to the opening of the micropyle, into which the pollen-tube has
penetrated, the separating cell-wall becomes absorbed, and the male or
sperm-cells are ejected into the embryo-sac. Guided by the synergidae
one male-cell passes into the oosphere with which it fuses, the two
nuclei uniting, while the other fuses with the definitive nucleus, or,
as it is also called, the endosperm nucleus. 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. The
endosperm-nucleus divides rapidly to produce a cellular tissue which
fills up the interior of the rapidly-growing embryo-sac, and forms a
tissue, known as endosperm, in which is stored a supply of nourishment
for the use later on of the embryo. It has long been known that after
fertilization of the egg has taken place, the formation of endosperm
begins from the endosperm nucleus, and this had come to be regarded as
the recommencement of the development of a prothallium after a pause
following the reinvigorating union of the polar nuclei. This view is
still maintained by those who differentiate two acts of fertilization
within the embryo-sac, and regard that of the egg by the first
male-cell, as the true or generative fertilization, and that of the
polar nuclei by the second male gamete as a vegetative fertilization
which gives a stimulus to development in correlation with the other.
If, on the other hand, the endosperm is the product of an act of
fertilization as definite as that giving rise to the embryo itself,
we have to recognize that twin-plants are produced within the
embryo-sac--one, the embryo, which becomes the angiospermous plant,
the other, the endosperm, a short-lived, undifferentiated nurse to
assist in the nutrition of the former, even as the subsidiary embryos
in a pluri-embryonic Gymnosperm may facilitate the nutrition of the
dominant one. If this is so, and the endosperm like the embryo is
normally the product of a sexual act, hybridization will give a hybrid
endosperm as it does a hybrid embryo, and herein (it is suggested) we
may have the explanation of the phenomenon of xenia observed in the
mixed endosperms of hybrid races of maize and other plants, regarding
which it has only been possible hitherto to assert that they were
indications of the extension of the influence of the pollen beyond the
egg and its product. This would not, however, explain the formation
of fruits intermediate in size and colour between those of crossed
parents. The signification of the coalescence of the polar nuclei is
not explained by these new facts, but it is noteworthy that the second
male-cell is said to unite sometimes with the apical polar nucleus,
the sister of the egg, before the union of this with the basal polar
one. The idea of the endosperm as a second subsidiary plant is no new
one; it was suggested long ago in explanation of the coalescence of
the polar nuclei, but it was then based on the assumption that these
represented male and female cells, an assumption for which there
was no evidence and which was inherently improbable. The proof of a
coalescence of the second male nucleus with the definitive nucleus
gives the conception a more stable basis. The antipodal cells aid more
or less in the process of nutrition of the developing embryo, and may
undergo multiplication, though they ultimately disintegrate, as do
also the synergidae. As in Gymnosperms and other groups an interesting
qualitative change is associated with the process of fertilization.
The number of chromosomes (see PLANTS: _Cytology_) in the nucleus of
the two spores, pollen-grain and embryo-sac, is only half the number
found in an ordinary vegetative nucleus; and this reduced number
persists in the cells derived from them. The full number is restored
in the fusion of the male and female nuclei in the process of
fertilization, and remains until the formation of the cells from which
the spores are derived in the new generation.
In several natural orders and genera departures from the course of
development just described have been noted. In the natural order
Rosaceae, the series Querciflorae, and the very anomalous genus
_Casuarina_ and others, instead of a single macrospore a more or less
extensive sporogenous tissue is formed, but only one cell proceeds to
the formation of a functional female cell. In _Casuarina_, _Juglans_
and the order Corylaceae, the pollen-tube does not enter by means
of the micropyle, but passing down the ovary wall and through the
placenta, enters at the chalazal end of the ovule. Such a method
of entrance is styled chalazogamic, in contrast to the porogamic or
ordinary method of approach by means of the micropyle.
