GAMETOPHYTE AND SPOROPHYTE OF ANGIOSPERMS.
=650. Male prothallium of angiosperms.=—The first division which
takes place in the nucleus of the pollen grain occurs, in the case of
trillium and many others of the angiosperms, before the pollen grain
is mature. In the case of some specimens of T. grandiflorum in which
the pollen was formed during the month of October of the year before
flowering, the division of the nucleus into two nuclei took place soon
after the formation of the four cells from the mother cell. The nucleus
divided in the young pollen grain is shown in fig. 385. After this
takes place the wall of the pollen grain becomes stouter, and minute
spiny projections are formed.
[Illustration: Fig. 385. Nearly mature pollen grain of trillium. The
smaller cell is the generative cell.]
[Illustration: Fig. 386. Germinating spores (pollen grains) of
peltandra; generative nucleus in one undivided, in other divided to
form the two sperm nuclei; vegetative nucleus in each near the pollen
grain.]
=651.= The larger cell is the vegetative cell of the prothallium,
while the smaller one, since it later forms the sperm cells, is
the generative cell. This generative cell then corresponds to the
central cell of the antheridium, and the vegetative cell perhaps
corresponds to a wall cell of the antheridium. If this is so, then the
male prothallium of angiosperms has become reduced to a very simple
antheridium. The farther growth takes place after fertilization. In
some plants the generative cell divides into the two sperm cells at
the maturity of the pollen grain. In other cases the generative cell
divides in the pollen tube after the germination of the pollen grain.
For study of the pollen tube the pollen may be germinated in a weak
solution of sugar, or on the cut surface of pear fruit, the latter
being kept in a moist chamber to prevent drying the surface.
=652.= In the spring after flowering the pollen escapes from the
anther sacs, and as a result of pollination is brought to rest on
the stigma of the pistil. Here it germinates, as we say, that is, it
develops a long tube which makes its way down through the style, and in
through the micropyle to the embryo sac, where, in accordance with what
takes place in other plants examined, one of the sperm cells unites
with the egg, and fertilization of the egg is the result.
[Illustration: Fig. 387. Section of pistil of trillium, showing
position of ovules (macrosporangia).]
[Illustration: Fig. 388. Mandrake (Podophyllum peltatum).]
=653. Macrospore and embryo sac.=—In trillium the three carpels
are united into one, and in dentaria the two carpels are also united
into one compound pistil. Simple pistils are found in many plants, for
example in the ranunculaceæ, the buttercups, columbine, etc. These
simple pistils bear a greater resemblance to a leaf, the margins of
which are folded around so that they meet and enclose the ovules or
sporangia.
[Illustration: Fig. 389. Young ovule (macrosporangium) of podophyllum.
_n_, nucellus containing the one-celled stage of the macrospore;
_i.int_, inner integument; _o.int_, outer integument.]
=654.= If we cut across the compound pistil of trillium we find
that the infoldings of the three pistils meet to form three partial
partitions which extend nearly to the center, dividing off three
spaces. In these spaces are the ovules which are attached to the
infolded margins. If we make cross-sections of a pistil of the
May-apple (podophyllum) and through the ovules when they are quite
young, we shall find that the ovule has a structure like that shown
in fig. 389. At _m_ is a cell much larger than the surrounding ones.
This is called the macrospore. The tissue surrounding it is called
here the nucellus, but because it contains the macrospore it must be
the macrosporangium. The two coats or integuments of the ovule are
yet short and have not grown out over the end of the nucellus. This
macrospore increases in size, forming first a cavity or sac in the
nucellus, the _embryo sac_. The nucleus divides several times until
eight are formed, four in the micropylar end of the embryo sac and
four in the opposite end. In some plants it has been found that one
nucleus from each group of four moves toward the middle of the embryo
sac. Here they fuse together to form one nucleus, the _endosperm
nucleus_ or _definitive nucleus_ shown in fig. 390. One of the nuclei
at the micropylar end is the egg, while the two smaller ones nearer
the end are the _synergids_. The egg-cell is all that remains of the
archegonium in this reduced prothallium. The three nuclei at the lower
end are the _antipodal_ cells.
