W. T. (William Thompson) Sedgwick.

An introduction to general biology online

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so drawn that the uninjured side lies behind. The broad posterior surface of S^
is seen covered with sieve-plates connecting with another sieve-tube. S^ on the
contrary, abuts by a smooth non-plated surface upon parenchymatous cells
which are seen through it. w, sections of walls bearing sieve-pits ; x, section of
a non-plated wall abutting upon parenchyma.

stem and branches. They arise endogenously from the main
stem or its branches, i.e., by an outgrowth of the internal tissues,
and not (as in the case of the false roots or rhizoids of the pro-
thallium, shortly to be described) by elongation .of superficial
cells of the epidermis. True roots, of which those of Pteris are
good examples, arise always as well from the fundamental and
fibro-vascular regions, and include all the systems found in the
stem itself. Hence cross- sections of Pteris roots differ but
slightly from those of the stem or the branches, and the root in
general is clearly a member of the plant body. As in all true
roots, the free end is covered by a special boring tip called the



root-cap., but this is apt to be lost in removing the specimen
from the earth.

The Embryonic Tissue or Meristem of the Rhizome. The
mature rhizome remains at the tip nearly undiiterentiated into
tissues. At tliis point the epidermis may ])e distinguished, but
it remains very delicate, and thu underlying cells continue to
grow and multiply, producing continued elongation of the mass.
In this way the apical bud is formed. Lateral buds are given
off right and left to constitute the embryos of leaves, branches,
or roots, wliich, always retaining their soft and delicate tips, are
capable of further growth.

Behind these ''growing points" the epidermis and otlier
tissues grow more and more slowly, and soon reacli their maxi-
mum size, whereupon rapid growth ceases. The power of
growth is henceforward mainly conlined to the apical buds, and
the growing tissue of which they are composed is known as em-
hryotiic {issue or meristem.

The Apical Cell of the Rhizome. Close examination reveals
the fact that each apical bud contains a remarkable cell which is
especially concerned in the function of growth, viz., the apical
cell.) which lies in a hollow at the apex of the bud. In the
apical buds of the rliizome or branches this cell has somewhat the



Fig. 55a. (After Hofmeister.)— Apical cell
of the rhizome in a vertical longitudinal
section, a.c^ apical cell ; /», hair ; m, meri-

Fig. 55n. (After Hofmeister.) —
Apical cell of tlie rliizome in hori-
zontal longitudinal section, a.c^
apical cell.

form of a wedge with its base turned forwards and its thin edge
backwards, the latter placed at right angles to a plane passing
through the lateral ridges. It continually increases in size, but
as it grows repeatedly divides so as to cut oif cells laterally



alternately on its riglit and left sides. These cells in turn con-
tinue to grow and divide, and thus give rise to two similar masses
of meristem, which together constitute the apical bud. From
the meristem by gradual, though rapid, changes the various tis-
sues of the adult rhizome are differentiated ; and longitudinal
sections passing through the lateral ridges show the mature
tissues fading out in a region of indifferent meristem about the
apical cell (Fig. 55b).

The apical cell lies at the bottom of a funnel-shaped depression at the
tip of the stem. It is shaped approximately like a thin, two-edged wedge
w'ith an arched or curved base turned forwards towards the centre of the
funnel-shaped depression. The thin edge of the wedge is directed back-
wards, and its sides, which are also curved, meet in a vertical plane above
and below. A longitudinal section taken through the plane of the lateral


Fig. 56. (After Sachs.)— A vertical transverse section through the apical cell, a.Ct
showing a boundary of hairs and a second apical cell, I, belonging to a leaf.

ridges therefore shows the apical cell in a triangular form as in Fig. 55b.
A section taken at right angles to this — i.e., vertical and longitudinal —
shows the cell to be approximately rectangular and quadrilateral (Fig.
55a), while a transverse vertical section shows it in the form of a bi-convex
lens (Fig. 56).

The funnel-shaped depression is compressed vertically, and its w^alls are
thickly covered with erect branching hairs, w^hich are closely fastened


Fig. 57.— Cross-section of an entire fertile leaflet, m.r, midrib ; v, veins ; ep, epi-
dermis ; ms, mesophyll ; sp, sporangia ; in, indusium.

together by a hardened mucilage secreted by the apical bud. These hairs
entirely close the mouth of the funnel and shut off the delicate young



portions at its base from the|Outer world. Protected by these hairs, the
end of the stem forces its wayfthrough the toughest clay without injury to
the delicate bud buried in its apex. (Hofmeister.)

