges for seed germination and flowering than did the other two species, and tolerated lower light intensity and higher osmotic pressure for growth and flowering than did C. longipedunculata. At low temperatures C. palustris flowered in both 8- and 16-h photoperiods; C. longipedunculata flowered only in 16-h photoperiods. Dormancy of C. palustris seed could be overcome with milder treatments than could dormancy of C. longipedunculata seed. Callitriche palustris, a highly protogynous species, was able to set normal amounts of viable seed at temperatures permitting formation of only pistillate flowers. Circumstantial evidence indicates that much of this species' reproduction in nature is apomictic. Apomixis would tend to preserve the highly tolerant genotypes this species has evolved.