Cone and Ovule Development in Callitris (Cupressaceae-Callitroideae)

Cones in Callitris consist of a series of sterile scale leaves, below two trimerous whorls of bracts. The shoot apex becomes the sterile central columella. Ovules first develop in the axils of the upper bracts as a unit triad, the two lateral ovules appearing before the median. The first unit is repeated one to three times in a basipetal (descending) sequence to a maximum of four units per bract. Subsequent to the development of the first ovular unit in the upper bracts, a similar basipetal sequence is initiated in the axil of each of the lower bracts, but usually with fewer units and frequent abortion of ovules. A potential maximum of 63 ovules exists. No separate ovule-supporting structure is developed. Pronounced transformation of the cone as seeds mature is the result of intercalary development, with marked radial expansion so that cone scales become valvate. The vascular and resin systems of the older cone become vastly augmented, but at the time of pollination there is no vascular system that can be used to identify a distinct ovule-supporting organ, other than transformed axial bundles that either supply the basal meristem of cone scales or provide the beginnings of bundle systems that are only elaborated after pollination.