Tracheary Elements in Ferns: New Techniques, Observations, and Concepts

2007·8.7 MB·English

by  Sherwin Carlquist

American Fern Journal 97(4):199-211 (2007)
New
Techniques,
Tracheary Elements Ferns:
in
and Concepts
Observations,
and Edward Schneider
Sherwin Carlquist L.
CA
Santa Barbara Botanic Garden, 1212 Mission Canyon Road, Santa Barbara, 93105
microscopy (SEM)
xylem were studied with scanning electron for
Abstract.—Longisections of
roots of Angiopteris, upright axes of Psilotum, and rhizomes of eight species of leptosporangiate
using
ferns of diverse habits and varied ecological preferences. In contrast to earlier studies
macerations, razor-blade sections of fixed material from living plants were prepared. All materials
showed membranes present on presumptive end walls of tracheids.
studied porose or reticulate pit
some Tracheid
were observed on walls of tracheids. to
non-porose lateral
Contrasting pits
parenchyma may have porose membranes on the tracheid side and nonporose pit
pairs pit
pit
membranes on parenchyma thus degree of porosity in a section can represent the degree to
the side;
membranes on tracheary element
which one primary wall or the other pared away. Reticulate pit
is
end walls are evidently widespread in ferns. Such cells should not be considered vessel elements,
membrane-free
membranes suggest degree of transition toward the
although the reticulate pit a
membranes do occur
True absent in perforations) in
vessels
perforations of typical vessels. (pit
produced
number The preparation methods of the present study
limited of fern genera.
roots in a
and must be altered accordingly.
from than did macerations, interpretations
results freer artifacts
Reports of lateral, multiple, and interrupted perforation plates in ferns are probably the result of
loss of pit membranes due to the oxidative action of maceration and should be rejected. Likewise,
membranes
dimorphism" (alternately wide and narrow pits) and "striate" (corrugated) pit in
"pit
probably always have secondary wall
represent True vessel elements in ferns
ferns artifacts.
end walls different from that of lateral walls.
architecture of
xylem
Key Words.—conductive tissue, scanning electron microscopy, tracheids, ultrastructure,
elements
Progress in study of the nature of tracheary in fern?
and With micrc
by techniques technology. light
gradual, limited
aquilinum
were reported Pteridiuin
macerations, vessels early in
Nephrodium Schot
and
filix-mas
(Russow, 1873; Bliss, 1939) (L.)
numb
microsco
Marsilea (White,
macerations revealed distinctive perforation plates in roots of
and White reported possible vessels in
Loyal Singh, 1978). (1962)
1961;
The
and Woodsia. report of possible
Phlebodium, Polystichum,
Astrolepis,
White
lem
between end and
which based on differences walls
an appearance in turn
is
secondary wall architecture.
walls in
lateral
we
we microscope (SEM),
Because had access to a scanning electron
xylem
endeavored confirm White's (1962) reports. In our studies of of
to
and Phlebodium and
Woodsia obtusa Torr. (Carlquist et aL, 1997)
(Spr.)
we
and obtained similar results. In
Polystichum (Schneider Carlquist, 1997),
SEM
we by
prepared
Woodsia used study of sections
obtusa,
the case of
by
tracheid walls
sectioning techniques. Excessive fracturing of
paraffin
AMERICAN
200 FERN VOLUME NUMBER
JOURNAL:
97
4 (2007)
paraffin sectioning in Pteridium (Carlquist and Schneider, 1997a) discouraged
ns from on
relying paraffin sectioning, however, and subsequent we
in
studies,
SEM
We
SEM
confirmed
with
4 /
from
walls secondary
lateral in wall architecture by
also differ lacking
pit
membranes
end
in walls but not in lateral wall pits. This was reported in
Pteridium and
(Carlquist Schneider,
1997a), Astrolepis and
(Carlquist
Schneider, Woodsia
1997b),
ilvensis and
(L.) R. Br. (Carlquist Schneider,
W.
1998a), scopulina Eat. (Schneider and and
Carlquist, 1998a), Marsilea
(Schneider and
Carlquist, 2000b). In these studies, as in those done during
this
period on vessels of Araceae and
