Developmental shoot morphology in Phyllocladus (Podocarpaceae)

Shoot architecture in the adult phase of Phyllocladus is established by a succession of units of extension that develop a system of permanent axes supporting photosynthetic units (phylloclades) each of which represents a branch complex with three branch orders. Seedlings have needle foliage leaves comparable to those of other conifers, but in adult plants all leaves are ephemeral, non-photosynthetic scales that for the most part subtend no axillary buds. Once rhythmic growth (usually seasonal) is established in the adult phase, each increment produces a whorl of phylloclades so that a regular tiered arrangement develops, with the tiers progressively reduced on outer units. In the resting terminal bud of permanent axes only scale primordia are present; with bud burst and beginning of extension of the unit the phylloclades are produced by syllepsis and complete the initiation and expansion of all axis orders in the short flushing cycle. Segments retain strict distichy throughout, but in a dorsiventral and not a lateral plane. Phylloclades may be either determinate, when the apex of the first-order axis develops as a terminal flattened segment, or indeterminate, when the apex retains radial symmetry and forms a resting bud that can continue axis extension as a permanent shoot in subsequent years. A phylloclade consequently only produces flattened lateral segments in its season of initiation. Reiteration from reserve buds is not possible, because none are produced in the adult phase, but is possible from detached meristems formed in the axils of needle leaves on juvenile shoots. Reiteration in the adult phase is thus possible only by axis dedifferentiation, that is, change from plagiotropy to orthoptropy. The distinctive massive vascular connection of the phylloclade is made possible by syllepsis. In this way the normal structural constraints of elaborate appendage development in conifers is fully overcome.