[Illustration: Fig. 390. Podophyllum peltatum, ovule containing mature
embryo sac; two synergids, and eggs at left, endosperm nucleus in
center, three antipodal cells at right.]
[Illustration: Fig. 391. Macrospore (one-celled stage) of lilium.]
=655. Embryo sac is the young female prothallium.=—In figs.
391-393 are shown the different stages in the development of the
embryo sac in lilium. The embryo sac at this stage is the young female
prothallium, and the egg is the only remnant of the female sexual
organ, the archegonium, in this reduced gametophyte.
=656. Fertilization.=—When the pollen tube has reached the embryo
sac (paragraph 652) it opens and the two sperm cells are emptied near
the egg. The first sperm nucleus enters the protoplasm surrounding the
egg nucleus and uniting with the latter brings about fertilization. The
second sperm nucleus often unites with the endosperm nucleus (or with
one or both of the “polar nuclei”), bringing about what some call a
second fertilization. Where this takes place in addition to the union
of the first sperm nucleus with the egg nucleus it is called _double
fertilization_. The sperm nucleus is usually smaller than the egg
nucleus, but often grows to near or quite the size of the egg nucleus
before union. See figs. 394 and 395.
[Illustration: Fig. 392. Two-and four-celled stage of embryo sac of
lilium. The middle one shows division of nuclei to form the four-celled
stage. (Easter lily.)]
=657. Fertilization in plants is fundamentally the same as in
animals.=—In all the great groups of plants as represented by
spirogyra, œdogonium, vaucheria, peronospora, ferns, gymnosperms, and
in the angiosperms, fertilization, as we have seen, consists in the
fusion of a male nucleus with a female nucleus. Fertilization, then, in
plants is identical with that which takes place in animals.
=658. Embryo.=—After fertilization the egg develops into a short
row of cells, the _suspensor_ of the embryo. At the free end the embryo
develops. In figs. 397 and 398 is a young embryo of trillium.
=659. Endosperm, the mature female prothallium.=—During the
development of the embryo the endosperm nucleus divides into a great
many nuclei in a mass of protoplasm, and cell walls are formed
separating them into cells. This mass of cells is the _endosperm_,
and it surrounds the embryo. It is the _mature female prothallium_,
belated in its growth in the angiosperms, usually developing only when
fertilization takes place, and its use has been assured.
[Illustration: Fig. 393. Mature embryo sac (young prothallium) of
lilium. _m_, micropylar end; _S_, synergids; _E_, egg; _Pn_, polar
nuclei; _Ant_, antipodals. (Easter lily.)]
[Illustration: Fig. 394. Section through nucellus and upper part of
embryo sac of cotton at time of entrance of pollen tube. _E_, egg; _S_,
synergids; _P_, pollen tube with sperm cell in the end. (Duggar.)]
=660. Seed.=—As the embryo is developing it derives its
nourishment from the endosperm (or in some cases perhaps from the
nucellus). At the same time the integuments increase in extent and
harden as the seed is formed.
[Illustration: Fig. 395. Fertilization of cotton. _pt_, pollen tube;
_Sn_, synergids; _E_, egg, with male and female nucleus fusing.
(Duggar.)]
[Illustration: Fig. 396.
Diagrammatic section of ovary and ovule at time of fertilization in
angiosperm. _f_, funicle of ovule; _n_, nucellus; _m_, micropyle; _b_,
antipodal cells of embryo sac; _e_, endosperm nucleus; _k_, egg-cell
and synergids; _ai_, outer integument of ovule; _ii_, inner integument.
The track of the pollen tube is shown down through the style, walls of
the ovary to the micropylar end of the embryo sac.]
