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Updated: May 31, 2025
Such groups may be large, as for instance, the monocotyledons, that have their leaves arranged in two opposite rows in many families, or small, as genera or subdivisions of genera.
According to systematic views of the monocotyledons the original prototype of the genus Iris must have had a whorl of six equal, or nearly equal perianth-segments and six stamens, such as are now seen in the more primitive types of the family of the lilies, as for instance in the lilies themselves, the tulips, hyacinths and others.
The dicotyledons are of more complex structure, and somewhat more perfect organization, than the monocotyledons; and some dicotyledonous families, such as the Compositæ, are rather more complex in their organization than the rest. But the differences are not of a marked character, and do not promise to throw any particular light upon the conditions and laws of vegetable life and development.
The most remarkable feature here is the class of Liorhizal Dicotyledons, which includes only the families of Nymphaeaceae and Gramineae. It is based upon the fact that the histological differentiation of the epidermis of their root is that generally characteristic of Monocotyledons, whilst they have two cotyledons the old view of the epiblast as a second cotyledon in Gramineae being adopted.
Among monocotyledons, the orders in which it is most remarkable are Liliaceae, Hemerocallideae, Asphodeleae, Irideae, and Commelineae.
Your attention has been directed to the monocotyledons as an example, where retrogression is everywhere so active that it can almost be said to be the prevailing movement.
As an attempt at a phylogenetic arrangement, Engler's system is now preferred by many botanists. More recently a startling novelty in the way of system has been produced by van Tieghem, as follows: Monocotyledons. Liorhizal Dicotyledons. Dicotyledons. INSEMINEAE. SEMINEAE. Unitegmineae. Bitegmineae.
But progression is nearly always accompanied by retrogression in the principal lines of evolution, as well as in the collateral branches of the genealogical tree. Sometimes it prevails, and the monocotyledons are obviously a reduced branch of the primitive dicotyledons.
The parasite monocotyledons take between the tropics the place of the moss and lichens of our northern zone. As we advanced, the forms and grouping of the rocks reminded us of Switzerland and the Tyrol.
In many Monocotyledons the terminal cell forms the cotyledonary portion alone of the shoot of the embryo, its axial part and the root being derived from the adjacent cell; the cotyledon is thus a terminal structure and the apex of the primary stem a lateral one a condition in marked contrast with that of the Dicotyledons. In some Monocotyledons, however, the cotyledon is not really terminal.
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