THE TENDRIL OF PARTHENOCISSUS INSERTA: DETERMINATION AND DEVELOPMENT

Tendrils on long shoots of Parthenocissus inserta occur in a regular pattern opposite the alternate distichous leaves at two successive nodes of each three nodes. Ontogenetic study shows that the tendril is initiated at the flank of the shoot apex during the second plastochron in an essentially axillary position. It is carried upward with growth of the internode above the axillant leaf and ultimately is situated opposite the next younger leaf. In a rhythmic pattern a different group of appendages is produced by the shoot apex at each node in the sequence of three. In acropetal order these are: at the tendrilless node the leaf subtends an axillary bud complex which in turn subtends a tendril; the leaf at the lower tendril‐bearing node directly subtends a tendril, and the leaf at the upper tendril‐bearing node subtends an axillary bud. Tendril primordia were not induced to develop as foliaceous shoots when cultured in vitro or in decapitation experiments, indicating that the meristem which becomes a tendril is determined early in its inception. Although built on a shoot pattern, the tendril is regarded as an organ sui generis with a possible relationship to the inflorescence. The morphological nature of the tendril is discussed in the light of theories postulated in the literature.