Dicotyledons sentence example
A tissue mother-cell of the xylem may, in the most advanced types of Dicotyledons, give rise to(I) a tracheid; (2) a segment of a vessel; (3) a xylem-fibre; or (4) a vertical file of xylem-parenchyma cells.
The tissue-elements just described are found only in the more complicated secondary vascular tissues of certain Dicotyledons.
In the secondary tissues of Dicotyledons we may have, as already described, considerably more differentiation of the cells, all the varieties being referable, however, on the one hand to the tracheal or sieve-tube type, on the other to the parenchyma type.
The formation of additional cambial cylinders or bands occurs in the most various families of Dicotyledons and in some Gymnosperms. They may arise in the pericycle or endocycle of the stele, in the cortex of the stem, or in the parenchyma of the secondary xylem or phloem.
It has been found in general that, while the number 5 occurs in the phyllotaxis of Dicotyledons, 3 is common in that of Monocotyledons.Advertisement
The advent in 1351 of Hofmeister's brilliant discovery of the changes proceeding in the embryo-sac of flowering plants, and his determination of the correct relationships of these with the Cryptogamia, fixed the true position of Gymnosperms as a class distinct from Dicotyledons, and the term Angiosperm then gradually came to be accepted as the suitable designation for the whole of the flowering plants other than Gymnosperms, and as including therefore the classes of Dicotyledons and Monocotyledons.
In the larger of the two great groups into which the Angiosperms are divided, the Dicotyledons, the bundles in the very young stem are arranged in an open ring, separating a central pith from an outer cortex.
Frequently, as in many Dicotyledons, the primary root, the original root of the seedling, persists throughout the life of the plant, forming, as often in biennials, a thickened tap-root, as in carrot, or in perennials, a much-branched root system.
In many Dicotyledons and most Monocotyledons, the primary root soon perishes, and its place is taken by adventitious roots developed from the stem.
This remarkable double fertilization as it has been called, although only recently discovered, has been proved to take place in widely-separated families, and both in Monocotyledons and Dicotyledons, and there is every probability that, perhaps with variations, it is the normal process in Angiosperms. After impregnation the fertilized oosphere immediately surrounds itself with a cell-wall and becomes the oospore which by a process of growth forms the embryo of the new plant.Advertisement
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.
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.
We readily recognize in them nowadays the natural classes of Dicotyledons and Monocotyledons distinguished alike in vegetative and in reproductive construction, yet showing remarkable parallel sequences in development; and we see that the Dicotyledons are the more advanced and show the greater capacity for further progressive evolution.
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.
This, however, does not entitle us to assume the origin of Monocotyledons from Dicotyledons, although there is manifestly a temptation to connect helobic forms of the former with ranal ones of the latter.Advertisement
Amongst Dicotyledons the gamopetalous forms are admitted to be the highest development and a dominant one of our epoch.
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.
The orders are carefully characterized, and those of Angiosperms are grouped in fourteen classes under the two main divisions Monocotyledons and Dicotyledons.
In his arrangement the last subdivision disappears, and the Dicotyledons fall into two groups, a larger containing those in which both calyx and corolla are present in the flower, and a smaller, Monochlamydeae, representing the Apetalae and Diclines Irregulares of Jussieu.
The seven series of Monocotyledons represent a sequence beginning with the most complicated epigynous orders, such as Orchideae and Scitamineae, and passing through the petaloid hypogynous orders (series Coronarieae) of which Liliaceae is the representative to Juncaceae and the palms (series Calycinae) where the perianth Ioses its petaloid character and thence to the Aroids, screw-pines and albuminous Dicotyledons the cotyledons act as the absorbents of the reserve-food of the seed and are commonly brought above ground (epigeal), either withdrawn from the seed-coat or carrying it upon them, and then they serve as the first green organs of the plant.Advertisement
Exalbuminous Dicotyledons usually store reserve-food in their cotyledons, which may in germination remain below ground (hypogeal).