=661. Perisperm.=—In most plants the nucellus is all consumed
in the development of the endosperm, so that only minute fragments
of disorganized cell walls remain next the inner integument. In some
plants, however, (the water-lily family, the pepper family, etc.,)
a portion of the nucellus remains intact in the mature seed. In such
seeds the remaining portion of the nucellus is the _perisperm_.
=662. Presence or absence of endosperm in the seed.=—In many of
the angiosperms all of the endosperm is consumed by the embryo during
its growth in the formation of the seed. This is the case in the rose
family, crucifers, composites, willows, oaks, legumes, etc., as in the
acorn, the bean, pea and others. In some, as in the bean, a large part
of the nutrient substance passing from the endosperm into the embryo is
stored in the cotyledons for use during germination. In other plants
the endosperm is not all consumed by the time the seed is mature.
Examples of this kind are found in the buttercup family, the violet,
lily, palm, jack-in-the-pulpit, etc. Here the remaining endosperm in
the seed is used as food by the embryo during germination.
[Illustration: Fig. 397. Section of one end of ovule of trillium,
showing young embryo in endosperm.]
[Illustration: Fig. 398. Embryo enlarged.]
[Illustration: Fig. 399. Seed of violet, external view, and section.
The section shows the embryo lying in the endosperm.]
[Illustration: Fig. 400. Section of fruit of pepper (Piper nigrum),
showing small embryo lying in a small quantity of whitish endosperm
at one end, the perisperm occupying the larger part of the interior,
surrounded by pericarp.]
=663. Outer parts of the seed.=—While the embryo is forming within the
ovule and the growth of the endosperm is taking place, where this is
formed, other correlated changes occur in the outer parts of the ovule,
and often in adjacent parts of the flower. These unite in making the
“seed,” or the “fruit.” Especially in connection with the formation of
the seed a new growth of the outer coat, or integument, of the ovule
occurs, forming the outer coat of the seed, known as the _testa_, while
the inner integument is absorbed. In some cases the inner integument
of the ovule also forms a new growth, making an inner coat of the seed
(rosaceæ). In still other cases neither of the integuments develops
into a testa, and the embryo sac lies in contact with the wall of
the ovary. Again an additional envelope grows up around the seed; an
example of this is found in the case of the red berries of the “yew”
(taxus), the red outer coat being an extra growth, called an _aril_.
In the willow and the milkweed an aril is developed in the form of
a tuft of hairs. (In the willow it is an outgrowth of the funicle,
= stalk of the ovule, and is called a funicular aril; while in the
milkweed it is an outgrowth of the micropyle, = the open end of the
ovule, and is called a micropylar aril.)
=664. Increase in size during seed formation.=—Accompanying this
extra growth of the different parts of the ovule in the formation of
the seed is an increase in the size, so that the seed is often much
greater in size than the ovule at the time of fertilization. At the
same time parts of the ovary, and in many plants, the adherent parts of
the floral envelopes, as in the apple; or of the receptacle, as in the
strawberry; or in the involucre, as in the acorn; are also stimulated
to additional growth, and assist in making the fruit.
=665. Synopsis of the seed.=
{ {Aril, rarely present.
{ {
{ {Ovular coats (one or two usually
{ { present), the _testa_.
{ {
{ {_Funicle_ (stalk of ovule), _raphe_
{ { (portion of funicle when bent on
{ { to the side of ovule),
{ {_micropyle_, _hilum_ (scar where
{_Ripened ovule._ { seed was attached to ovary).
{ {
{ {_Remnant of the nucellus_ (central
_The seed._ { { part of ovule); sometimes
{ { nucellus remains as
{ {_Perisperm_ in some albuminous
{ seeds.
{_Endosperm_, present in albuminous seeds.
{
{_Embryo_ within surrounded by endosperm when this is
{ present, or by the remnant of nucellus, and by the
{ ovular coats which make the _testa_. In many seeds
{ (example, bean) the endospermis transferred to the
{ cotyledons which become fleshy(exalbuminous seeds).