_Embryology._
The result of fertilization is the development of the ovule into
the seed. By the segmentation of the fertilized egg, now invested by
cell-membrane, the embryo-plant arises. A varying number of transverse
segment-walls transform it into a pro-embryo--a cellular row of which
the cell nearest the micropyle becomes attached to the apex of the
embryo-sac, and thus fixes the position of the developing embryo,
while the terminal cell is projected into its cavity. 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. In some Monocotyledons, however, the cotyledon is not
really terminal. The primary root of the embryo in all Angiosperms
points towards the micropyle. The developing embryo at the end of the
suspensor grows out to a varying extent into the forming endosperm,
from which by surface absorption it derives good material for growth;
at the same time the suspensor plays a direct part as a carrier of
nutrition, and may even develop, where perhaps no endosperm is formed,
special absorptive "suspensor roots" which invest the developing
embryo, or pass out into the body and coats of the ovule, or even into
the placenta. In some cases the embryo or the embryo-sac sends
out suckers into the nucellus and ovular integument. As the embryo
develops it may absorb all the food material available, and store,
either in its cotyledons or in its hypocotyl, what is not immediately
required for growth, as reserve-food for use in germination, and by so
doing it increases in size until it may fill entirely the embryo-sac;
or its absorptive power at this stage may be limited to what is
necessary for growth and it remains of relatively small size,
occupying but a small area of the embryo-sac, which is otherwise
filled with endosperm in which the reserve-food is stored. There are
also intermediate states. The position of the embryo in relation to
the endosperm varies, sometimes it is internal, sometimes external,
but the significance of this has not yet been established.
[v.02 p.0012]
The formation of endosperm starts, as has been stated, from the
endosperm nucleus. Its segmentation always begins before that of the
egg, and thus there is timely preparation for the nursing of the young
embryo. If in its extension to contain the new formations within it
the embryo-sac remains narrow, endosperm formation proceeds upon the
lines of a cell-division, but in wide embryo-sacs the endosperm is
first of all formed as a layer of naked cells around the wall of the
sac, and only gradually acquires a pluricellular character, forming
a tissue filling the sac. The function of the endosperm is primarily
that of nourishing the embryo, and its basal position in the
embryo-sac places it favourably for the absorption of food material
entering the ovule. Its duration varies with the precocity of the
embryo. It may be wholly absorbed by the progressive growth of the
embryo within the embryo-sac, or it may persist as a definite and more
or less conspicuous constituent of the seed. When it persists as
a massive element of the seed its nutritive function is usually
apparent, for there is accumulated within its cells reserve-food, and
according to the dominant substance it is starchy, oily, or rich in
cellulose, mucilage or proteid. In cases where the embryo has stored
reserve food within itself and thus provided for self-nutrition, such
endosperm as remains in the seed may take on other functions, for
instance, that of water-absorption.
Some deviations from the usual course of development may be noted.
Parthenogenesis, or the development of an embryo from an egg-cell
without the latter having been fertilized, has been described in
species of _Thalictrum_, _Antennaria_ and _Alchemilla_. Polyembryony
is generally associated with the development of cells other than the
egg-cell. Thus in _Erythronium_ and _Limnocharis_ the fertilized
egg may form a mass of tissue on which several embryos are produced.
Isolated cases show that any of the cells within the embryo-sac may
exceptionally form an embryo, _e.g._ the synergidae in species of
_Mimosa_, _Iris_ and _Allium_, and in the last-mentioned the
antipodal cells also. In _Coelebogyne_ (Euphorbiaceae) and in _Funkia_
(Liliaceae) polyembryony results from an adventitious production
of embryos from the cells of the nucellus around the top of the
embryo-sac. In a species of _Allium_, embryos have been found
developing in the same individual from the egg-cell, synergids,
antipodal cells and cells of the nucellus. In two Malayan species of
_Balanophora_, the embryo is developed from a cell of the endosperm,
which is formed from the upper polar nucleus only, the egg apparatus
becoming disorganized. The last-mentioned case has been regarded
as representing an apogamous development of the sporophyte from the
gametophyte comparable to the cases of apogamy described in Ferns. But
the great diversity of these abnormal cases as shown in the examples
cited above suggests the use of great caution in formulating definite
morphological theories upon them.