The sequence of orders in the polypetalous subdivision of Dicotyledons undoubtedly represents a progression from simpler to more elaborate forms, but a great drawback to the value of the system is the inclusion among the Monochlamydeae of a number of orders which are closely allied with orders of Polypetalae though differing in absence of a corolla.
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.
A modification of Eichler's system, embracing the most recent views of the affinities of the orders of Angiosperms, has been put forward by Dr Adolf Engler of Berlin, who adopts the suggestive names Archichlamydeae and Metachlamydeae for the two subdivisions of Dicotyledons.
The most remarkable feature here is the class of Liorhizal Dicotyledons, which includes only the families of Nymphaeaceae and Gramineae.Advertisement
Ovular characters determine the grouping in the Dicotyledons, van Tieghem supporting the view that the integument, the outer if there be two, is the lamina of a leaf of which the funicle is the petiole, whilst the nucellus is an outgrowth of this leaf, and the inner integument, if present, an indusium.
It was the English botanist Robert Brown who first recognized this important distinguishing feature in conifers and cycads in 1825; he established the gymnospermy of these seed-bearing classes as distinct from the angiospermy of the monocotyledons and dicotyledons.
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 MonocoFIG.8.
Short and reticulately-pitted tracheal cells, similar to tracheids, often occur in the circummedullary region of cycadean stems. In an old stem of Cycas, Encephalartos or Macrozamia the secondary wood consists of several rather unevenly concentric zones, while in some other genera it forms a continuous mass as in conifers and normal dicotyledons.
It is in the nature of the secondary xylem that the Coniferales are most readily distinguished from the Dicotyledons and Cycadaceae; the wood is homogeneous in structure, consisting almost entirely of tracheids with circular or polygonal bordered pits on the radial walls, more particularly in the late summer wood.Advertisement
These three orders are closely allied and form the series Sarraceniales of the free-petalled section (Choripetalae) of Dicotyledons.
Of the 4100 known plants - of which about three-fourths are endemic - composing the Madagascar flora, there are 3492 Dicotyledons, 248 Monocotyledons and 360 Acotyledons.
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.
The symmetry which is most commonly met with is trimerous and pentamerous - the former occurring generally among monocotyledons, the latter among dicotyledons.
It is present in the greater number of Dicotyledons.Advertisement
A division takes place, by which four cells are formed in each, the exact mode of division differing in dicotyledons and monocotyledons.
In Monocotyledons, as in grasses, there is often only one, while in Dicotyledons they number from three upwards; when numerous, the pores are either scattered irregularly, or in a regular order, frequently forming a circle round the equatorial surface.
Lester Ward records no fewer than 737 distinct forms, consisting chiefly of Ferns, Cycads, Conifers and Dicotyledons, the Ferns and Cycads being con fined mainly to the Older Potomac, FIG.
Whatever doubt may be left as to the exact botanical position of these early Lower Cretaceous Angiosperms, it is clear that both Monocotyledons and Dicotyledons are represented by several types of leaves, and that the flora extended over wide areas in North America and Greenland, and is found again at a few points in Europe.
The leading characteristic of this Middle Potomac flora is the proportion of Dicotyledons.Advertisement
The genera of Dicotyledons represented are Quercus, Sassafras, Platanus, Celastrophyllum, Cissites, Viburnites.
According to this catalogue, the true Laramie flora includes about 250 species, more than half of which are deciduous forest trees, herbaceous Dicotyledons, Monocotyledons and Cryptogams, all being but poorly represented.
The Dicotyledons include several water-lilies, a somewhat doubtful Trapa, and many genera of forest trees still common in America.
The Senonian strata have yielded 118 species, 21 of which are Cryptogams, 1 i Conifers, 5 Monocotyledons, 75 Dicotyledons.