=666. Parts of the ovule.=—In fig. 401 are represented three
different kinds of ovules, which depend on the position of the ovule
with reference to its stalk. The funicle is the stalk of the ovule,
the hilum is the point of attachment of the ovule with the ovary, the
raphe is the part of the funicle in contact with the ovule in inverted
ovules, the chalaza is the portion of the ovule where the nucellus and
the integuments merge at the base of the ovule, and the micropyle is
the opening at the apex of the ovule where the coats do not meet.
[Illustration: Fig. 401.
_A_, represents a straight (orthotropous) ovule of polygonum; _B_, the
inverted (anatropous) ovule of the lily; and _C_, the right-angled
(campylotropous) ovule of the bean. _f_, funicle; _c_, chalaza;
_k_, nucellus; _ai_, outer integument; _ii_, inner integument; _m_,
micropyle; _em_, embryo sac.]
Comparison of Organ and Member.
=667. The stamens and pistils are not the sexual organs.=—Before
the sexual organs and sexual processes in plants were properly
understood it was customary for botanists to speak of the stamens and
pistils of flowering plants as the sexual organs. Some of the early
botanists, a century ago, found that in many plants the seed would not
form unless first the pollen from the stamens came to be deposited on
the stigma of the pistil. A little further study showed that the pollen
germinated on the stigma and formed a tube which made its way down
through the pistil and into the ovule.
This process, including the deposition of the pollen on the stigma,
was supposed to be fertilization, the stamen was looked on as the male
sexual organ, and the pistil as the female sexual organ. We have found
out, however, by further study, and especially by a comparison of the
flowering plants and the lower plants, that the stamens and pistils are
not the sexual organs of the flower.
=668. The stamens and pistils are spore-bearing leaves.=—The
stamen is the spore-bearing leaf, and the pollen grains are not
unlike spores; in fact they are the small spores of the angiosperms.
The pistil is also a spore-bearing leaf, the ovule the sporangium,
which contains the large spore called an _embryo sac_. In the ferns
we know that the spore germinates and produces the green heart-shaped
prothallium. The prothallium bears the sexual organs. Now the fern leaf
bears the spores and the spore forms the prothallium. So it is in the
flowering plants. The stamen bears the small spores—pollen grains—and
the pollen grain forms the prothallium. The prothallium in turn forms
the sexual organs. The process is in general the same as it is in the
ferns, but with this special difference: the prothallium and the sexual
organ of the flowering plants are very much reduced.
=669. Difference between organ and member.=—While it is not
strictly correct then to say that the stamen is a sexual organ, or male
organ, we might regard it as a _male member_ of the flower, and we
should distinguish between _organ_ and _member_. It is an _organ_ when
we consider _pollen production_, but it is not a sexual organ. When we
consider _fertilization_ it is _not a sexual organ, but a male member_
of the flower which bears the small spore.
The following table will serve to indicate these relations.
Stamen = spore-bearing leaf = male member of flower.
Anther locule = sporangium.
Pollen grain = small spore = reduced male prothallium and
sexual organ.
So the pistil is not a sexual organ, but might be regarded as the
female member of the flower.
Pistil = spore-bearing leaf = female member of flower.
Ovule = sporangium.
Embryo sac = large spore = female prothallium containing the egg.
The egg = a reduced archegonium = the female sexual organ.
Progression and Retrogression in Sporophyte and Gametophyte.
=670. Sporophyte is prominent and highly developed.=—In the
angiosperms then, as we have seen from the plants already studied, the
trillium, dentaria, etc., are sporophytes, that is they represent the
spore-bearing, or sporophytic, stage. Just as we found in the case of
the gymnosperms and ferns, this stage is the prominent one, and the
one by which we characterize and recognize the plant. We see also that
the plants of this group are still more highly specialized and complex
than the gymnosperms, just as they were more specialized and complex
than the members of the fern group. From the very simple condition
in which we possibly find the sporophyte in some of the algæ like
spirogyra, vaucheria, and coleochæte, there has been a gradual increase
in size, specialization of parts, and complexity of structure through
the bryophytes, pteridophytes, and gymnosperms, up to the highest types
of plant structure found in the angiosperms. Not only do we find that
these changes have taken place, but we see that, from a condition of
complete dependence of the spore-bearing stage on the sexual stage
(gametophyte), as we find it in the liverworts and mosses, it first
becomes free from the gametophyte in the members of the fern group, and
is here able to lead an independent existence. The sporophyte, then,
might be regarded as the modern phase of plant life, since it is that
which has become and remains the prominent one in later times.
