Sydney Howard Vines.

A student's text-book of botany online

. (page 44 of 83)
Online LibrarySydney Howard VinesA student's text-book of botany → online text (page 44 of 83)
Font size
QR-code for this ebook

the absorption of more or less of the tissue of the nucellus, more
especially towards the micropylar end. It commonly attains
such a size that little or none of the nucellar tissue remains, and
it may even project beyond the micropyle (e.g. Santalum, Torenia
asiaticaf Fig. 285) ; and in many gamopetalous Dicotyledons it

Digitized by



developes tnbalar outgpx>wths, which penetrate into and destroy
the tissae of the integument (e.g, Bhinanthns, LathrsBa, some
LabiataB). In some cases, however (e.g. Grjmnosperms, Scitamineae,
most Njmph8Bace», Piper), the macrospore does not grow to such
an extent, so that a considerable mass of nucellar tissue is left,
which persists to some extent in the seed as perisperm, its cells
being then filled with nutritive substances. This may be due, as
in the Gymnosperms, to the fact that the macrospore is covered,
towards the micropyle, by a mass of nucellar tissue formed by
the growth and repeated division, both periclinal and anticlinal,
of either the tapetal cell, or of the apical epidermal cells of the
ovule, or of both ; or, as in the other cases, to the fact that the
macrospore, in its growth, which is relatively slight, does not
absorb the chalazal portion of the nucellar tissue.

General Histology. The following are the principal characteris-
tic features : — The apical growth of shoot and root is only excep-
tionally effected by means of a single apical cell : the small-celled
moristem of the growing-point of the stem is more or less distinctly
differentiated into dermatogen. periblem, and plerome, so that the
stem has a true epidermis : the epiblema of the root is either the
persistent innermost layer of the original many-layered endodermis
(most Dicotyledons, Gymnosperms), or it is the external layer of
the cortex (Monocotyledons ; see p. 154) : stem and root are mono-
stelic, with but few exceptions (p. 152) : the vascular bundles of
the stem are generally collateral : both root and stem generally
present secondary growth in thickness (except Monocotyledons,
and a few other cases) by means oE a normal cambium-ring (for
abnormal cases, see p. 204) : the growing-points of the lateral roots
are developed from the pericycle of the parent root (see p. 186).

The Emhryogeny of the Sporophyte. The sporophy te is developed
from the fertilised oosphere in the ovule. The development of the
embryo is not continuous, but is in two stages, which may be con-
veniently distinguished as the intra-seminal and the extra-'seminal.
The intra-seminal stage includes the whole of the development
which the embryo undergoes during the conversion of the ovule
into the ripe seed — that is, during what is known as the '* ripening
of the seed.'' The extra-seminal stage includes the development
of the embryo which follows the sowing of the seed ; — that is, the
escape of the embryo from the seed, and the gradual development
of the characters of the adult plant. The interval between these
two stages may be brief, or it may extend over many years if the

Digitized by



seed be kept dry. The ** germination " of the seed when sown is
simply the resumption of development by the embryo in conse-
quence of exposure to the necessary conditions of moisture,
warmth, etc.

In most Phanerogams, each oospore gives rise to a single em-
bryo ; but in most -Gymnosperms each oospore gives rise to more
than one embryo (four or many), thus exhibiiing polyembryony .

The primary development of the embryo is either holoblastic or
meroblastic (see p. 13) ; meroblastic embryogeny is common among

In some exceptional cases (Cycads, Ginkgo, Ephedra) the
embryogeny begins with free cell-formation in the oospore (see p.

Generally speaking, the oospore of holoblastic plants divides
into two by a transverse wall : the upper of the two cells remains
coherent to the micropylar end of the embryo-sac and developes
into the suspensor, an embryonic orgfan which is a characteristic
feature of the embryogeny of Phanerogams, which bears at its
lower end the other cell, termed the emhryO'Cell, from which the
whole or a considerable part of the body of the embryo is de-
veloped. In meroblastic plants, the suspensorial cell and the
embryo-cell are developed in a somewhat similar though more
complicated manner, from the embryogenic portion of the oospore
(see Gymnosperms, p. 71).

It is in comparatively few plants that the suspensor contributes
nothing to the development of the permanent members of the
embryo. This is necessarily the case in those plants (enumerated
below) in which no suspensor is developed ; it is also the case
in some plants in which a suspensor is present {e.g. plants
with massive suspensors, such as Geranium, Tropwolum, many
Grasses; also most LegnminossB). Here the embryo is de-
veloped entirely from the embryo-cell. In some cases (Vicie»,
Conifera&) the embryo-cell, on the other hand, contributes to the
elongation of the suspensor. In many cases, however, the lowest
cell of the suspensor contributes in part (e.g. Capsella, Fig. 286) or
entirely (e.g, Alisma, Fig. 287) to the construction of the embryo.

