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The flowers of amur maple occur in oblong clusters circular in section about 1.5" long and 1" across on the tips of the branches; flower clusters don't appear until after the leaves have emerged. There are two kinds of flowers in each cluster, both yellowish white in color, about 1/8" across, both with five sepals and five petals (each 1.5-2 mm long); the petals' tips curve in over the central parts of the flower. One flower type is staminate with eight stamens (no ovary or style) within the ring of sepals and petals. The other flower type is "perfect" with both eight stamens tipped with light yellow anthers and with a distinct central style rising above the stamens that bears a split tip that curls back. Minnesota Wildflowers [https://www.minnesotawildflowers.info/tree/amur-maple] states that the style splits in two, but the amur maple present in Jackson Park has perfect flowers whose styles either split in two or split in three — both morphs are present in the same flower cluster. Fertilized perfect flowers produce a fruit that is a pair of winged seeds (samaras) joined seed-to-seed with the wings making an angle of less than 30° (much less than most maples). Initially green in color, the samaras develop reddish highlights before turning bright reddish pink in mid to late summer. When the fruit matures in early to mid-autumn, each samara is 1-1.5" long and turns light brown.

Acer

Acer ginnala

Amur maple [Blooms: Apr–May]

Boxelder is dioecious — trees either produce all male (staminate) or all female (pistillate) flowers. Neither male or female flowers have petals; both consist of drooping, hairy filaments that bear either 5 reddish-brown anthers (male) or a greenish-white, deeply-forked style (female). The flowers are wind pollinated. Boxelder wood is soft and brittle so broken branches are common, but the trees can live to be a century old.

Acer

Acer negundo

boxelder [Blooms: Apr-May]

Norway maple produces separate male and female flowers, either on different trees or on the same tree. Flowers are produced in umbels of 10-30 flowers 2-3" across, either as the leaves are just unfolding or slightly before. Individual flowers are about 6-8 mm across; each has five 1/8" long sepals, five ~1/2" long petals, both of which are greenish yellow, and a brownish central disc. Male (pistillate) flowers have eight fertile stamens; female flowers have a green pistil with a pair of basal wings and a pair of styles, all surrounded by eight sterile stamens. Pollination occurs primarily by insects (bees). Fertilized female flowers produce pairs of winged fruits (samaras) joined at the seeds with an angle between the samaras of slightly less than 180°. The mature fruits are 1.5-2.5" long and turn brown in the autumn.

Acer

Acer platanoides

Norway maple [Blooms: Apr]

Perfect flowers are rare in red maple; individual trees may produce all male flowers, all female flowers, or both male flowers and female flowers on the same tree. Male flowers occur on the previous year's twigs (before the leaves open) as dense clusters 3/4-1" across composed of 1/8" long sessile flowers surrounded by short bracts with hairy margins; each flower has five sepals and five petals (making up a red or sometimes yellow bell-shaped corolla), and 5-8 slender, projecting stamens two to three times longer than the sepals. The female flowers also occur on the previous year's twigs before the leaves open as dense (3/4-1" across) clusters of initially sessile flowers, but the pedicels of the flowers soon elongate (to 1/2-2") and the flowers begin to droop; each 1/8" long flower has five sepals and five petals (making up a red or sometimes yellow bell-shaped corolla), and a two celled, hairless ovary with a pair of robust, arching red styles in the center. The flowers are wind pollinated. Fertilized flowers are soon transformed into a pair of red, winged samaras (i.e, maple seeds) joined tip to tip at a 45-90° angle. At maturity, the samaras are each 3/4-1" long. Like silver maple, the seeds of red maple can germinate in the same year they form (i.e., they don't need to overwinter to sprout).

Acer

Acer rubrum

red maple [Blooms: Mar-Apr]

Sugar maple trees may produce male and female flowers on separate trees or may produce male and female flowers on the same tree, even on the same branch. Male flowers are produced in drooping umbels, with 3-4" long, hairy pedicels. Individual male flowers are about 1/8" long consisting of a yellowish-green calyx with five teeth and 6-8 stamens with yellow anthers (in young flowers) that extend well outside the boundary of the calyx; petals are absent. Female flowers are also produced in drooping umbels; they are 1-2" long (including the hairy pedicel), but the flower itself is only 1/8" long consisting of a yellowish-green calyx with five teeth, a two-celled ovary, and a divided style that extends beyond the margin of the calyx. Again, petals are absent. The female flowers are wind pollinated. Fertilized flowers produce pairs of winged fruit (samaras) each about 1" long, with the two fruits at a 60-90° angle to each other. The samaras mature over the fall and must be exposed to an extensive interval of sub-freezing temperatures during the winter in order to germinate.

Acer

Acer saccharum

sugar maple [Blooms: Apr–May]