DIVISION V.
ANGIOSPERMÆ.
See pages [3] and [224]. To this Division belong the majority of the Flowering-plants. They are divided into two parallel classes, the Monocotyledons and the Dicotyledons, which differ from each other not only in the number of cotyledons, which, with a few exceptions, is one in the former, two in the latter, but also in the internal structure of the stem, the venation of the leaves, the number of the parts of the flower, etc. Assuming that these two classes have sprang from a common origin, it is amongst the Helobieæ in the first, and amongst the Polycarpicæ in the second class that we might expect to find closely allied forms, which might reasonably be supposed to have varied less from this original type. As for the rest, they seem to stand quite parallel, without exhibiting any close relationship. It is scarcely proved that the Monocotyledons are the older class.
[Our knowledge of the forms included under the Angiosperms has recently been considerably increased by Treub (Ann. d. Jar. Bot. d. Buitenzorg, 1891), who has shown that the Casuarinas differ in many important points from the typical Angiosperms. Among other characters the pollen-tube is found to enter the ovule near the chalaza and therefore at the opposite end to the micropyle, and Treub therefore suggests that these plants should be placed in a subdivision termed Chalazogams.
According to this view the principal divisions of the Angiosperms would be represented thus:—
Angiospermæ.
| Sub-division. | Sub-division. |
| Chalazogames. | Porogames. |
| Class. | Classes. |
| Chalazogames. | Monocotyledones, Dicotyledones. |
More recently Nawaschin (Bull. Acad. Imp. Sci. St. Petersb., ser. iii., xxxv.) has shown that Betula, and Miss Benson (Trans. Linn. Soc., 1894) that Alnus, Corylus, and Carpinus also belong to the Chalazogams.
Our knowledge, however, is still so incomplete that one would hesitate to accord the full systematic value which Dr. Treub attaches to his discovery until the limits of the Chalazogamic group are better defined; and it would hardly be justifiable to include the Casuarinas and the above-noted genera in one family.]
Class 1. Monocotyledones.
The embryo has only one cotyledon; the leaves are as a rule scattered, with parallel venation; the vascular bundles of the stem are closed, there is no increase of thickness. The flower is typically constructed of five 3-merous whorls, placed alternately.