_Fruit and seed._
As the development of embryo and endosperm proceeds within the
embryo-sac, its wall enlarges and commonly absorbs the substance of
the nucellus (which is likewise enlarging) to near its outer limit,
and combines with it and the integument to form the _seed-coat_; or
the whole nucellus and even the integument may be absorbed. In some
plants the nucellus is not thus absorbed, but itself becomes a seat of
deposit of reserve-food constituting the _perisperm_ which may coexist
with endosperm, as in the water-lily order, or may alone form a
food-reserve for the embryo, as in _Canna_. Endospermic food-reserve
has evident advantages over perispermic, and the latter is
comparatively rarely found and only in non-progressive series. Seeds
in which endosperm or perisperm or both exist are commonly called
_albuminous_ or _endospermic_, those in which neither is found are
termed _exalbuminous_ or _exendospermic_. These terms, extensively
used by systematists, only refer, however, to the grosser features
of the seed, and indicate the more or less evident occurrence of a
food-reserve; many so-called exalbuminous seeds show to microscopic
examination a distinct endosperm which may have other than a nutritive
function. The presence or absence of endosperm, its relative amount
when present, and the position of the embryo within it, are valuable
characters for the distinction of orders and groups of orders.
Meanwhile the ovary wall has developed to form the fruit or pericarp,
the structure of which is closely associated with the manner of
distribution of the seed. Frequently the influence of fertilization is
felt beyond the ovary, and other parts of the flower take part in
the formation of the fruit, as the floral receptacle in the apple,
strawberry and others. The character of the seed-coat bears a definite
relation to that of the fruit. Their function is the twofold one of
protecting the embryo and of aiding in dissemination; they may also
directly promote germination. If the fruit is a dehiscent one and the
seed is therefore soon exposed, the seed-coat has to provide for the
protection of the embryo and may also have to secure dissemination. On
the other hand, indehiscent fruits discharge these functions for the
embryo, and the seed-coat is only slightly developed.
_Dissemination._
Dissemination is effected by the agency of water, of air, of
animals--and fruits and seeds are therefore grouped in respect of this
as hydrophilous, anemophilous and zooidiophilous. The needs for these
are obvious--buoyancy in water and resistance to wetting for the
first, some form of parachute for the second, and some attaching
mechanism or attractive structure for the third. The methods in which
these are provided are of infinite variety, and any and every part of
the flower and of the inflorescence may be called into requisition to
supply the adaptation (see FRUIT). Special outgrowths, arils, of the
seed-coat are of frequent occurrence. In the feature of fruit and
seed, by which the distribution of Angiosperms is effected, we have a
distinctive character of the class. In Gymnosperms we have seeds, and
the carpels may become modified and close around these, as in _Pinus_,
during the process of ripening to form an imitation of a box-like
fruit which subsequently opening allows the seeds to escape; but
there is never in them the closed ovary investing from the outset the
ovules, and ultimately forming the ground-work of the fruit.
_Germination of Seed._
Their fortuitous dissemination does not always bring seeds upon a
suitable nidus for germination, the primary essential of which is a
sufficiency of moisture, and the duration of vitality of the embryo is
a point of interest. Some seeds retain vitality for a period of many
years, though there is no warrant for the popular notion that genuine
"mummy wheat" will germinate; on the other hand some seeds lose
vitality in little more than a year. Further, the older the seed the
more slow as a general rule will germination be in starting, but there
are notable exceptions. This pause, often of so long duration, in
the growth of the embryo between the time of its perfect development
within the seed and the moment of germination, is one of the
remarkable and distinctive features of the life of Spermatophytes. The
aim of germination is the fixing of the embryo in the soil, effected
usually by means of the root, which is the first part of the embryo
to appear, in preparation for the elongation of the epicotyledonary
portion of the shoot, and there is infinite variety in the details
of the process. In 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. The part of the stem below the cotyledons (_hypocotyl_)
commonly plays the greater part in bringing this about. Exalbuminous
Dicotyledons usually store reserve-food in their cotyledons, which
may in germination remain below ground (_hypogeal_). In albuminous
Monocotyledons the cotyledon itself, probably in consequence of its
terminal position, is commonly the agent by which the embryo is thrust
out of the seed, and it may function solely as a feeder, its extremity
developing as a sucker through which the endosperm is absorbed, or
it may become the first green organ, the terminal sucker dropping
off with the seed-coat when the endosperm is exhausted. Exalbuminous
Monocotyledons are either hydrophytes or strongly hygrophilous plants
and have often peculiar features in germination.
