=935. Order, class, group.=—In like manner the lily family, the iris family, the amaryllis family, and others which show characters of close relationship are united into an _order_ which has broader limits than the family. This order is the lily order, or order _Liliales_. The various orders unite to make up the _class_, and the classes unite to form a _group_. =936. Variations in usage of the terms class, order, etc.=—Thus, according to the system of classification adopted by some, the angiosperms form a _group_. The group angiosperms is then divided into two _classes_, the _monocotyledones_ and _dicotyledones_. (It should be remembered that all systematists do not agree in assigning the same grade and limits to the classes, subclasses, etc. For example, some treat of the angiosperms as a class, and the monocotyledons and dicotyledons as subclasses; while others would divide the monocotyledons and dicotyledons into classes, instead of treating each one as a class or as a subclass. Systematists differ also in usage as to the termination of the ordinal name; for example, some use the word _Liliales_ for _Liliifloræ_, in writing of the order.) [Illustration: Fig. 497. _A._ Cross-section of the stem of an oak tree thirty-seven years old, showing the annual rings. _rm_, the medullary rays; _m_, the pith (medulla). _B._ Cross-section of the stem of a palm tree, showing the scattered bundles.] =937. Monocotyledones.=—In the monocotyledons there is a single cotyledon on the embryo; the leaves are parallel-veined; the parts of the flower are usually in threes; endosperm is usually present in the seed; the vascular bundles are usually closed, and are scattered irregularly through the stem as shown by a cross-section of the stem of a palm (fig. 497), or by the arrangement of the bundles in the corn stem (fig. 57). Thus a single character is not sufficient to show relationship in the class (nor is it in orders, nor in many of the lower grades), but one must use the sum of several important characters. =938. Dicotyledones.=—In the dicotyledons there are two cotyledons on the embryo; the venation of the leaves is reticulate; the endosperm is usually absent in the seed; the parts of the flower are frequently in fives; the vascular bundles of the stem are generally open and arranged in rings around the stem, as shown in the cross-section of the oak (fig. 497). There are exceptions to all the above characters, and the sum of the characters must be considered, just as in the case of the monocotyledons. =939. Taxonomy.=—This grouping of plants into species, genera, families, etc., according to characters and relationships is _classification_, or _taxonomy_. To take Trillium grandiflorum for example, its position in the system, if all the principal subdivisions should be included in the outline, would be indicated as follows: