When the Dicotyledons appear in the upper cretaceous beds, representatives of the three great groups [Apetalae, Monopetalae, Polypetalae] appear together in the same deposit.
The Ordinal form, if it ever existed, must necessarily be much older than the period of the upper Cretaceous rocks, that is than the period to which the earliest known Dicotyledons belong.
In the dicotyledons root and shoot are represented as springing from the same point, and in monocotyledons from opposite poles in the seed.
The distinction between dicotyledons and monocotyledons is accurately set forth, though the stress is laid not so much on the cotyledonous character of the seed as on the relation of root and shoot.
In many Dicotyledons and most Monocotyledons, the primary root soon perishes, and its place is taken by adventitious roots developed from the stem.
Well-defined polypetalous and gamopetalous genera sometimes occur in the same order, and even Monocotyledons and Dicotyledons are classed together where they have some striking physiological character in common.
But there is no sound basis for the assumption that the Dicotyledons are derived from Monocotyledons; indeed, the palaeontological evidence seems to point to the Dicotyledons being the older.
Exalbuminous Dicotyledons usually store reserve-food in their cotyledons, which may in germination remain below ground (hypogeal).
Eichler, attempted to remove this disadvantage which since the time of Jussieu had characterized the French system, and in 1883 grouped the Dicotyledons in two subclasses.
In Dicotyledons the shoot of the embryo is wholly derived from the terminal cell of the pro-embryo, from the next cell the root arises, and the remaining ones form the suspensor.
Amongst Dicotyledons the gamopetalous forms are admitted to be the highest development and a dominant one of our epoch.
The origin of the Monocotyledons and Dicotyledons remains one of the most difficult and attractive problems of Palaeobotany.
The dicotyledonsare of more complex structure, and somewhat more perfect organization, than the monocotyledons: and some dicotyledonous families, such as the Compositae, are rather more complex in their organization than the rest.
In Monocotyledons it is usual for the staminal whorl to be double, it rarely having more than two rows, whilst amongst dicotyledons there are often very numerous rows of stamens.
A division takes place, by which four cells are formed in each, the exact mode of division differing in dicotyledonsand monocotyledons.
Nilson has observed leaves ofDicotyledons mixed with leaves of Cycadites.
The Gymnospermous dicotyledonsare represented by Coniferae (Cupressinae, Abietineae, Taxineae, and Gnetaceae.
Monocotyledons andDicotyledons existed in the Upper Cretaceous times.
This description of the analogies of the pollen and embryo-sac of dicotyledons assumes that the general vegetative structure of this class of plants is equivalent to the asexual generation of the higher cryptogams.
Besides the groups already mentioned, there are several families of dicotyledons whose affinities are very doubtful.
The second group (Sympetalæ or Gamopetalæ) comprises those dicotyledons whose flowers have the petals more or less completely united into a tube.
As many of thedicotyledons are trees or shrubs that lose their leaves annually, special leaves are developed for the protection of the young leaves during the winter.
Nearly all of the dicotyledons may be placed in one of two great divisions distinguished by the character of the petals.
The number of the dicotyledons is very large, and very much the greater number of living spermaphytes belong to this group.
Brongniart, who was the first to investigate in detail the anatomy of a cycadean stem, recognized an agreement, as regards the secondary wood, with Dicotyledons and Gymnosperms, rather than with Monocotyledons.
Dicotyledons and amongst Monocotyledons, together with several Cryptogams, have now been described.
This was, however, retained for the specimens belonging to the Monocotyledons and Cryptogams, while those of the Dicotyledons were arranged in the new and spacious building of No.
Passing on to the Tertiary period the numbers increase, till they reach their maximum in the Miocene, where more than 2000 species of dicotyledons have been discovered.
True dicotyledons appear very much later, in the Cretaceous period, and only in its upper division, if we except a single species from the Urgonian beds of Greenland.
So, the ferns, lycopods, conifers, and monocotyledons of the palaeozoic and mesozoic rocks, have developed into the marvellous wealth of forms of the higher dicotyledons that now adorn the earth.
The ordinal form, if it ever existed, must necessarily be much older than the period of the Upper Cretaceous rocks, that is, than the period to which the earliest known dicotyledons belong.
I may add that the new discoveries made since 1884 would probably tend to increase the proportionate number of dicotyledons in the newer groups.
The class of dicotyledons shows itself there in its fullest development; it is the epoch of flowers.
Dicotyledons are divided into three main groups, the division being based on the structure of the flowers.
In the Dicotyledons the vascular bundles of the stem are united into a ring which surrounds a central pith; the veins of the leaves form a network, and the parts of the flower are arranged in whorls of four or five.
Another fact, which might indicate that the Dicotyledons have taken possession of comparatively unoccupied regions only, is that they are very poorly represented, so far as higher groups are concerned, in aquatic conditions.
This tropical display of Dicotyledons further shows the great prominence of America in the display of forms.
In the case of Dicotyledonsthis is different, for while they are found in the tropics, they are found in the other regions as well, and have better chances for intermingling than the other groups.
In looking over the display of Dicotyledonsin the tropics of the eastern and western hemispheres, it becomes evident that there is no such difference between the forms of the two regions as in the groups previously mentioned.
If, as now seems probable, the Angiosperms were derived from ancestors allied to the Cycads, it would naturally follow that the Dicotyledons were first evolved, for their structure has most in common with that of the Cycadophyta.
Very few of these classes really represent natural groups, and the chief of all distinctions among Flowering Plants, that between Dicotyledons and Monocotyledons, which was foreshadowed by Albertus, is almost lost to sight.
It seems clear from this expression that he realised that there was an anatomical distinction betweenDicotyledons and Monocotyledons.
He is thus led to make a rough separation between the classes which we now call Dicotyledons and Monocotyledons.
In this flora there is the predominance of Dicotyledons in number and variety; there are few Monocotyledons.
The Tertiary period is characterised by the abundance of Angiospermous Dicotyledons and of Monocotyledons, more especially of Palms.
There is a small number ofDicotyledons with gamopetalous corollas.
Although the majority ofDicotyledons belong to the most highly developed and most perfect plants, still the lowest division of them is directly allied to the Gymnosperms, and particularly to the Gnetaceae.
It was not until the chalk period that the second and more perfect class of the Dicotyledons appeared, namely, the group with corollas (Dichlamydeae, or Corolliflorae).
Dicotyledons have bark, wood, and pith, and grow by producing a new ring of wood outside the old.
Flowering plants are divided first into these two classes, the Dicotyledons and the Monocotyledons.
The parallel-veined leaves of monocotyledons have stems without distinction of wood, bark and pith; the netted-veined leaves of dicotyledons have exogenous stems.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "dicotyledons" in a variety of sentences. We hope that you will now be able to make sentences using this word.