The Proteaceae are also missing; but other Dicotyledons occur in profusion, many of them being remarkable for the large size of their deciduous leaves.Advertisement
Among the flowering plants are Dewalquea, a ranunculaceous genus already mentioned as occurring in the Upper Cretaceous, and numerous living genera of forest-trees, such as occur throughout the Tertiary period, and are readily comparable with living forms. Saporta has described about seventy Dicotyledons, most of which are peculiar to this locality.
The Dicotyledons need further study.
The plants include a Fern, Onoclea hebridica, close to a living American form; four Gymnosperms belonging to the genera Cryptomeria, Ginkgo, Taxus and Podocarpus; Dicotyledons of about 30 species, several of which have been figured.
The Irish strata yield two ferns; 7 Gymnosperms, Cupressus, Cryptomeria, Taxus, Podocarpus, Pinus (2 species), Tsuga; and leaves of about 25 Dicotyledons.
Among the noticeable Dicotyledons are the Myricaceae, Proteaceae, Laurineae, Bombax, the Judas-tree, Acacia, Ailanthus, while the most plentiful forms are the Araliaceae.Advertisement
Among the Dicotyledons, the Leguminosae take the first place with 131 species, including Acacia, Caesalpinia and Cassia, each represented by several forms. The occurrence of 90 species of Amentaceae shows that, as the climate became less tropical, the relative proportion of this group to the total flora increased.
It has since been shown by other observers that this double fertilization Occurs in many other Angiosperms, both Dicotyledons and Monocotyledons, so that it is probably of general occurrence throughout the group (see ANGIOSPERMS).
Among the Dicotyledons may be mentioned Platanus, Acer (?), Quercus (?), Viburnum, Alnus, Magnolia, Corylus (?), Castanea (?), Zizyphus, Populus and the nettlelike Boehmeria antiqua.
The anatomical construction of these plants presents many peculiarities which have given rise to discussion as to the allocation of the order among the dicotyledons or among the monocotyledons, the general balance of opinion being in favour of the former view.
The embryo consists of an axis bearing one (Monocotyledons) or two (Dicotyledons) cotyledons, which protect the stem bud (plumule) of the future plant, and ending below in a radicle.
The typical structure of the vascular cylinder of the adult primary stem in the Gyrnnosperms and Dicotyledons is, like that of the higher ferns, a hollow cylinder of vas- Structure of cular tissue enclosing a central parenchymatous pith.
The outermost is the caiyptrogen, which gives rise to the root-cap, and in Dicotyledons to the piliferous layer as well.
Other undoubted Dicotyledons, though of uncertain affinity, of similar age have now been detected in North America.
The parts of the flower are most frequently arranged in fives, or multiples of fives; for instance, a common arrangement is as follows, - five sepals, succeeded by five petals, ten stamens in two sets of five, and five or fewer carpels; an arrangement in fours is less frequent, while the arrangement in threes, so common in monocotyledons, is rare in dicotyledons.
He separated flowering from flowerless plants, and divided the former into Dicotyledons and Monocotyledons.
The name Ambrosia was also applied by Dioscorides and Pliny to certain herbs, and has been retained in modern botany for a genus of plants from which it has been extended to the group of dicotyledons called Ambrosiaceae, including Ambrosia, Xanthium and Iva, all annual herbaceous plants represented in America.
Such bases occur almost exclusively in the dicotyledons, generally in combination with malic, citric, tartaric or similar plant-acids.
The genus Myrica is the type of a small, but widely distributed order, Myricaceae, which is placed among the apetalous families of Dicotyledons, and is perhaps most nearly allied to the willow family.
Amongst Dicotyledons this is very rare.
The leaves of Monocotyledons have generally this kind of venation, while reticulated venation most usually occurs amongst Dicotyledons.
This sheath is comparatively rare in dicotyledons, but is seen in umbelliferous plants.
They are not common in dicotyledons with opposite leaves.
Such arrangements are common in Dicotyledons.