The suspensor may be regarded, in most cases, simply as a
temporary organ of the embryo : but it occasionally presents such
a degree of independence of growth, that it assumes the character
rather of a proembryonic organism, making the embryogeny
heteroblastic, than of a mere organ (see p. 14).

Digitized by



The following are noteworthy peculiarities in the naorphology and physiology
of the suspensor. It is generally a filament consisting of a longer or shorter
single row of cylindrical cells, sharply defined from the rest of the embryo : in
some oases it consists of several such rows {e,g, Glaucinm, Viciea?) : in others
it is massive (see above), consisting of a number of cells covering the posterior
end of the embryo, and not sharply defined from it: sometimes it consists
of a single cell {e.g. Funkia) : in some plants {e.g. Vieieie) the segments of
the suspensor are coenocytic. Rarely, it is diflferentiated at a relatively late
stage of embryogenj {e.g. Cytisui Laburnum^ and some other LeguminossQ). Its
common function is, by its growth, to force the embryo into the nutritive tissue
of the seed, and it is usually attached by its upper end to the mioropylar end
of the embryo-sac : but it is not unfreguently adapted, more particularly when
the embryo-sac contains little or no store of nutriment, as an organ of ab-
sorption. Thus in some Orchids {e.g^ Anacamptis pyramidalis, Platanthera
bifolia. Orchis latifolia)^ the filamentous suspensor grows through the wall of the
embryo-sac, and out at the micropyle, reaching the wall of the ovary where it
buries itself in the tissue of the placenta, from the cells of which it absorbs
nutriment for the embryo attached to its other end in the embryo-sac. Again,
in other Orchids (PhalaBuopsis, Vanda), the primitive suspensorial cell divides
longitudinally into six cells which grow out into long filaments, both upwards
and downwards, enveloping the embryo but not leaving the ovule, which act as
absorbent organs. In Tropseolum, the suspensor produces two lateral branches,
one of which bores through the wall of the ovule into the cavity of the ovary,
acting as an anchor for the embiyo ; the other penetrates the wall of the ovule,
where it is in contact with the placenta, and, entering the placental tissue, acts
as an absorbent organ. In Gnetum the suspensor branches and bears an
embryo at the end of each branch. When the suspensor is massive, it is itself
a depository of nutrient substances for the use of the embryo.

No suspensor is developed in the following plants : Pistia Stratiotfi, Liitera
ovata, Bpipactis paluttrit and latifoliaf Cypripedium speetabile, among Mono-
cotyledons; CorydalU cava, and certain Leguminous plants, such as the
MimosesB and some Hedysarese, among Dicotyledons ; Ginkgo, among Gymno-

In those plants which have no saspensor, the development of
the embryo from the oospore is simple. The oospore divides
bj a transverse (basal) wall into two; then by a longitudinal
wall into fonr; and then by a second longitudinal wall, at
right angles to both the preceding, into eight cells, octant-s of a
sphere : generally speaking, from the half of the oospore next
the micropyle the primary root is developed, from the other half
the growing-point of the primary stem and the (one or two)
primary leaves or cotyledons. The early stages of the embryogeny
are essentially the same in those plants in which, though a sus-
pensor is present, it does not contribute to the structure of the
embryo, though here it is the embryo-cell that divides into octants.

Digitized by




In those plants which have a saspensor which contnbutes to
the embryo, the embryo. cell is not a complete sphere. It divides,


*^ ^d




FiQ. 396. — Vmbryogeny of Dicotyledons as represented by Cap$ella Bursa-Pastoria (dia-
grammatic, after Goebel and Honstein). A-D Sacoessive stages: stttp. snspensor; tmh,
embryo ; 1-1, 2-2, octant>walls ; a lowest cell of saspensor, dividing in B to form the hypo-
physial cell h ; in C the hypophysial cell has divided into two, h^ and h,, the former cod-
stitating the periblem, the latter the dermatogen, of the growing-poiut of the primary
root ; in D, H, has ondergone a periclinal division to form the primitive root-cap : dderma-
togen ; c periblem ; pi. plerome ; cot. cotyledons, between which lies the growing-point of
the primary stem.

as a rale, into two by a longitudinal wall, then transversely, and
then in a plane to both the preceding, into octants ; but while