[v.02 p.0013]
_Vegetative reproduction._
Distribution by seed appears to satisfy so well the requirements of
Angiosperms that distribution by vegetative buds is only an occasional
process. At the same time every bud on a shoot has the capacity
to form a new plant if placed in suitable conditions, as the
horticultural practice of propagation by cuttings shows; in nature we
see plants spreading by the rooting of their shoots, and buds we know
may be freely formed not only on stems but on leaves and on roots.
Where detachable buds are produced, which can be transported through
the air to a distance, each of them is an incipient shoot which may
have a root, and there is always reserve-food stored in some part of
it. In essentials such a bud resembles a seed. A relation between
such vegetative distribution buds and production of flower is usually
marked. Where there is free formation of buds there is little flower
and commonly no seed, and the converse is also the case. Viviparous
plants are an illustration of substitution of vegetative buds for
flower.
_Phylogeny and taxonomy._
The position of Angiosperms as the highest plant-group is
unassailable, but of the point or points of their origin from the
general stem of the plant kingdom, and of the path or paths of their
evolution, we can as yet say little.
Until well on in the Mesozoic period geological history tells us
nothing about Angiosperms, and then only by their vegetative organs.
We readily recognize in them now-a-days 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. There is no doubt that the phylum of Angiosperms has not
sprung from that of Gymnosperms.
Within each class the flower-characters as the essential feature
of Angiosperms supply the clue to phylogeny, but the uncertainty
regarding the construction of the primitive angiospermous flower gives
a fundamental point of divergence in attempts to construct progressive
sequences of the families. Simplicity of flower-structure has appeared
to some to be always primitive, whilst by others it has been taken to
be always derived. There is, however, abundant evidence that it may
have the one or the other character in different cases. Apart from
this, botanists are generally agreed that the concrescence of parts of
the flower-whorls--in the gynaeceum as the seed-covering, and in the
corolla as the seat of attraction, more than in the androecium and the
calyx--is an indication of advance, as is also the concrescence
that gives the condition of epigyny. Dorsiventrality is also clearly
derived from radial construction, and anatropy of the ovule has
followed atropy. We should expect the albuminous state of the seed
to be an antecedent one to the exalbuminous condition, and the recent
discoveries in fertilization tend to confirm this view. Amongst
Dicotyledons the gamopetalous forms are admitted to be the highest
development and a dominant one of our epoch. Advance has been along
two lines, markedly in relation to insect-pollination, one of which
has culminated in the hypogynous epipetalous bicarpellate forms with
dorsiventral often large and loosely arranged flowers such as occur
in Scrophulariaceae, and the other in the epigynous bicarpellate
small-flowered families of which the Compositae represent the most
elaborate type. In the polypetalous forms progression from hypogyny
to epigyny is generally recognized, and where dorsiventrality with
insect-pollination has been established, a dominant group has been
developed as in the Leguminosae. The starting-point of the class,
however, and the position within it of apetalous families with
frequently unisexual flowers, have provoked much discussion. In
Monocotyledons a similar advance from hypogyny to epigyny is observed,
and from the dorsiventral to the radial type of flower. In this
connexion it is noteworthy that so many of the higher forms are
adapted as bulbous geophytes, or as aerophytes to special xerophilous
conditions. The Gramineae offer a prominent example of a dominant
self-pollinated or wind-pollinated family, and this may find
explanation in a multiplicity of factors.
