Douglas Houghton Campbell.

A university text-book of botany online

. (page 22 of 50)
Online LibraryDouglas Houghton CampbellA university text-book of botany → online text (page 22 of 50)
Font size
QR-code for this ebook


thalliahave been observed to develop
from the margin or from the surface
of a leaf, or in some cases from the
transformation of a young sporan-
gium into a prothallium.

Distribution of Pteridophytes. —
The sporophyte in most Pterido-
phytes is terrestrial, but a few
forms are aquatics. Thus Isoetes
lacustris is completely submersed, while Salvinia and Azolla are
floating forms. MarsiJia is usually amphibious. The Pteridophytes,
like the HepaticsB, reach their greatest development in the moist
mountain forests of the Tropics, where they constitute an important
and most beautiful feature of the vegetation. A small number,
like the little Gold-back Peru {Gymnogramme triangularis)^ are
adapted to a dry region, and can endure complete desiccation with-
out injury.

The living Pteridophytes are usually divided into three classes,
Filicales, Equisetales, and Lycopodiales. Of these the first com-
prises much the greater number of existing species. The Equise-
tales, which during the Palaeozoic age were numerous and varied,
now comprise but a single genus. The Lycopodiales were also at
this period much better developed than they are at present. There




Fig. 210. — Lycopodium clavatum,
A, leafy shoot having two cones
composed of sporophylls, sp (re-
duced) . B , a single sporophy 11 , with
sporangium , sp , enlarged . C, cross-
section of the stem.



Digitized by



Google



246 BOTANY



are also a number of fossil types of Pteridophytes which are not
assignable to any of the three existing classes.

Class I. Filigales

The Filicales, or Ferns, are cosmopolitan, but much more abundant
in the moist Tropics than elsewhere. In northern regions a few spe-
cies like Pteridium aqtiilinum or Oamunda regalis may be abundant
enough to attract notice, but most of the northern Ferns are compar-




Fio. 211. — Spore-divisioD in EquUetum, A, D, J£, E. telmateia (X400). B, C,
E, Hmo9um, more highly magnified. A, group of four sporogenous cells. S, C,

' first nuclear division; in B, multipolar spindle. D, second nuclear divlBion.
E, diyision of the ceU into the young spores. {B, C, after Ostb&hout.)

atively insignificant. In the Tropics, however, especially in the
mountains, they often occur in great numbers and variety, and some
of the Tree-ferns are among the most striking of all plants.

The Gametophyte

The gametophyte (Fig. 212) in most Ferns is a flat, green thallus,
which in exceptional cases (e.g, Vittana «p.) may reach several centi-
metres in length and branch repeatedly.

The ripe spore usually shows three membranes, the inner cellulose



Digitized by



Google



PTERroOPHYTA



247



membrane (Intine), the cuticularized exine or " Exospore," and the
outermost sculptured coat or perinium ("Epispore") which some-




E.:^




Fio. 2\2. — Struthiopteris Oermanica. A^ B, germinating spores, with perinium
removed. ( X 300) . C, young gametophyte ( X 100) . D, £, older stages with apical
cell, z (X dOO). Ff small female gametophyte, seen from below ; r, rhizoids ; ar,
archegonia.

times becomes detached from the exospore. In case the spore con-
tains chlorophyll at maturity the germination is usually rapid ; in



Digitized by



Google



248 BOTANY



Osmunda the first division of the spore may take place within less
than twenty-four hours. Where chlorophyll is not present the pro-
cess of germination is retarded, as chlorophyll must be developed
before any further growth takes place.

Germination. — In most Ferns the first division in the germinating
spore (Fig. 212) cuts off a small cell, which at once lengthens and
forms the first root-hair, from a larger green cell which gives rise to
a row of cells varying in length under different conditions. In the
terminal cell of the row a two-sided apical cell is formed by inter-
secting oblique walls, and the gametophyte rapidly forms a flat
thallus. New root-hairs grow out from the lower side, and fasten
it to tihe earth. At this stage the yoimg gametophyte closely resem-
bles a simple thallose Liverwort, such as Metzgeria or Aneura, and
as in these, the apical cell lies in an indentation of the margin of the
heart-shaped thallus, caused by the rapid growth in the outer cells of
the young segments cut off from the apical cell.

Sooner or later the two-sided apical cell is divided by a transverse
wall, and from this time inner or basal segments are regularly cut
off, which undergo horizontal divisions, so that back of the apex the
thallus forms a cushion of tissue, upon whose lower surface the arche-
gonia are later developed. If this thickening begins early, as in
Osmimda and Marattia, a thickened midrib is developed. The root-
hairs develop little or no chlorophyll, but the other cells contain
numerous discoid chromatophores.

