This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Trees or shrubs, with milky sap, alternate dentate and often lobed, 3-nerved leaves, fugacious stipules, and small monoecious or dioecious flowers, in axillary ament-like spikes, the pistillate spikes ripening into a succulent aggregate fruit. Staminate flowers with a 4-parted perianth, its ovate segments somewhat imbricated, and 4 stamens, the filaments inflexed in the bud, straightening and exserted in anthesis. Pistillate flowers with a 4-parted persistent perianth, which becomes fleshy in fruit, a sessile ovary, and 2 linear spreading stigmas. Fruiting perianth enclosing the ripened ovary (achene) the exocarp succulent, the endocarp crustaceous. Endosperm scanty; embryo curved. [The ancient name of the mulberry; Celtic mor.]
Leaves rough above, pubescent beneath; fruit purple; spikes l'-1 1/2' long.
Leaves smooth and glabrous, or very nearly so, on both sides; fruit nearlywhite; spikes 5"-7" long.
Morus rubra L. Sp. PI. 986. 1753.
A tree, attaining a maximum height of about 650 and a trunk diameter of 70, the bark brown and rough. Leaves ovate or nearly orbicular in outline, scabrous above, persistently pubescent beneath, or when young almost tomentose, acuminate at the apex, rounded, truncate or cordate at the base, serrate-dentate or 3-7-lobed, 3'-5' long; petioles slender, 7"-18" long; staminate spikes drooping, 1 1/2'-3' long; pistillate spikes spreading or pendulous in fruit, 1'-1 1/2' long, 4"-5" in diameter when mature, slender-peduncled, dark purple-red, delicious.
In rich soil, Vermont and Ontario to Michigan and South Dakota, south to Florida and Texas. Wood soft, weak, compact, durable; color light yellow; weight per cubic foot 37 lbs. April-May. Fruit ripe in June-July.
Morus alba L. Sp. PI. 986. 1753.
A small tree, sometimes 400 high and with a trunk 30 in diameter, the bark light gray, rough, the branches spreading. Leaves ovate, thin, smooth, glabrous and somewhat shining on both sides, acute or abruptly acuminate at the apex, rounded, truncate or cordate at the base, varying from serrate to variously lobed, 2'-6' long; petioles slender, shorter than the blades; staminate spikes slender, drooping, about 1' long; pistillate spikes oblong or sub-globose, drooping, 5"-7" long, 3" in diameter and white or pinkish when mature, not as succulent as those of the preceding species.
Sparingly escaped from cultivation, Maine and Ontario to Florida. Introduced from the Old World for feeding silkworms. May. Fruit ripe July-Aug.
Morus nigra L., the black-mulberry of Europe, with smooth leaves and black fruit, has escaped from cultivation in the southeastern states and has been recorded from as far north as New York.