Sexual behaviour of the genus Equisetum, subgenus Hippochaete

When grown for long periods in axenic single spore cultures, gametophytes of the four European species of subgenus Hippochaete are initially male or female. Females invariably produce antheridia as they get older, but archegonium formation by males is a much rarer event and was only observed in E. hyemale and E. scirpoides. Male gametophytes are smaller, grow more slowly than the females and in E. hyemale and E. variegatum are often short-lived. The proportions of male and bisexual individuals are relatively constant within each species but vary between species. Likewise species differ in the rate at which females later produce antheridia. Gametophytes grow in three ways: activity of antheridial meristems which are totally committed to the formation of male tissues, activity of cushion meristems which may produce archegonia for an indefinite period or switch to antheridium formation, and proliferation from lamellae. Either archegonia or antheridia may arise on the adventitious branches resulting from lamellar proliferation, but the type of sex organ produced is strongly influenced by the culture medium, the sex and age of the parent tissue and the particular species in question. Only by this means are archegonia formed on initially male individuals. The sexual behaviour of the subgenus Hippochaete is compared in some detail with that of the subgenus Equisetum and several subgeneric characteristics are outlined.