Both archegonia and antheridia are borne upon the same plant in
most Ferns, but some are dioecious (e.g. StrtUhiopteris Germanica).
In the latter the male plants are smaller and less regular in form
than the females. Small male plants are not uncommon in species
which also produce large hermaphrodite prothallia.

The largest gametophytes are found in certain tropical Ferns,
especially species of Vittaria and Hymenophyllum, in which the
gametophyte may reach a length of several centimetres and fork
repeatedly, like a thallose Liverwort, which it closely resembles.
These prothallia often increase in numbers rapidly by the develop-
ment of gemmae, and thus they may occur in great numbers, forming
thick mats upon the trunks of trees, or upon rocks.

The Sexual Organs

Antheridium. — The antheridium in the lower Ferns shows a slight
analogy with that of the Anthocerotales, especially those forms in
which a single antheridium is developed in each cavity. As in the
Anthocerotales, the cell from which the antheridium develops arises
by a transverse division of a superficial cell into an outer and an
inner cell. The latter, which in the Anthocerotales gives rise to the



Digitized by



Google



PTERIDOPHYTA



249



whole antheridium, in the lower or eusporangiate Ferns, develops
at once into the mass of sperm-cells, while the outer cell serves as
the covering for these (Fig. 235). It is not impossible that this type
of antheridium may have been derived from that of the Anthocero-
tales by a suppression of the sterile cells of the endogenous anther-
idium, whose peripheral cells are replaced by the superficial celU
covering the cavity in which the antheridium is situated.




FiQ. 2\3.^8truthiopteris Oermanica. Development of antheridium. ^-C, vertical
sections (x 600). Z>, nearly ripe sperm-cells. Ej spermatozoid (X1200).

In the more specialized Ferns (Leptosporangiatae), especially the
Polypodiaceae, the antheridium (Fig. 213) projects as a nearly spherical
body, in which the cell-divisions are very regular. In the Polypo-
diace» the hemispherical mother-cell is first divided by a funnel-
shaped wall (Fig. 213, A-C), whose smaller end is usually in
contact with the basal wall of the antheridium. The next waJl is
dome-shaped, and its base is in contact with the first-formed wall*



Digitized by



Google



250



BOTANY



Finally a concave wall is formed above the dome-shaped one and
meeting it. The young antheridium now consists of a central cell
enclosed by three peripheral cells, the two lower being ring-shaped,
the upper one somewhat lenticular. These outer cells contain chro-
matophores which are absent from the central cell.

In the lower Leptosporangiatse — e.g, Osmunda, Hymenophyllum
— there are more than thre« peripheral cells, and tiiere may be a
special opercular cell, as there is in the Marattiacese.

The first division of the central cell is usually vertical, and this is
followed by several other divisions, so that there are usually thirty-
two to sixty-four sperm-cells finally developed, although the number




Fig. 21^, ^ Struthiopteria Oermanica, Deyelopment of archegonimn. A, longitudi-
nal section of apex of prothallium ; apical ceU, x (x210). B-D, archegonia
(X 430) ; h, neck canal-ceU.

is not always the same. Previous to the last division but two, the
blepharoplasts, from which the cilia are developed, make their ap-
pearance.

The dehiscence of the antheridium is caused either by the rupture
of the cover-cell, or a small opercular cell is thrown off. The great
distention of the peripheral cells then forces out the separated sperm-
cells, whose membrane soon completely dissolves and sets free the
spermatozoid. In the typical Ferns the spermatozoids are relatively
large and consist of a spiral band, tapering at the forward end, from
which the numerous cilia extend. The larger posterior coils are
mainly composed of the nucleus of the sperm-cell, and enclose a deli-
crate vesicle containing the remains of the cytoplasm of the sperm-



Digitized by



Google



PTERmOPHYTA 261



cell. This may become much distended, and often contains small
granules of starch.

Archegoninm. — In the typical Ferns the archegonium (Fig. 214)
is formed upon the lower surface of the thickened cushion back of
the apex. In Botrychium it is borne upon the upper surface of the
subterranean gametophyte, and in Osmunda the archegonia forms
two rows along the sides of the midrib. The mother-cell divides
into two by a transverse wall, and the inner cell usually divides
again into an inner or basal cell, and a central cell from which the
egg-cell and canal-cells are formed. The outer cell, which corre-
sponds to the terminal or cover-cell of the Liverwort archegonium,
divides by cross-waUs into four cells, which, by a series of transverse
divisions, give rise to the four-rowed neck characteristic of the




Fio. 215.—^, Osmunda cinnamomeat section of recently fertilized archegoninm
(X 450). A spermatozoid has penetrated the nnoleos of the egg, and several are in
the space above the egg. B, Onoclea sensibilia. Egg fourteen hours after the
penetration of the spermatozoid, which is still recognizable within the egg-nncleus
(X 900) . (B, after Shaw.)