Fig. 58.— Cross-section, still more enlarged, passing through the midrib of a leaflet.
In the centre the circular fibro-vascular bundle, supported, especially above and
below, by thickened prosenchyma ( p). On either side the parenchymatous, mes-
ophyll cells (shaded) and the intercellular spaces (j.j.) opening by stomata (st)',
epidermis (ep).

The Aerial Part of the Brake. The Frond or Leaf.

The external form of the leaf has been descril)ed on p. 109,
and it now remains to consider its internal structure. The
lamina is to be regarded as a flattened and ahered portion of the
stipe, made thin and delicate in order to present a large surface
to the light and the air. The stipe, in turn, is a prok^ngation
of the rhizome, so that the whole plant body is a continuous
mass, throughout wdiicli extend tlie three systems of tissue vir-
tually unchanged. Tlie transverse and longitudinal sections of
the stipe show only minor points of difference from correspond-
ing sections of the rhizome. In the leaf, however, all tliree



systems undergo great changes. The epidermis becomes very
thin, dehcate, and transparent ; the fibro-vasciilar bundles break
up into an extremely fine and complex network forming the

Fig. 59.— Cross-section of part of a leaflet showing the microscopic structure, ep,
epidermis ; st, stomata ; r.s, intercellular spaces between the mesophyll-cells,
which are filled with (shaded) chlorophyll-bodies lying in the protoplasm.

veins I the sclerotic tissues become transparent and are found
only along the veins. The cells of the fundamental parenchyma
alter their form, lose their starch, and become filled with bright-
green, rounded bodies, called the cliromatopliores or GMoropJiyll-
hodies^ which are composed of a protoplasmic basis colored by a
pigment known as chlorophyll. The green fundamental paren-
chyma of the leaf is sometimes called the mesophyU.

A cross-section of a leaflet (p. 109) is shown in Fig. 57.
The finer structure of the leaflet is shown in Fiffs. ^S and 59.
On the outside is the epidermis (ep) ; within, the mesophyll and
midrib — the latter composed of thickened epidermal and sclerotic
fundamental tissue, and a large fibro- vascular bundle.

The mesophyll^ or leaf -parenchyma, consists of irregular cells



which are loosely arranged on the lower side, leaving very large
intercellular spaces, but are closely packed, and leave few or no
intercellular spaces, on the iip])er (sunny) side. The cells have
very thin walls, contain protoplasm and a large central space

Fig. 60.— Epidermis from the under side of a leaflet, showing? wa\'y cells ; elonprated
(pro,sencAjj/ma^)».s) cells over the veins; and stomata with tlieir guard-cells, sty
stomata and guard-cells ; v, veins covered by thick and prosenchymatous epi-
dermal cells. Intermediate stages between wavy and straight cells are also
shown. (Surface view.)

(vacuole) filled with sap, and numerous chlorophyll-bodies im-
bedded in the protoplasm. These are especially numerous in




the upper part of the leaf, as might be expected from theit
functions in connection witli the action of light (see page 147).

The epidermis^ or shin of the leaf, consists of translucent,
greatly flattened cells having peculiar wavy outlines and rela-
tively thick walls (Figs. 58-61). Upon
the veins they become elongated, and
their walls are considerably thickened,
especially upon the midrib (Fig. 58,
They generally contain large, distinct
nuclei, and often considerable proto-
j^lasm. The wavy epidermal cells,
particularly in young plants, contain
some chlorophyll and starch, though
1-c* in this respect the fern is somewhat

In the rhizome the epidermis forms
a continuous layer over the whole sur-

(After Sachs.)— Epi- facc. In the leaf, however, this is not
dermal cells of ptcris flabei- ^]^g ^^^^ ^l^g epidermis ou the lower

tola, showing the development ^ ^ ^ ^

'• of stomata. A, very young side being perforated by holes leading
Z?::-rL.!lima.t,ric^ ^^to the interior and known as months

mother-cell; sx, sudsidiary or stOlJiata (singular, StomO) (Fig. 61).
cell ; g.c, guard-cell ; St, stoma, ^m ii i x -xxi n

Ihese holes do not pass into the ceils,
but are gaps or breaks between certain cells of the epidermis,
and open directly into the intercellular spaces, of wliich they are,
in fact, the ends. That portion of the intercellular labyrinth
which directly underlies the stoma is sometimes called the respira-
tory cavity. Each stoma is l)ounded, as in most plants, by two
curving guard-cells^ which are generally nucleated, and, unlike
epidermal cells generally, contain abundant chlorophyll-bodies
and starch.

