Eastern shade-tolerant conifers

Thuja occidentalis L. - eastern white cedar, or eastern thuja

Image 1. First-year seedlings in September: note that above the two cotyledons, needle-like leaves are produced in pairs with each succeeding pair being at right angles to the one below it; when first branching occurs the pairs of leaves along the branch and sometimes along the continuation of the main axis become more scale-like - in fact, with successive branching, the degree of "scalelikeness" of the leaves tends to increase. All leaves above the cotyledons are neoformed, and all branching is sylleptic: no buds are formed.

Image 2. Pollen cones in early to mid-April, some cones have shed their pollen and pollen sacs are shrivelling, and one cone has yet to shed its pollen and its pollen sacs are still pink and turgid: note the positions of the pollen cones - each terminates a relatively long (with four or five sets of two pairs of leaves), minor, side shoot of a side-branch complex.

Image 3. Pollen cones in early to mid-April: note that each pollen cone consists of three pairs of oppisitely arranged microsporophylls each bearing four pollen sacs; just before pollen shedding the cone is pushed clear of the last leaf-pair below it by extension of the cone axis, the cones all tend to assume an upward or erect orientation, pollen can be seen scattered over the shoot surfaces.

Image 4. A pollen cone just before elongation of its lower axis and pollen shedding (early April): the shield-shaped brown to black surface of a microsporophyll and its four pink pollen sacs below it are clearly evident.

Image 5. An "old", spent, pollen sac in early May showing the shrivelled pollen sacs still in place below their respective microsporophylls and the extended axis below the cone: such cones gradually shrivel, but generally remain in place for weeks or even months. Note that the leaf immediately below the cone has the same form as the other leaves, but is more yellow.

Image 6. A shoot system in October showing at the tips of many relatively short shoots with two or three sets of two pairs of leaves yellowy-brown structures that are parts of preformed seed cones.

Image 7. Seed cones at the receptive stage of development in early to mid-April: the exposed tips of urn-shaped ovules can be seen to the inside of the ovuliferous scales (two ovules per scale); at the bottom left, and top right, exuded drops of moisture can be seen at ovule tips.

Image 8. A view from above of a seed cone at the receptive stage: the tips of the ovuliferous scales are dark-coloured, the innermost pair is too poorly developed to have associated ovules, the scales of the next pair each have just one developed ovule, but those of the next two pairs each have two ovules, seven of the ten ovules have drops of exuded moisture onto which pollen may land and then be drawn into, or pass into, the micropyle of the erect ovule: note how the ovules are extended sideways, these extensions are the beginnings of the bilateral wings that eventually are parts of the seeds to come.

Image 9. A seed cone in mid-May, one month after the time of pollination: scale growth is occurring rapidly and ovules (also growing) have been overgrown by the scales (ovuliferous scales with the bracts fused to them).

Image 10. Seed cones in late May, during the period of rapid cone enlargement: note that cone orientation remains erect.

Image 11. A seed cone in early June.

Image 12. The summit of a sapling bearing many seed cones, in mid-July when most cone expansion has been completed: note the pattern of seed-cone distribution and the general extension of the branching systems beyond the cones.

Image 13. Densely clustered seed cones in mid-July in a year of abundant cone production: note that individual cones are erect, but their massed weight bends the supporting branch down.

Image 14. Seed cones turning yellowish as they dry during maturation, mid-August.

Image 15. Seed cones in the final stages of maturation, in mid-September: most are orange-brown, and a few are beginning to "open" as their scales spread apart; when that occurs seeds can be whipped out and dispersed by the wind.

Image 16. Mature, partially open seed cones in early October.

Image 17. Seeds of Thuja occidentalis: note the bilateral seed wing and the presence of some resin "blisters" on some seeds, the scale-bar is in millimetres.

Information provided by:
Dr. G.R. Powell
Faculty of Forestry and Environmental Management at UNB