Pteridophytes. In the Polypodiacese the two posterior rows remain
shorter than the anterior ones, and the neck is curved backward,
probably an adaptation for facilitating the entrance of the sperma-
tozoids. In the lower Leptosporangiatae, and all the Eusporangiatae,
the neck is straight. The base of the archegonium is always coherent
with the surroiinding tissue, as in the Anthocerotales.

The middle cell of the original three becomes pointed above, and
this portion is cut off as the neck canal-cell, which subsequently
divides more or less completely into two. A second transverse, or
concave division-wall, cuts off the ventral canal-cell from the egg,
which later contracts so as not to fill the cavity of the venter. The
walls of the canal-cells, as in the Bryophytes, become mucilaginous,
and effect the opening of the ripe archegonium, when water is



Digitized by



Google



252 BOTANY



applied. As already stated, the attractive substance thrown out has
been shown to be malic acid.

The Embryo

In the common Ferns the first division of the embryo does not
occur for a week or more after fertilization. The globular embrya
(Fig. 216) then divides by a nearly vertical " basal " wall into two
cells, an epibasal (anterior) and an hypobasal (posterior). Each of




Fio. 216. — Af B, Osmunda Claytoniana. C, O. cinnamomea. A, vertical section-
of an S-celled embryo ( x 260) . B, median longitudinal section of an older embryo,
showing the primary organs ; stem, st ; leaf, / ; root, r ; foot,/. C, two transverse
sections of an embryo, showing the apical cells of stem and root; the first root-cap-
ceU has not yet been cnt off.

these is next divided by a transverse wall into two usually equal
parts, and this establishes the primary organs of the sporophyte.
Of the epibasal quadrants, the outer one becomes the cotyledon, or
primary leaf ; the inner one, the stem-apex. Of the hypobasal quad-
rants, the outer gives rise to the primary root, the other to the foot.
Stem and Cotyledon. — Following the quadrant-walls are the octant-
walls, which are not always exactly median in position, this being
especially the case in the root-quadrant, where one octant is usually
noticeably smaller than the other. Each octant is a tetrahedron, and
the next divisions in all of them are parallel to the lateral faces of
the octant-cells. These divisions persist in one of the stem and leaf
octants which assume the function of apical cells for these organs^



Digitized by



Google



PTERIDOPHYTA 253



In the foot, the apical growth is of very brief duration, and the
divisions do not show any definite succession.

Root. — In the root-quadrant, the larger of the two octants at once
assumes the role of an apical cell, the smaller one undergoing very
little further growth. After one or two series of lateral segments,
apericlinal wall cuts off the first cell of the root-cap, and thence-
forth there are four series of segments, as in the roots of the mature
sporophyte.

Cotyledon. — The primary leaf or cotyledon behaves at first much
like the root. One of the octants develops faster than the other,
and the growth is also apical; but, of course, no cap-cells are
formed, and later the tetrahedral apical cell is replaced by a two-
sided one, and the leaf begins to assum.e its characteristic flattened
form.

The establishment of the separate growing-points in the embryo
soon causes the growing organs to project, and the embryo loses
the globular form found in the early stages. Growth is rapid in both
root and leaf, and these presently break through the overlying tissue
of the gametophyte. The root turns downward and penetrates the
earth, and the leaf, expanding and growing upward, spreads its flat-
tened blade to the light-rays. So soon as this is accomplished, the
young sporophyte, which has hitherto obtained its nourishment from
the gametophyte by means of the foot,
now is completely independent, and
the gametophyte soon dies, leaving the
sporophyte rooted in the ground.

The cotyledon in most Ferns is fan-
shaped (Fig. 217), due to an early
dichotomy of the apex, which is re-
peated several times.

Of the two stem-octants, one becomes
at once the tetrahedral apical cell of
the permanent stem, the other develops
into the second leaf, whose growth is
much like that of the cotyledon, but
which becomes larger.

During its early growth, the embryo Fio^ 217. - 0«mt£nda Claytoniana.
^ ° - , /i 1 • ^- Young sporophyte attached to

IS protected by the enveloping tissue the gametophyte (x 6).
of the archegonium venter, which
forms a calyptra like that found in the Bryophytes.