=671. The gametophyte once prominent has become degenerate.=—On
the other hand we can see that just as remarkable changes have come
upon the other phase of plant life, the sexual stage, or gametophyte.
There is reason to believe that the gametophyte was the stage of plant
life which in early times existed almost to the exclusion of the
sporophyte, since the characteristic thallus of the algæ is better
adapted to an aquatic life than is the spore-bearing state of plants.
At least, we now find in the plants of this group as well as in the
liverworts, that the gametophyte is the prominent stage. When we reach
the members of the fern group, and the sporophyte becomes independent,
we find that the gametophyte is decreasing in size, in the higher
members of the pteridophytes, the male prothallium consisting of only a
few cells, while the female prothallium completes its development still
within the spore wall. And in selaginella it is entirely dependent on
the sporophyte for nourishment.
=672.= As we pass through the gymnosperms we find that the
condition of things which existed in the bryophytes has been reversed,
and the gametophyte is now entirely dependent on the sporophyte for
its nourishment, the female prothallium not even becoming free from
the sporangium, which remains attached to the sporophyte, while the
remnant of a male prothallium, during the stage of its growth, receives
nourishment from the tissues of the nucellus through which it bores its
way to the egg-cell.
=673.= In the angiosperms this gradual degradation of the male
and female prothallia has reached a climax in a one-celled male
prothallium with two sperm cells, and in the embryo sac with no clearly
recognizable traces of an archegonium to identify it as a female
prothallium. The development of the endosperm subsequent, in most
cases, to fertilization, providing nourishment for the sporophytic
embryo at one stage or another, is believed to be the last remnant of
the female prothallium in plants.
=674. The seed.=—The seed is the only important character
possessed by the higher plants (the gymnosperms and angiosperms) which
is not possessed by one or another of the lower great groups. With the
gradual evolution of the higher plants from the lower there has been
developed at certain periods organs or structural characters which
were not present in some of the lower groups. Thus the thallus is the
plant body of the algæ and fungi, so that these two groups of plants
are sometimes called _Thallophytes_. In the Bryophytes (liverworts and
mosses) the thallus is still present, but there is added the highly
organized archegonium in place of the simple female gamete or oogonium,
or carpogonium of the algæ and fungi, and the sporophyte has become
a distinct though still dependent structure. In the Pteridophytes
the thallus is still present as the prothallium, archegoina are also
present, and while both of these structures are retrograding the
sporophyte has become independent and has organized for the first time
a true vascular system for conduction of water and food. In the
gymnosperms and angiosperms the thallus is present in the endosperm;
distinct, though reduced, archegonia are present in most gymnosperms
and represented only by the egg in the angiosperms; the vascular system
is still more highly developed while the seed for the first time is
organized, and characterizes these plants so that they are called seed
plants, or _Spermatophytes_.
Variation, Hybridization, Mutation.
=674a. Variation.=—It is a well-known fact that plants as well
as animals are subject to variation. Under certain conditions, some
of which are partly understood and others are unknown, the progeny of
plants differ in one or more characters from their parents. Some of
these variations are believed to be due to the influence of environment
(see Parts III and IV). Others are the result of the crossing of
individuals which show greater or lesser differences in one or more
characters, or the crossing of different species (_hybridization_). The
most profound variations are those which spring suddenly into existence
(_mutation_).
