Eastern shade-tolerant conifers

Tsuga canadensis (L.) Carr. - eastern hemlock

Image 1. A distal lateral shoot that developed far more vigorously in the previous year than the terminal and some other distal lateral shoots: the shoot elongated some 30 cm, and developed along it several substantial sylleptic shoots (in axils of leaves) and many much weaker ones; this shows (1) the typical manner of development of vigorously growing shoots (2) that laterally situated shoots frequently take over from terminal shoots the stronger developmental role, and (3) that formation of true lateral buds is rare (and generally restricted to the most-proximal and the most-distal positions) on the more vigorous shoots.

Image 2. The underside, in April, of a relatively weak shoot of the previous year, and the distal portion of the 2-year-old shoot from the terminal bud of which it extended: the buds in axils of leaves are pollen-cone buds, those on the 2-year-old shoot have developed from latent buds formed when the shoot extended (and represent an unusual occurrence among the genera of the Pinaceae), there is a vegetative terminal bud on the 1-year-old shoot. The bands embracing several lines of stomata on either side of the midrib on the undersides of the leaves are evident, as are the petioles of the leaves, and the leaf cushions (or pulvini) on which the leaves are individually seated.

Image 3. Pollen cones recently emerged from their buds and enlarging in mid-May: note the two pollen cones (left), which have emerged from buds that were latent through their first year of existence, and the still unexpanded vegetative bud terminating the 1-year-old shoot.

Image 4. Pollen cones similar to those of No. 3 with swollen pollen sacs shortly before pollen shedding (mid- to late May): note the presence of a terminally situated pollen cone on the 1-year-old shoot, as well as pollen cones on the 2-year-old shoot.

Image 5. Pollen cones at the pollen-shedding stage in late May: in the abundant cone year depicted, pollen cones occurred in axils of some leaves on the upper sides of shoots as well as in the more usual underside positions.

Image 6. Pollen cones shortly after pollen shedding (early June) when pollen sacs have split open and been emptied of pollen, and cone axes have extended below the cones to push the cones clear of bud scales: note the presence of a terminal pollen cone.

Image 7. A seed-cone bud terminating the shoot on the left, and a comparable vegetative bud terminating the shoot on the right, mid- to late April: each shoot also has lateral vegetative buds in leaf axils.

Image 8. A seed cone just after its emergence from its (terminal) bud: the view, in mid-May, shows the upper side of the shoot along which the upper sides of leaves originating along the sides and beneath the shoot but spreading bi-laterally are seen as uniformly dark, shiny green, and much longer than the more erect leaves originating along the shoot's upper surface that display their undersides.

Image 9. A seed cone at about the receptive stage (there are some pollen grains adhering to ovuliferous-scale surfaces) in late May or early June: note how the receptive cone is more or less pendent at the end of the rather weak shoot which, itself, tends to hang down (this orientation is different from that of receptive seed cones in other genera of Pinaceae).

Image 10. A seed cone in late June: the cone is in the stage of rapid growth, cone scales are closely appressed to one another, bracts subtending the cone scales are visible only at the cone's base below which an elongating, pubescent cone stalk is evident.

Image 11. A full-grown seed cone in late July.

Image 12. Full-grown seed cones in late July: each terminates a shoot of the previous year that was of moderate vigour.

Image 13. Maturing seed cones in mid-August: some of the cone scales are beginning to show signs of drying at their tips.

Image 14. A mature, almost dry, but still closed seed cone in early October.

Image 15. A seed cone in mid-October beginning to "open" as cone scales spread apart to permit seeds to drop out of the pendent cone.

Image 16. An "open" seed cone in late October: the tip of a loosened seed wing (and therefore seed) is evident beneath a cone scale in the middle of the cone.

Image 17. Upper, lower, and edge views of seeds and their wings: it can be seen that the bases of the seed wings extend fully over one side of the seeds and "grip" two edges around the other side of the seeds where several resin-filled "blisters" are also evident (the blisters would be ruptured if the bases of the seed wings are pried loose; hence, for seed storage the flat parts of the wings are broken away and discarded), the scale-bar is in millimetres.

Image 18. The underside of a relatively weak, overwintered shoot with an enlarging terminal vegetative bud in which inner, green bud scales have elongated, late May.

Image 19. Clusters of preformed leaves recently emerged from a terminal and a distal lateral bud: at this stage in early June neoformed leaf production is under way around the apical meristem at the new shoot's tip in the centre of each of the preformed leaf clusters.

Image 20. New shoots elongating in mid-June: the preformed leaves that were in clusters 7 days earlier are now being spread out as the shoot axes elongate, and as neoformed growth occurs at the shoots' tips.

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