Digitized by




the four anterior octants are octants of a sphere, this is not the
case with the four truncated posterior octants abutting on the
suspensor. In some cases, the transverse division precedes the
longitudinal. From the anterior octants are developed, in Dicotyle-
dons generally (Fig. 286), the two cotyledons and the growing-


Fio. 287.-'Embryogeny of Monocutyledon^, as represented by A\wn.a VXaxdaqo (diaf^rain-
malic, after Qoebel, Hanstein, and Famintzin). A-Q Successive stages : a embryo>cell ;
b lowest cell of snspensor, siwp. : (he products of the repeated transverse division of b are
Indicated (c, d, <• /) in D and C. In C, a has given rise to the single terminal cotyledon ;
c to the growing-point of the primary stem ; d and e form the hypocotyl ; the growing-
. point of the root is developed from/; ep dermatogen. P is a mature embryo, less highly
magnified : cot, cotyledon ; nt. growing-point of stem ; H.vp. hypocotyl. The nuclei of
the cells are indicated in A and B.

point of the primary stem, bat the growing-point of the primary
root is supplied from the last cell of the suspensor (Fig. 286
A^ a) which divides ti'ansversely into two (Fig. 286 B) and con-
tributes the cell ^, the hypophysis, to complete the root- end of the

Digitized by



embrjo. In MonoootyledonB, on the other hand, the erabrjo-cell
gives rise, as a rale (Fig. 287 A and C, a), only to the single ter-
minal cotyledon ; whilst the last cell of the suspensor (Fig. 287
-^» ^) gives rise to the growing-point of the stem, which is here
lateral (Fig. 287 0, c; A «0» ^^^ to that of the root by a hypo-
physial cell (J).

In two cafles only {Cephalotaxus Fortunei, Araucaria brasilianat both Gymno-
sperms) are the cotyledons and the growing-point of the primary stem
developed endogenously : here they are at first covered by some cells at the
apex of the embryo, which are eventually thrown off.

Id a few exceptional Monocotyledons {e.g. Dioscoreaoeie, Commelynaceffi) the
growing-point of the primary stem is developed, not laterally, but apically,
and the cotyledon is lateral. In some Dicotyledons (e,g. Camm BuVbocatianumy
Hananeulut Ficaria) the embryo is pseudo-monocotyledonout ; that is, only one
cotyledon is developed though two are originally indicated.

In the Gymnosperms, the number of cotyledons varies from one of fifteen.

With regard to the histological differentiation of the embryo,
the first step, after the division into octants, is the formation of
periclinal walls marking off a superficial layer, which is the
dermatogen (Figs. 286, 287) ; this differentiation proceeds from
the anterior end, or apex, backwards towards the posterior end of
the embryo. In those plants in which the root-end of the embryo
is formed by a hypophysial cell contributed by the suspensor
(Fig. 286 2?, h), the dermatogen- layer is completed by the peri-
clinal division of the hypophysial cell, the inner cell forming the
periblem of the growing-point, the out«r forming the dermatogen
which undergoes further periclinal division to form the primitive
root-cap. In the meantime, anticlinal and longitudinal walls
have also been formed, so that the embryo, as it increases in size,
consists of an increasing number of cells. The degree of histo-
logical differentiation attained varies widely : in the highest forms
(Fig. 286 D) a cylinder of plerome is differentiated in the axis
of the embryo, so that the three primary tissue- systems, der-
matogen, periblem, and plerome, are clearly defined.

The degree of morphological differentiation attained by the
embryo in its intra-seminal development also varies widely, as
does also the size of the embryo. In the ripe seed of most Orchids
and parasitic plants {e.g. Orobanche, Monotropa, etc.), the body of
the embryo presents no differentiation into members. In most
plants, the embryo, in the ripe seed, consists of the following
members: (a) one, two, or several cotyledons; (&) a primary

Digitized by



stem beanng the cotyledon or cotyledons, but not projecting be-
yond them, termed the hypocotyl, passing posteriorly into (c) the
primary root or radicle. In some plants {e.g. Triticum and other
Grasses, Phaseolns, Vicia, Amygdalus, etc.) the primary stem has
elongated beyond the insertion of the cotyledon or cotyledons,
and bears the rudiments of fatnre foliage-leaves : this portion of
the primary shoot is termed the plumule or epicotyl.

The size and texture of the cotyledons vary with the functions
which they have to perform. When, as in exalbuminous seeds,
such as peas and beans, the cotyledons are themselves the store-
houses in which food is deposited for the nutrition of the embryo
during its extra-seminal development, they are relatively large,
thick, and fleshy ; but when, as in albuminous seeds {e.g. Ricinus,
Grasses, etc.), the food is stored in the endosperm, the cotyledons
are absorbent organs and, though still relatively large, are not
thick and fleshy.

In a few Phanerogams (e.g. Utricularia, which never deyelopes
any root, Buppia rostellata, Wolffia arrhiza) no primary root is
developed or even indicated.

The extra-seminal development of the embryo may be briefly
stated as follows : — The first event is the elongation of the hypo-
cotyl, with the result that the radicle passes, through the micropyle,
out of the seed into the soil, where it becomes firmly attached :
where (as in some Gymnosperms, Grasses, Tropceolum) the growing,
point of the root is developed deep in the tissue of the embryo,
the radicle, before it can escape from the seed, has to penetrate
this more or less considerable mass of tissue which can be seen,
on examining the germinated seed, as a collar, termed the
coleorhiza, surrounding the base of the radicle. The other mem-
bers then escape from the seed, the coat of which becomes more
or less split. In those cases in which the growth of the hypocotyl
is active, the cotyledons appear above the surface of the soil, that
is, they are epigean {e.g. Cucurbita, Ricinus, Radish, Sunflower,
Scarlet Runner, etc., most Gymnosperms), either leaving the seed-
coat in the soil, or carrying it up to the surface. In those cases in
which the growth of the hypocotyl is comparatively slight, the
cotyledons do not reach the surface of the soil, that is, they are
hypogean {e.g. Vicia Faha, Pea, Grasses, etc.) : here it is the
epicotyl (plumule) which grows rapidly, and is the first member
to appear above ground. The part which first appears above
ground, whether it be hypocotyl, epicotyl, or cotyledon, usually

Digitized by



does 80 in the form of an arch, so that the apex is not exposed
to injury whilst the member is forcing its way np through the

Epigean cotyledons become green in colour, and in many cases
(e.g. Sunflower, Radish) assume the appearance, and discharge
the functions, of foliage-leaves; but they do not ever precisely re-
semble, either in size or form, the true foliage- leaves of the plant
to which they belong.

Vegetative Propagation is common among Phanerogams, by





Fio. 288.— Germination of pollen-Rrain of Ulium UaHagon (x760: after Gnignard). A
Young pollen-grain: c centrospherea; n resting nucleuf. B Commencement of germina-
tion : n the dividing nudeua ; o centrospheres. C Cell-formation has taken place, result-
ing in the formation of the generative cell g; n nacleus of remainder of pollen-grain (*'.«.
vegetative cell).

means of bulbs {e.g, Lily, Onion, and many other Monocotyledons),
tubers (Potato), tuberous roots (Dahlia), etc.

B. The Gambtophytb. As all Phanerogams are heterosporous,
the sexual generation is represented by two individuals, a male

V. s. B. GO

Digitized by



and a female, developed respectivel j from the microspore and the
macros pore.

The Male Prothallium is, in all cases, filamentous and relatively
small, consisting of but few cells. The first indication of its
development is the division of the nnclens of the mici'ospore,
which may take place even before the microspore escapes from
the microsporanginm, and this is followed by cell- formation. In
the Angiosperms (Fig. 288) the cell-formation is simple, consisting
in the aggp:*egation of protoplasm round one of the two nuclei, with-

Fio. 289.— DeTolopment of male prothalUum from the pollen-grain of the Tew : A early
Btage. B Later stage: tt atalk-cell; an antheridial cell; n nucleua of the large vegetative
cell which ha« grown out into the pollen-tube. In B the geiierative cell is travelling down
the pollen-tube as a preliminary to fertilisatton. ( x SfiO : after Belajeff.)

out any formation of cell- wall, so that a small primordial cell,
the generative cell, is formed, floating freely in the protoplasm
of the microspore which, with the other nucleus, constitutes
the vegetative cell. In the Gymnosperms the process is rather
more complicated. In the simplest case {e,g. the Yew, Fig
289) the microspore divides into two cells, separated by a cell-
wall ; of these the one, the antheridial cell, undergoes division
into two, a stalk-cell (st) and a generative cell (an); whilst the
other remains as an undivided vegetative cell. In some cases,

Digitized by




however (e.g. Larch, Ginkgo, Fir, Ephedra), generally three cells
are successively cut off by parallel septa (Fig. 290) : of these,
the two first formed are merely vegetative prothallial ceils, and
undergo disorganisation, whilst the last is the antheridial cell, and
undergoes division into a generative cell and a stalk-ceil. In
some cases (Ginkgo, Ephedra, Cycads) the second prothallial cell
persists ; and it appears that in the Gnetacete the antheridial cell
does not divide to form a stalk-cell bat is actually the generative

