01-Floral formula
Here is the floral formula for the Aceraceae. Flowers are usually imperfect and the plants may be either monoecious or dioecious. There is a trend from insect to wind pollination.

02-Map
Distribution map; from Heywood. There are only two genera of Aceraceae, and most species belong to the genus Acer. This family is most common in the North Temperate Zone.

03-Tree
All members of the Aceraceae are woody plants, either trees or shrubs, with one species being a woody vine. Taken by K. R. Robertson at the Arnold Arboretum of Harvard University.

04-Winter twig
The leaves are opposite, hence the leaf scars are opposite on winter twigs. Photo of Acer saccharum by K. R. Robertson from trees cultivated at the University of Illinois.

05-Leaves
The opposite leaves are most often palmately lobed and palmate veined. Some species have beautiful fall color. Photo of Acer rubrum by K. R. Robertson from trees cultivated at the University of Illinois.

06-Inflorescence
There is a trend from insect pollination to wind pollination in the genus Acer. The first example to be examined is the Norway maple, Acer platanoides, which is insect pollinated. You can see these flowers have conspicuous petals. Photo by K. R. Robertson in Cambridge, Massachusetts.

07-Male flower
There 5 sepals and 5 petals seen in this male flower of Acer platanoides. In the center of the flower is a large green nectar disk to attract insect pollinators. Note that there are 8 (eight) stamens, a common number in the Aceraceae. There is NO rudimentary gynoecium present. Photo by K. R. Robertson and D. L. Nickrent from plants cultivated in Champaign, Illinois.

08-Female flower
This female flower of Acer platanoides has 5 sepals, 5 petals, 8 staminodia, a large nectar disk, and a syncarpous gynoecium of 2 carpels. Photo by K. R. Robertson and D. L. Nickrent from plants cultivated in Champaign, Illinois.

09-Female flower l.s.
Note the syncarpous gynoecium of 2 carpels. The ovary has two prominent wings, one on each side. Photo by K. R. Robertson and D. L. Nickrent from plants cultivated in Champaign, Illinois.


10-Fruits
The fruits of Acer are schizocarps that break apart into 2 mericarps. Since each mericarp is winged they are also samaras. Photo of Acer platanoides by K. R. Robertson of plants cultivated in Champaign, Illinois.

11-Silver maple trunk
Trees of Silver maple, Acer saccharinum, have light gray, scaly bark. Photo by K. R. Robertson of a tree at the University of Illinois.

12-Male inflorescences
The flowers of silver maple, Acer saccharinum, are adapted to wind rather than insect pollination. The plant bloom very early in spring before leaves appear. In this photo, many winter buds have opened to reveal several male flowers within each bud. Photo by K. R. Robertson and D. L. Nickrent from plants cultivated in Champaign, Illinois.

13-Male & female inflor.
In this photo, winter buds of Acer saccharinum are shown with female flowers contained in the buds on the left and male flowers on the right. Photo by K. R. Robertson and D. L. Nickrent from plants cultivated in Champaign, Illinois.

14-Male & female flowers
The female flowers of Acer saccharinum have 2 long styles that project outward to catch windblown pollen. The male flowers have very long filaments to place the anthers out where wind can pick up the pollen grains. Petals and a nectar disk are absent. Photo by K. R. Robertson and D. L. Nickrent from plants cultivated in Champaign, Illinois.

15-Box elder leaves
Unlike most maples, the leaves of box elder, Acer negundo, are ternate or pinnately compound. These leaves somewhat resemble those of poison-ivy, which belongs to a different family, the Anacardiaceae. Photo by K. R. Robertson in Highland Park, Illinois.

16-Male inflorescences
The male flowers of Acer negundo have very long pedicels so the flower dangle to release pollen to the wind. Photo by K. R. Robertson at the Arnold Arboretum of Harvard University.

17-Male & female flowers
These flowers of Acer negundo are extremely specialized for wind pollination. Both male and female flowers have long pedicels, and the female flowers have long styles as well. Note the winds on the ovaries. The perianth is very reduced.  Photo by K. R. Robertson and D. L. Nickrent from plants cultivated in Champaign, Illinois.

18-Sugar maple
Early in spring, sap starts running in sugar maple, Acer saccharum. Trees can be tapped to collect the sap, which contains a weak solution of sucrose. Photo by K. R. Robertson at Homer Lake, Champaign County.

19-Boiling maple sap
The collected sap is then boiled to make maple syrup; it takes about 40 gallons of sap to make one gallon of syrup. Photo by K. R. Robertson at Homer Lake, Champaign County.

20-Acer griseum
Many species of maples are cultivated. This is the paperbark maple, Acer griseum, which is native to China.  It has this beautiful copper colored peeling bark. There are several trees on this species on the U of I campus. Photo by K. R. Robertson at the Arnold Arboretum of Harvard University.