The guard-cells are capable of clianging their form accord-
ing to the amount of light, the hygroscopic state of the atmos-
phere, and other circumstances, and thus open or close the hole
or stoma between them. This action is of great importance in
the physiology of the plant (transpiration, p. 117).

In Pteris cretica and P. fldbellata the stomata develop as follows : A
young epidermal cell is divided by a curved partition into two cells, one of
which (Fig. 61) is called the initial cell of the stoma {i.c). This is again


divided by a curved partition into the mother-cell of the stoma (Fig. 61,
m.c) and a subsidiary cell (Fig. 61, s.c).

The mother-cell is then bisected into the two guard-cells, and the stoma
appears as a chink between them (Fig. 61, B).

The veins are the fibres or threads which constitute the
framework of the leaf. Each consists, essentially, of a small
iibro-vascular bundle branching from that of the midrib (Figs.
57, 58, 62). Above and below them the mesophyll and epi-
dermal cells are generally thickened and prosenchymatous, in this
way contributing alike to the form and the function of the
*' vein."

Fig. 63. (After Luerssen.)— Venation of a leaflet of Pteris aquilina.

Their arrangement (veining or venation) is definite, and depends on the
mode of branching of the fibro-vascular strand which constitutes the prin-
cipal part of the midrib. Secondary strands (nerves) proceed from this at
an acute angle, then turn somewhat abruptly towards the edge of the
leaflet (or lobe), making an arch which is convex towards the distal ex-
tremity of the midrib (Fig. 62).

From this point, after branching once or twice, the delicate veins run
parallel to each other to the edge of the leaflet, where they join one another
or anastomose. This form of venation is known as Nervatio Neuropteri-
dis, and is more easily seen in the leaf of Osmunda regalis (cf. Luerssen,
Rabe7ihorsVs Kryptogamen-Flora (1884), III., s. 12).



Reproduction and Development of the Brake or Fern,

Reproduction. Unlike tlie earthworm, the fern reproduces
both by gamogenesis (sexually) and agamogefiesis (asexually).
Pteris possesses two modes of asexual reproduction, viz., the
detachment of entire branches from the rhizome and the con-
sequent establishment of independent plants, as already men-
tioned (p. Ill), and the formation of " adventitious buds " from
the bases of the leaf- stalks (Fig. 46). But besides these the
fern has a quite different method of reproduction, in which a
process of agamogenesis regularly alternates with gamogenesis
{alternation of generations). The folio whig brief outhne of
this important process may help to guide the student through
the subsequent detailed descriptions.

Upon some of the leaves are formed organs called sijorangia
(Figs. 57, 63, 64), which produce numerous reproductive cells
called spores. The spores become detached from the parent and
develop into indejDendent plants, the prothallia (Fig. 70), w^hich
differ entirely in appearance from the fern and ultimately pro-
duce male and female germ-cells. The female cell of the pro-
thallium, if fertilized by a male cell, develops into an ordinary
' ' fern, ' ' which again produces spores asexually. The forma-
tion and development of the spores is evidently a process of
agamogenesis., and the fern proper is therefore neither male nor
female — i.e., it is sexless or asexual. The formation and de-
velopment of the germ-cells, on the contrary, is a process of
gamogenesis j and the prothallium is a distinct sexual plant,
being both male and female (Jiermajyhi'odite or hisexual). In
general terms this is expressed by calling the ordinary fern the
spore-bearer, or sporophore^ and the prothallium the egg-
bearer, or oopJiore. The life -history of the fern, broadly




Fig. 63. (After Suminski.)— Spo-
rangium of Pteris serrulata. p,
pedicel ; c, capsule ; a, annulus ;
s, spore.

speaking, consists tlierefore in an alternation of the sporophore
(asexual generation) with the oopliore (sexual generation) ; that
is, it consists of an alternation of
generations. An essentially similar
alternation of sporophore with oophore
occurs in all higher plants, though in
most cases it is so disguised as to es-
cape ordinary observation.

The Sporangia and Spores. The
sporangia of Pteris (Figs. 63, 64)
arise upon a longitudinal thickening
of tissue situated on the under side of
the leaflets near their edges, and in-
cluding a marginal anastomosis of the
veins. This swelling is known as
the receptacle. Hairs are not uncom-
mon upon the under side of the leaf,
and some are found upon or near the
receptacle. On the latter arise structures, at first superficially
similar to hairs, which become enlarged at the tip, and finally
develop into the sporangia. Meanwhile the edge of the leaflet
is bent down and under so as to make a longitudinal band of
thin tissue composed of epidermis known as the oider veil or
indusium (Fig. 64, o.i). A similar thin sheet of epidermis
grows down from the under side of the leaf, and passing out-
wards to meet the former, constitutes the inner veil or true
indusium (Fig. 64, B^ i.i).

