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:
- Group, Angiosperms.
- Class, Monocotyledones.
- Order, Liliales.
- Family, Liliaceæ.
- Genus, Trillium.
- Species, grandiflorum.
In the same way the position of the toothwort would be indicated as follows:
- Group, Angiosperms.
- Class, Dicotyledones.
- Order, Papaverales.
- Family, Cruciferæ.
- Genus, Dentaria.
- Species, diphylla.
But in giving the technical name of the plant only two of these names are used, the genus and species, so that for the toothwort we say Dentaria diphylla, and for the white wake-robin we say Trillium grandiflorum.
940. Kingdom and Subkingdom.—Organic beings form altogether two kingdoms, the Animal Kingdom and the Plant Kingdom. The Plant Kingdom is then divided into a number of subkingdoms as follows: 1st, Subkingdom Thallophyta, the thallus plants, including the Algæ and Fungi; 2d, Subkingdom Bryophyta, the moss-like plants, including the Liverworts and Mosses; 3d, Subkingdom Pteridophyta, the fern-like plants, including Ferns, Lycopods, Equisetum, Isoetes, etc.; 4th, Subkingdom Spermatophyta, the seed plants, including Gymnosperms and Angiosperms. Subkingdoms are divided into groups of lower order down to the classes. So there are subclasses, subfamilies or tribes, subgenera, and even subspecies. But taking the principal taxonomic divisions from the greater to the lesser rank, the order would be as follows: