| Phyllotaxis is the study of the arrangement of primordia on the shoot apical meristem or mature organs on the stem. Phyllotactic patterns can change during the ontogenetic process and is called pattern transition. The main goal of this study is to examine different phyllotactic and transition patterns of the seedlings of Thuja occidentalis. Scanning electron and optical microscopy were used to examine dissected shoot tips of seedlings and take measurements of a variety of parameters. Four phyllotactic patterns were observed on the main stem of T. occidentalis: tetracussate, tricussate, (3, 5) spiral and decussate. Therefore, 4 types of phyllotactic pattern transitions were documented: tetracussate to decussate, tetracussate to tricussate, tricussate to (3, 5) spiral and (3, 5) spiral to decussate. Only 1 phyllotactic pattern was observed on the side branches of T. occidentalis: decussate. For each phyllotactic pattern, the following phyllotactic parameters were measured or calculated using histological techniques: divergence angle, plastochrone ratio, leaf insertion angle, parameter Gamma and apical angle of the shoot apical meristem. The results from histological techniques indicate that: divergence angle, plastochrone ratio and leaf insertion angle fit with their empirical ranges for all the patterns; divergence angle and plastochrone ratio fit with some theoretical models for the (3, 5) spiral pattern of the main stem; parameter Gamma only matched the theoretical prediction for the (3, 5) spiral pattern of the main stem; using Gamma = √( l1l2)/R 0 is a better way to calculate parameter Gamma. This study is the first to use a scanning electron microscopy three-dimensional reconstruction method to study the shoot apical meristem at an early stage of development. Four new phyllotactic parameters were introduced: volume of the shoot apical meristem, surface area of the shoot apical meristem, projected area of the shoot apical meristem and height of the shoot apical meristem. The results from the scanning electron microscopy three-dimensional reconstruction method indicate that: different phyllotactic patterns observed on T. occidentalis are not significantly different for these 4 parameters; the assumption that the shoot apical meristem can be approximated as a cone may not be valid. The results from histological techniques and scanning electron microscopy three-dimensional reconstruction method combined indicate that: the apical angle of the shoot apical meristem decreased significantly as the seedlings of T. occidentalis grow and undergo pattern transitions in both main stem and side branches. |