In the Y-shaped space thus formed the sporangia are de-

A superficial (epidermal) cell enlarges and becomes divided into a
proximal (basal) cell and a distal (apical) cell (Fig. 65, a). The former de-
velops into the iwiuve pedicel or stalk of the sporangium ; the latter gives
rise to the head or capsule within which the spores are formed (cf. Fig. 63>.
The pedicel arises from the original pedicel-cell by continued growth and
subdivision until it consists of three rows of cells somewhat elongated.
The rounded capsule-cell is next transformed by four successive oblique
divisions into four plano-convex "parietal cells" and a tetrahedral central
cel>, the archesporium, enclosed by the others. The capsule-cell is thus
divided by three planes inclined at about 120° (Fig. 65, 6, c). A fourth
(Fig. 65, d, e) passes nearly parallel to the top of the capsule and cuts off



from it the central cell or archesporiiim. In the parietal cells further
divisions follow, perpendicular to the surface, while the archesporium gives
rise to four intermediate or tapetal cells, parallel to the original parietal
group (Fig. 65, g). The sporangium now consists of a central tetrahedral
archesporium bounded by four tapetal cells, which in turn are enclosed by
the parietal cells, at this time rapidly multiplying by divisions perpen-
dicular to the exterior. Owing to the peculiar position of the planes of


Fig. 64. (From Luerssen, after Burck.)— Indusiaand receptacle of Pterisaquilina;
B (diagrammatic), seen from below ; A, in the section of the edge of a leaflet, o.i,
outer (false) indusium ; i.i^ inner (true) indusium ; r, receptacle ; s, young


division the whole capsule is now somewhat flattened, and it becomes still
more so by the formation along the edge of a peculiar structure called the
ring or annulus^ whose function is the rupturing of the capsule and the
liberation of the spores. The annulus is formed by a number of parallel
transverse partitions (Fig. 65,/, h, i,J), which subdivide the peripheral
cells of one edge of the capsule until a certain number of cells have been
formed. These then project upon the capsule (Fig. 65, J) and form an in
complete ring (Fig. 65, k).

Meanwhile the tapetal cells sometimes subdivide so as to form a double
row (Fig. 65, h), and soon afterwards are absorbed, space being thus left



Fig. 65. (After Luerssen.)— Development of the sporangia of Aspklmm Filix mas,
which is closely similar to that of Pterin, a, the young sporangium standing
upon the epidermis-cell from which it has just been divided; r, the proximal
cell cut off from the sporangium to form the pedicel and support the capsule;
a, 1, the first partition in the capsule; b, 1 and 2, the first and second partitions;
c, 1, 2, 4, the first, second, and fourth partitions ; d and e are cross-sections of the
capsule showing the oV^lique position of the partitions, and especially that of the
third ; /, a later stage ; 3, the origin of the tapetal cells and the formation of tlie
archesporium ; /«, division of the tapetal cells and the formation of the spore
mother-cells; I, four spores as they originate in the spore mother-cells; i, j, fc,
the annulus and ripe sporangium, in surface view ; j), peripheral cells; ar,
archesporium ; t, tapetal cells ; an, annulus.



for the growth and enlargement of the archesporium. The latter now
divides — first into 2, then into 4, 8, and finally 16 cells, the mother-cells
of the S2)ores. These remain for a time closely united, but eventually
separate and again subdivide, each into 4 daughter-cells (Fig. 65, I). The
64 cells thus formed are the asexual sjjoi^es. In their mature state they
have a tetrahedral form and certain external markings, indicated in Figs.
63, 66. Each spore acquires a double membrane, viz., an inner, endo-
sporium, delicate and white, and an outer, exosjjormm, yellowish brown,
hard, and sculptured over the surface with very close and fine, but
irregular, warty excrescences.

Germination of the Spores. Development of the Prothallium.
In the brake the spores ripen in July or August and are set
free by rupture of the sporangium under
the strain exerted by the elastic annulus, as
indicated in Fig. 63. Germination of the
spores normally occurs only after a considera-
ble period (perhaps not before the following
spring) ; it begins by a rupture of the exospo-

Fio. 66. (After
Suminski.) —
Single spore of
Pteris scrrula-

Fig. 67. (After Suminski.)— Germinat-
ing spores of Pteris serrulata. A, in an
early stage ; B, after the appearance
of one transverse partition ; s, spore ;
p, protonema ; r, rliizoid.

