The Rosaceae comprises approximately 3,400 species. It is found throughout the world (on land), except in the arctic, but it has its greatest diversity in the north temperate regions. There are 81 species in Shaw's manual, 29 of which are introduced. The introduced taxa include such economically important genera as Malus (apples), Pyrus (pears), Prunus (plums, cherries, almonds, apricots), Rubus (raspberries, blackberries), and Fragaria (strawberries). Several native species also produce fruits that humans find edible and palatable; many more are appreciated by birds and other animals. The family is also important as a source of ornamental trees, shrubs, and herbaceous plants; think of Potentilla, Prunus, Geum, Alchemilla, Sorbus, Spiraea, and, of course, Rosa.
Vegetatively, members of the Rosaceae include annual and perennial herbs as well as shrubs and trees. The leaves are usually alternate and without stipules, but a few species have opposite leaves and some have no stipules. The leaves may be simple or compound, but generally have dentate or serrate margins. Pubescence, if present, is simple.
Flowers of the Rosaceae are marked by the presence of a hypanthium. This is easiest to see and understand in large flowers, such as those of Malus, but it occurs in all members of the family. The other distinguishing characteristics of the family are its radially symmetric flowers with 5 separate petals, many stamens (i.e., more than twice as many as the petals), and the presence of stipules. This combination of characters will enable you to recognize all members of the family in northern Utah, so long as you bear in mind that a taxon may differ in one or two ways from the rest of the family and still be included in it.
Subfamilies. Within the Rosaceae, there is considerable diversity in the gynoecium. Indeed, the gynoecial characters are used to distinguish different subfamilies. The enthusiastic among you may wish to become familiar with the subfamilies, so I have summarized those that occur in our region below. The less enthusiastic (or those more pressed for time) should concentrate on remembering the distinguishing characteristics of the family, and remember that the gynoecium varies a lot.
Subfamily Spiraeoideae. Gynoecium superior, of 2-5 simple pistils with 1 or 2 ovules each. Fruit an achene (1 seed, non-splitting) or follicle (more than one seed, fruit splitting). Examples: Spiraea, Aruncus, Holodiscus.
Subfamily Rosoideae. Gynoecium superior, of many simple pistils, often on a conical receptacle (think of strawberries); each pistil with 1-2 ovules. Fruit achenes (as in Fragaria and Malus) or drupelets (as in Rubus). What part do we enjoy most when eating strawberries? raspberries? Feel free to check your answer with me.
Subfamily Prunoideae. Gynoecium superior, of 1 simple pistil which has one ovule. Fruit a drupe. Think of plums, apricots, and cherries. Why is there only a tiny blip at the non-stalk end of a cherry but a lot of dried up stuff at the top of an apple or pear?
Subfamily Maloideae. Ovary inferior (why have I suddenly switched to "ovary" rather than "gynoecium"?), of 1 compound pistil having 2-5 carpels. Fruit a pome, comprising the fleshy hypanthium that has become fused to the ovary wall.
The Rosaceae retains several features that are thought to have characterized the earliest flowering plants, e.g., many stamens, simple pistils in most subfamilies, and radially symmetric flowers with separate petals, but the hypanthium is definitely an advancement. The family is also considered taxonomically difficult.. This means that taxonomists often do not agree on what should go in its genera and/or species. The problem arises because our classification scheme is neat and tidy, but the breeding habits of the Rosaceae are not. Apomixis and polyploidy are common, generally mixed with normal reproductio. Their reproductive "untidiness" leads to patterns of variation that defy neat classification schemes. That's life.
Self-quiz, to be taken after you find members of other families with many stamens: How would you distinguish them from the Rosaceae?