Tissues of the Bmbrjro. — The young sporophyte is composed of perfectly
uniform parenchyma, but as the organs develop, there soon becomes evident a
separation of the tissue elements into definite tissue systems. A single layer of
epidermal cells is generally evident at an early period, and somewhat later the
axis of each of the primary organs shows a strand of elongated cells, especially



Digitized by



Google



264



BOTANY



conspicuous in the root and leaf. These are at first composed of thin-walled
elements (procambium), but later some of them begin to show the characters of
the elements found in the older vascular bundles — these being met with for the
first time among the Pteridophytes. The first recognizable elements are short
spiral or reticulate tracheids, which appear near the junction of the young bun-
dles in the middle of the embryo, and develop from this point toward the apices
of the elongating members.

Vascular Bundles. — The completed vascular bundle of the young stem shows
a central mass of tracheary tissue, some of whose cells have the scalariform
markings found in the tracheids of the older stem. Around these are several
TOWS of cells forming the phloem, but at this stage perfect sieve-tubes cannot be
made out. The endodermis, or bundle-sheath, is also much less evident than in
the older 8poroph3rte.

The tracheary tissue of the cotyledon is composed entirely of spiral tracheids,
and, like the stem-bundle, the sieve-tissue and endodermis are poorly developed.

The bundle of the primary root is
'* monarch ** ; i.e. there is a single strand
of primary wood, and as in the other or-
gans, the other elements of the bundle
are not well developed.

Ground - Tissue. — The tissue lying
around the vascular bundles is usually
known as the ground-tissue. This
remains very much like the original
parenchyma, but in the lamina of the
leaf it forms the spongy mesophyll,
which is the principal green tissue of
the plant, and its spaces communicate
with the external atmosphere by means
of the stomata developed in the epi-
dermis.




Fio. 218. — Adiantum pedatum. A,
rhizome, with young leaf, I, and base
of an older one, I' ; x, stem-apex,
slightly enlarged. B, leaf-segment,
showing the venation, and sori, «,
covered with the marginal indosinm.

podially, or adventitious buds
leaf-bases. A conspicuous case



The Mature Spobophtte

The sporophytes of the various
Ferns differ much in size. In
some of the HymenophyllaceaB
there is a slender creeping stem
with upright leaves less than a
centimetre in length. Some of
the Cyatheaceae are Tree-ferns,
with upright stems ten to fifteen
metres in height, and leaves four
to five metres long. Ferns of
temperate regions usually have a
subterranean stem, which forms
an upright or creeping rhi-
zome. This may branch mono-
may be developed from the old
of this adventitious budding is



Digitized by



Google



PTERIDOPHYTA 266

seen in Stnithiopteris, where numerous stolons develop from the old
leaf-bases.

The Stem

The growth of the stem, in the typical Ferns, is due to the division
of a single tetrahedral apical cell, which in unbranched stems is the
direct descendant of the orginal stem-quadrant of the embryo. The
segmentation of the apical cell is usually slow, and it is generally
impossible to determine the exact relation of the leaves and lateral
branches to the primary segments of the apical cell.

Early divisions in the young segments separate a central cylinder,
in consequence of which ttie vascular bundles and pith (when pres-




PiG. 219. — A-Cy Adiantum emarginatum, A^ longitudinal section of stem-apex
(X25); X, apical cell; Z, young leaf. B, apex of the same (x 180). C, cross-
section of the stem-apex (X 180). Z>, young leaf of Stnithiopteris Oermanica,
showing apical cell, x,

ent) are delimited from the cortex. In case there is a single axial
bundle, the stem is " monostelic " ; if more than one vascular bundle
is present, the stem is " polystelic." The ground-tissue may remain
permanently parenchymatous, or it may develop sclerenchyma, which
is characteristic of the stems of many Ferns. The typical scleren-
chyma (Fig. 221) is made up of cells with very thick striated and
pitted walls of a golden or dark-brown color.

Vascular Bundles. — The vascular bundles of the stem, in most
Ferns, form a hollow network within which lies the pith. The
spaces between the bundles are the " foliar-gaps," and it is at these



Digitized by



Google



266



BOTANY



points that the bundles are given off to the leaves. The bundles are
usually concentric in structure, but in the Ophioglossacese and Osmun-



c



Fiu. 220.—^, Fteridium aqwXiwam, GroBB-section of vascular bundle from the
rhizome; en, endodermis; «, sieve-tubes; ^ scalariform vessels. B, part of two
large scalariform trachesB. C, sieve-tube of IStruihiopttris Oermanica (X 375).
{At B, after Atkinson.)

daceae they are truly collateral. In some of the larger species of
Botrychium there is a genuine secondary growth, with a true cam-
bium, like that in the stem of normal
Dicotyledons or Conifers. In the
typical Ferns (Fig. 220) a section
of a stem-bundle appears circular
or oval. It is clearly separated
from the ground-tissue by a well-
marked bundle sheath or endo-
dermis, composed of cells with
radially folded walls. The endbder-
mis is the innermost layer of the
cortex. Within this are one or two
layers of cells forming the "Peri-
cycle." The tracheary tissue is
made up of large prismatic tracheids, with conspicuous narrow trans-
verse pits — the "scalariform" elements which are typical of the



Fio. 221. — Aneimia hirsuta, Scleren-
chyma from the rhizome, showing
the lamination of the cell-walls and
pits (X 250).



