Some of the chief subdivisions of these large groups have been decided upon the fact that a considerable number of plants in them have some character in common, not found in the remaining plants of the group. Among the monocotyledons, for instance, there is a large class of plants that have tiny flowers between dry, chaffy scales, bear no true petals or sepals, all wind pollinated and are all commonly, though incorrectly, called grasses. These include, strictly speaking, two groups; one, the true grasses in which the stem is mostly hollow and the fruit a grain, while the other, with solid stems and bearing achenes for fruits, are the sedges. The grasses form one family and the sedges another, but while they differ in the characters just mentioned they agree in having flowers of the same general type. Families of plants are thus groups of genera, placed together in the scheme of classification, because they are more like one another than like any other such group. Among the grasses, for instance, are corn, wheat, rice, bamboo, orchard grass, Kentucky blue grass, sugar cane, and hundreds of others, all belonging to different genera, but all those genera grouped into a single family because of their generally similar flowers. Just as the Kentucky blue grass has a generic name (_Poa_) and a specific one (_pratensis_), the families of plants must also bear names, usually derived from the generic name of one of the chief genera in it. Because _Poa_ is a large and important genus of the grasses, the family is named after it, with the addition of _ceæ_. _Poaceæ_ is thus the family name of all the grasses. Among the sedges one of the commonest genera is _Cyperus_, including many species of the galingale or earth almond. From this genus the sedge family has been named _Cyperaceæ_ (Figure 87). So the rose family is the _Rosaceæ_, the violet family is _Violaceæ_, and so on through all the three hundred or more families which contain all the flowering plants so far discovered. Going back for a moment to the _Poaceæ_ and _Cyperaceæ_, the fact that these two large families are different from each other, but have some characters in common, fixes them as both belonging to one _order_. Orders are thus groups of one or more plant families, all differing one from another, but obviously related and having some characters in common. The order containing the grasses and sedges is named for one of the families in it with the ending _ales_. Thus _Poales_ include _Poaceæ_ and _Cyperaceæ_. _Rosales_ include _Rosaceæ_ and several families.