=674b. Hybridization.=—Two different species are “crossed” where
the egg-cell of one species is fertilized by the sperm of another
species. The progeny resulting from such a cross is a _hybrid_. Hybrids
sometimes resemble one parent, sometimes another, sometimes both. Where
the parents differ only in respect to one character of an organ or
structure, there is a regular law in respect to the progeny if they are
self-fertilized. In the first generation all the individuals are alike
and resemble one of the parents, and the special differential character
of that parent is called the _dominant_ character. In the second
generation 75% possess the dominant character, while 25% resemble
the other original parent, and its differential character is called
_recessive_. These are _pure_ recessives, since successive generations,
if self-fertilized, are always recessive. Of the 75% which show the
dominant character in the second generation, one-third (or 25% of the
whole number) are pure dominants if self-fertilization is continued,
while 50% are really “cross breds” like the first generation, and
if self-fertilized split up again into approximately 25 dominants,
50 cross breds, and 25 recessives. This is what is called Mendel’s
law. Where the original parents differ in respect to more than one
character, the result is more complicated (see Mendel’s Principles of
Heredity; also de Vries, Das Spaltungsgesetz der Bastarde, Ber. d.
deutsch. bot. Gesell., 18, 83, 1900).
=674c. Mutation.=—This term is applied to those variations which
appear so suddenly that some of the progeny of two like individuals
differ from all the others to a marked degree. Some of these mutations
are so different as to be regarded as new species. Some of the
primroses show mutations, and Œnothera gigas is a mutation from Œnothera
lamarkiana (see de Vries, Die Mutationstheorie, Leipzig).
=675.= TABLE SHOWING HOMOLOGIES OF SPOROPHYTE AND GAMETOPHYTE IN
ANGIOSPERMS.
TERMS CORRESPONDING TO THOSE USED IN PTERIDOPHYTES. COMMON TERMS.
Sporophyte { = Higher plant.
Spore-bearing part { = Stamens and
{ carpels.
--------------------------------------------------------------------
{ {Anther.
Sporophyte {Microsporophyll = Stamen {Filament.
{
{Microsporangium = Pollen sac,
{ usually
{ two or four.
--------------------------------------------------------------------
{Microspore at maturity = Pollen grain.
{ usually of 2 or 3 cells
{ {young male prothallium}
{
{1. Large cell (part of = Vegetative cell.
{ antheridium wall?), with
{ its nucleus surrounded
{ by wall of spore
Male gametophyte {2. Small cell with nucleus, = Generative cell.
{ no wall, floating in
{ protoplasm of large cell
{ is the central cell of
{ antheridium
{ (male sexual organ)
{Mature male prothallium = Pollen grain
{ with tube.
{Antheridium cell divided, = Paternal cells,
{ 2 sperm cells or generative
{ cells.
--------------------------------------------------------------------
{ {Stigma.
{Macrosporophyll = {Carpel or {Style.
Sporophyte { {simple pistil {Ovary.
{
{
{Macrosporangium, covered = {Nucellus, covered
{ by 1 or 2 coats { by 1 or 2
{ { coats = ovule.
--------------------------------------------------------------------
{Macrospore, cell in end of = Uninuclear state
{ macrosporangium, does of embryo sac.
{ not become free, cavity
{ enlarges
{Macrospore divides into
{ 8 cells to form young
{ female prothallium = Embryo sac.
{
Female gametophyte. {Remnant of archegonium, = Maternal cell,
{ egg (female sexual organ) or germ cell.
{Growing part of prothallium = {Two polar nuclei
{ { fused, making
{ { endosperm
{ { nucleus.
{Mature female prothallium = {Endosperm,
{ { developed by
{ { many divisions
{ { of endosperm
{ { nucleus.
--------------------------------------------------------------------
Young sporophyte {After fecundation of egg, egg divides
surrounded by { to form embryo. Embryo in endosperm
parts of the { (sometimes latter nearly or quite
gametophyte and { absent) surrounded by coats = Seed.
new growth of old{
sporophyte {
Young sporophyte surrounded by remnants of gametophyte and new
parts of old sporophyte (remains of endosperm and of nucellus,
and ovular coat) = the seed.