Digitized by



Google



PTERIDOPHYTA 257




Ferns. Two strands of much smaller tracheids, with spiral or retic-
ulate thickenings, occupy the foci of the elliptical section. These
are the primary tracheids, " Protoxylem," and from these the devel-
opment of the tracheary tissue proceeds centripetally.

The phloem, which completely surrounds the xylem, is composed
of elongated walled cells, some of which are developed into sieve-
tubes. These have numerous sieve-plates upon their lateral walls.
Vessels, t.6., tracheary elements composed of several fused cells, are
rare in the Ferns.

The Leaf

Where the stem is prostrate, leaves are developed upon the dorsal
side only. Where it is upright, the leaves usually form a crown at
its summit. In their early stages, „

the growth of the leaf is usually
apical, generally from a two-sided
cell. In Osmunda the apical cell
of the young leaf is tetrahedral.
Later the growth is chiefly basal.
The segmentation of the apical
cell is much like that of the stem,
and the separation of the primary
tissues is accomplished in much
the same way, and takes place
very early.

The leaves in most young Ferns
are dichotomously branched, but
this is not usually the case in the

mature leaf, although it may be; p,^ 222.-^, B. Struthiopteris Ger^
e.g., species of Gleichenia, Tri- monica; ^, first; 5, second, leaf of
chomanes, Adiantum pedcUum, etc. yoong sporophyte, showing dichoto-
Ttr,,^!. ^rv-rv ^.x^«,^«i«. 4.\.r. 1 «« «.^„ mous veuation ( X 3) . CWoodwardia
Much more commonly the leaves radicans, areolated venation (x 2).
are pinnately divided, and the

branching is monopodial, the primary divisions corresponding to the
two series of segments of the apical cell.

The growth of the leaf is very slow in many Ferns, especially
those of cooler regions, where it often takes three years for the com-
plete development of a single leaf. A section through the apex of
the stem of such a Fern will show two, and sometimes three, complete
series of leaves, representing as many seasons' growth. The lamina
remains rudimentary until the season preceding its expansion, when
it rapidly develops, so that it is completely formed by the end of the
growing season, and is ready to expand very quickly in the following
spring. This accounts for the extraordinary rapidity with which the
leaves of many Ferns expand in the spring or early summer.



Digitized by



Google



268 BOTANY



The early growth of the leaf is much stronger upon the outer side,
so that most Ferns show the marked inward rolling of the leaf which
is so characteristic of these plants. In the Ophioglossaceae, how-
ever, the young leaves are usually folded straight in the bud.

A few Ferns, e.g, Scolopendrium, Asplenium nidusy etc., have simple
leaves, but usually they are pinnately compound, or decompound, the
leaves of many Tree-ferns being among the most complex and beauti-
fully segmented known. The leaf commonly has a well-marked stalk
(" Stipe "), which when young is often covered with thin, chaffy scales
or"Pale8B," and these sometimes are tipped with a glandular cell.



A.



^* B.



Fio. 223. — Polypodium falcatum. Gross-section of a leaf cattinfj^ across a vein; $t,
section of a stoma; m, mesophyll (x 200). B, section of yonnfj^ sorus (x 75).

Hairs are less common, but occur upon some Ferns. They are espe-
cially conspicuous upon the young leaves of Oamunda dnnamomea.

Venation. — The venation of the leaves is usually pinnate, but the
ultimate divisions are generally dichotomous. Sometimes connect-
ing veins, enclosing reticulations, are developed (Fig. 222). The
venation is of some importance in classification.

£pidermi8. — The epidermis of the leaf is composed of flat cells
with strongly undulating outline, and, unlike the epidermal cells of
most vascular plants, they contain chlorophyll. Stomata are usually
developed upon the lower epidermis only, but may occur upon the
upper surface in some instances. The Hyraenophyllaceae differ from
the other Ferns in having the lamina of the leaf reduced to a single



Digitized by



Google



PTERIDOPHYTA



Online LibraryDouglas Houghton CampbellA university text-book of botany → online text (page 22 of 50)