Though best known for his artificial (or sexual) system, Linnaeus
was impressed with the importance of elaborating a natural system of
arrangement in which plants should be arranged according to their true
affinities. In his _Philosophia Botanica_ (1751) Linnaeus grouped the
genera then known into sixty-seven orders (_fragmenta_), all except
five of which are Angiosperms. He gave names to these but did not
characterize them or attempt to arrange them in larger groups.
Some represent natural groups and had in several cases been already
recognized by Ray and others, but the majority are, in the light
of modern knowledge, very mixed. 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.
Work on the lines suggested by the Linnaean _fragmenta_ was continued
in France by Bernard de Jussieu and his nephew, Antoine Laurent,
and the arrangement suggested by the latter in his _Genera Plantarum
secundum Ordines Naturales disposita_ (1789) is the first which can
claim to be a natural system. The orders are carefully characterized,
and those of Angiosperms are grouped in fourteen classes under the two
main divisions Monocotyledons and Dicotyledons. The former comprise
three classes, which are distinguished by the relative position of the
stamens and ovary; the eleven classes of the latter are based on the
same set of characters and fall into the larger subdivisions Apetalae,
Monopetalae and Polypetalae, characterized respectively by absence,
union or freedom of the petals, and a subdivision, _Diclines
Irregulares_, a very unnatural group, including one class only. A.P.
de Candolle introduced several improvements into the system. 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 dichlamydeous group is subdivided into three, Thalamiflorae,
Calyciflorae and Corolliflorae, depending on the position and union of
the petals. This, which we may distinguish as the French system,
finds its most perfect expression in the classic _Genera Plantarum_
(1862-1883) of Bentham and Hooker, a work containing a description,
based on careful examination of specimens, of all known genera of
flowering plants. The subdivision is as follows:--
DICOTYLEDONS.
Polypetalae:
Thalamiflorae.
Disciflorae.
Calyciflorae.
Gamopetalae:
Inferae.
Heteromerae.
Bicarpellatae.
Monochlamydeae in eight series.
Monocotyledons in seven series.
Of the Polypetalae, series 1, Thalamiflorae, is characterized by
hypogynous petals and stamens, and contains 34 orders distributed in 6
larger groups or cohorts. Series 2, Disciflorae, takes its name from
a development of the floral axis which forms a ring or cushion at the
base of the ovary or is broken up into glands; the ovary is superior.
It contains 23 orders in 4 cohorts. Series 3, Calyciflorae, has petals
and stamens perigynous, or sometimes superior. It contains 27 orders
in 5 cohorts.
Of the Gamopetalae, series 1, Inferae, has an interior ovary and
stamens usually as many as the corolla-lobes. It contains 9 orders
in 3 cohorts. Series 2, Heteromerae, has generally a superior ovary,
stamens as many as the corolla-lobes or more, and more than two
carpels. It contains 12 orders in 3 cohorts. Series 3, Bicarpellatae,
has generally a superior ovary and usually two carpels. It contains 24
orders in 4 cohorts.
The eight series of Monochlamydeae, containing 36 orders, form groups
characterized mainly by differences in the ovary and ovules, and are
now recognized as of unequal value.
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 loses
its petaloid character and thence to the Aroids, screw-pines and
others where it is more or less aborted (series Nudiflorae). Series
6, Apocarpeae, is characterized by 5 carpels, and in the last series
Glumaceae, great simplification in the flower is associated with a
grass-like habit.
[v.02 p.0014]
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. The German systematist, A.W. 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. The earlier Choripetalae embraces the Polypetalae
and Monochlamydae of the French systems. It includes 21 series, and
is an attempt to arrange as far as possible in a linear series those
orders which are characterized by absence or freedom of petals. The
second subclass, Gamopetalae, includes 9 series and culminates
in those which show the most elaborate type of flower, the series
Aggregatae, the chief representative of which is the great and
wide-spread order Compositae. 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. Dr. Engler is the principal editor
of a large series of volumes which, under the title _Die naturlichen
Pflanzenfamilien_, is a systematic account of all the known genera
of plants and represents the work of many botanists. More recently
in _Das Pflanzenreich_ the same author organized a series of complete
monographs of the families of seed-plants.
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_.