Fig. 68. ^After Sumin-
ski.)— Very young pro-
thallium of Pteris,
showing the spore (s),
two rhizoids (r), and
the enlarging extrem-

rium which is probably immediately due to an unbibition of
water. The spore bursts irregularly along the borders of the
pyramidal surfaces, and from the opening thus formed the endo-
sporium protrudes as a papilla filled with protoplasm in which
numerous chlorophyll-bodies soon aj^pear.

This papilla is known as the ijrotonema.^ or first portion of
the prothallium (Fig. 67). It develops very quickly into a stout
cylindrical protrusion divided into cells joined end to end.
Close to the spore one or more rhizoids are put down from the



growing protonenia to serve as anchors and roots. At the oppo-
site or distal end longitudinal partitions soon appear (Fig. 68),
whieli speedily convert this portion into a broad flat plate at
first only one cell thick, but eventually several cells tliick al(jng
the median line. This thickening is the so-called "cushion"
(see Fig. 70). The whole prothalliuni is now somewhat spatulate
(Fig. 69), but by further grow^th anteriorly, by an apical cell or
otherwise, the wider end becomes
still more ilattened and heart-
shaped or even kidney -shaped.
Numerous rhizoids (so-called be-
cause they are not morphologi-
cally true roots) are put down,
and the whole structure assumes
approximately the appearance in-
dicated in Fig. 70. The spore-
membranes and proton ema soon
fall away, and the prothallium
enters upon an independent exist-
ence, being rooted by its rhizoids
and having an abundance of
chlorophyll. In the broad thin
plate of tissue no subdivision into
stem and leaf exists, and the
plant body closely resembles the
' ' thallus ' ' of one of the lowest
plants. Since it is the precursor
of the ordinary "fern," it is
called the '' prothallus " or '''pro-
thallium. ' '

The cushion forms a prominence on the lower side ; upon
its posterior part most of the rhizoids are borne.

Sexual Organs of the Prothallium. The prothallia of ferns
are as a rule bisexual or hermaphrodite ; that is, each individual
possesses both male and female organs. But the latter appear
somewhat later than the former, and poorly nourished prothallia
often bear only male organs, though they will frequently develop
female organs also if placed in better circumstances.

The Antheridia^ or male organs, are hemispherical promi-

FiG. 69, (After Suminski.)— Older pro-
thallium, showing two rhizoids, three
young antheridia, and numerous



nences occurring upon the posterior part and the under side of
the prothallium, often among the rhizoids. When fully formed
(Fio-s. 70, 71) an antheridium consists of a mass of rounded cells
{sperinatozoid mother-cells) enveloped by a membrane one cell
In tliickness.



Fig. 70. (After Suminski, slightly modified.)— Adult prothallium of Fter\B serrulata
seen from below, showing the rhizoids (r) at the posterior end, the depression at
the anterior end ; the cushion near the latter bearing (in this case) four arche-
gonia. Among the rhizoids are the (spherical) antheridia. The chlorophyll-
bodies only are shown in the cells of the broad plate of tissue constituting the
prothallium. Just above the anterior depression is seen a rrothallium of the
natural size.

Fig. 71. (After Strasburger.)— Mature an-
theridium of Pterin- sej7tt?<xf ft. p, periphe-
ral cells; m, mother-cells of the sper-

Fig. 72.— Diagram to illustrate the ori-
gin of an antheridium. A, very
young stage; B, older; a, original
epidermal cell enlarged ; b, mother-
cell of the entire antheridium.



The mode of origin of the mother-cells differs considerably in different
ferns, but in all cases is essentially as follows : An ordinary cell on the
lower side of the prothallium swells and forms a hemispherical or dome-
shaped projection, which is soon separated by a partition from the original
cell (Fig. 72). Further divisions then follow in the dome-shaped cell such
that a central cell is left, surrounded by a
layer of peripheral cells (Fig. 73). By re-
peated divisions the central cell splits up
into the spermatozoid mother-cells (Fig. 71).

Within each mother-cell the proto-
plasm arranges itself in a peculiar
spiral body, the sjyermatozoid^ which
is the male germ^cell.

When the mature antheridium is
moistened, the peripheral cells swell
and thus press out the mother-cells
and spermatozoids (Fig. 74). The
latter escape from the mother-cells and swim about very actively
in the water. They appear as naked single cells, of a peculiar

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Online LibraryW. T. (William Thompson) SedgwickAn introduction to general biology → online text (page 12 of 20)