In both Angiosperms and Gymnosperms, the pollen-tube is
formed by
the o u t-
growth of
the large
cell : in both
cases the
cell (after
being set
free when
enters the ^.

with the ve-
getative nu-
cleus, and,
in Gymno-
with the nu-
cleus of the
stalk - cell ;
the vegeta-

becomes disorganised (Fig. 2fc9 By n), whilst the generative cell
undergoes division into two; either into two equal generative
cells, as is generally the case, or into two unequal cells only one
of which is generative (e.g. Taxus). More than one pollen-tube
may be developed from the microspore (Fig. 283).

Thus the male individual in the Phanerogams is a prothallium

Fio. 280.— Development of the male pi 1

PolleD-gi-ain in which cell- division is proceeding whilst still in the
pollen-sac ; n nacleas : pr three prothallial cells, the innermost of
which is the antheridial cell. B Older pollen-grain developing a
pollen 'tube; pr prothallial cells; n nucleus of pollen>tnbe. (x 510:
after Strasburger.)

Digitized by




consisting of but few cells, and the antheridinm is at most two-
celled : the generative cell represents a spermatozoid-mother-cell,
the protoplasm of Avhich is not, however, differentiated into a
apermatozoid, but simply constitutes a male cell.

The male cell is a small nucleated primordial cell in the pollen -
tube, and is either the original generative cell itself, or a product
of ifcs division. It is eventually extruded through the apex of
the pollen-tube.

The Female Prothallium is developed in the interior of the
macrospore (embryo-sac) in a similar manner to that of the
heterosporous Pteridophyta : but, in the Phanerogams it does not
at any period project fi-om the macrospore as it does in the
Pteridophyta, though this occurs exceptionally in the Cycadaceae
among Gymnosperms, and in Avicennia among Angiosperms.

The development of the prothal-
lium (or endosperm) is simple in the
Gymnosperms. The nucleus of the
maci'ospore divides; repeated nu-
clear division takes place, until a
large number of nuclei are formed
which lie in the protoplasm round
the wall of the macrospore ; between
these nuclei cell-Avalls are developed,
so that a cellular tissue is produced,
the cells of Avhich grow and multi-
ply by division until the cavity of
the macrospore is entirely filled with
this tissue which constitutes the
prothallium. In Guetum, however,
the development of the prothallium
is not completed until fertilisation
has taken place.

In the Angiosperms the develop-
ment of the prothallium is more
complicated in that it generally
takes place in two stages, the one
preceding, the other following, fer-
tilisation. The nucleus of the ma-
crospore divides into two : of these
the one travels to the micropylar pole, the other to the chalazal
pole, of the macrospore ; each nucleus then divides, and each of


Pig. 291.— The female prothallinm
of Gymnosperma, shown in a long^iia>
dinal section of the ovnle ( x about
15 ; diagrrammatic) : u integument ;
m micropyle. K Niicellus (macrospo-
rangium). B Bmbryo-sac (macro-
spore); e female prothallium (endo-
sperm), in which are situated, towards
the micropyle, two archegonia, c,
with neck H; ps pollen-tube entering
the neck of the left archegonium;
p pollen-grain seated on the apex of
the nucellus.

Digitized by





the four so formed divides again, so that eight nuclei are formed,
four at the micropylar, and fonr at the chalazal pole of the
macrospore ; one nneleas is then conveyed from each pole to-
ward the centre of the macrospore, where the two nuclei meet
and fuse into one which is termed the definitive nucleic of the
macrospore or embryo-sac. Three nuclei now lie at each pole,
and around these aggregation of protoplasm takes place, so that
cells are formed : those at the chalazal pole soon acquire a cell-
wall, and are termed antipodal
cells: those at the micropylar
end do not form any cell- wall ;
one of them is the female re-
productive cell or oosphere,
the other two are sterile
(though in rare cases they
are fertile), and are termed
the syfiergidce, the three to-
gether constituting the egg-
apparatus. This is the extent
to which the development of
the female prothallium takes
place previously to fertilisa-
tion. In most Angiosperms
the structure of the prothal-
lium is completed by the for-
mation, after fertilisation has
taken place, of additional cel-
lular tissue : this process is

Online LibrarySydney Howard VinesA student's text-book of botany → online text (page 44 of 83)