(Carlquist Schneider, 1998b; Schneider and
we
Carlquist, 1998b), thought that macerations provided reasonably
reliable
membrane
material
for establishing presence
pit or absence. All of the four
genera above were
listed regarded having
as vessels.
subsequent on
In studies fern tracheary elements done with macerations, we
therefore applied similar interpretive and
criteria (Carlquist Schneider, 1998c,
1999, 2000a, 2000b, 2000c,
2001; Carlquist, Schneider
et ah, 1999, 2000;
Schneider and
Carlquist, 1998c, 1998d, 1999a, 1999b,
1999c, 2000a); the fern
groups
studied
in these respective papers lack differences between end walls
and We
walls secondary
lateral wall
in architecture. observed porose
pit
membranes, which we
at that time considered indicative of vessel presence, on
We
pits of tracheids of all of these ferns. also observed absence
of
pit
membranes. we
Since those studies, became concerned some may
that
artifacts
We
have
resulted from the maceration
process. resolved
to reinvestigate
selected ferns using with aqueous
fixation ethanol solutions, sectioning by
hand
with and
razor blades, drying in These methods had provided
air.
membranes
reliable preservation of pit in tracheary elements such workers
for
Meyl
Matkrial and
Mktiiods
All materials studied were from rhizomes
(stems) of ferns except
for
which
Angiopteris, in roots were studied, and Psilotum, which
in subaerial
portions of upright axes were selected. Roots of the ferns available us other than
to
wen;
Angiopteris
too slender to be readily sectioned by hand.
Suitable portions
mm
were 70%
aqueous
fixed in ethanol. Sections 1-2 thick were by hand
cut
using
single-edged These
razor blades. have
sections the advantage of withstand-
ing the sectioning process better than thinner sections, and our observations with
SEM
confirmed
primary
that delicate walls sectioned well and were not damaged.
No
more
than
six or eight sections were cut with a given razor blade, because
sharpness
deteriorates rapidly. Sections were placed between
with
glass slides
gentle pressure applied in order to prevent curling during and were
drying,
air-
dried on warming
a table. Dried sections were mounted on aluminum
stubs,
sputter coated with gold, and examined with 2600N SEM.
a Hitachi
Cultivated material from commercial was
sources used
for the study of
Blechnum
brasiliense Desv., Cyathea cooperi Domin,
Muell.)
Davallia
(F.
CARLQUIST TRACHEARY ELEMENTS
& SCHNEIDER: IN FERNS. 201
fejeensis Hook. Pellaea falcata (R. Br.} Fee, Platycerium bifurcation (Cav.) C. Chr.
,
The specimen of Angiopteris evecta (G. Forst.) Hoffm. was cultivated in the
tropical greenhouse of the University of California, Santa Barbara. Psilotum
nudum common weed complex
Beauv.
a in the horticultural that
(L.) P. is at
Polypodium become
institution. californicum Kaulf. has naturalized in the Santa
Cyrtomium
Barbara Botanic Garden. falcatum Presl adventive in gardens of
is
The specimen Woodsia was
Santa Barbara. of obtusa (Spr.) Torr. collected in the
wild by George Yatskievich and George Taylor along a roadcut in Irons Co.,
Herbarium specimens
Missouri (for details, see Carlquist et 1997). (collection
a/.,
numbers by Sherwin used study have been deposited
Carlquist) of ferns in this in
the herbarium of the Santa Barbara Botanic Garden (SBBG).
Results
and "polypod"
Illustrations descriptions are presented for the ferns (Pryer et
2004) followed by Cyatheaceae, then the eusporangiate
aL, ferns.
first,
—
Blechnum show
brasiliense 1-6 Longisections of tracheids a re-
fFigs.
j.
some membranes some
ticulate pattern in pit (Fig. Tearing of of the strands
1).
various patterns readily apparent but the reticulate
in is (Figs. 1, 4,
5),
membranes are remarkably intact despite sectioning and handling procedures.
membrane
most
Porosities or spaces in the pit reticulum are readily visible in
areas superimposed above apertures, because of the greater contrast with
pit
dark background such locations However, the reticulate
a at (Figs.
1, 2, 5).
membrane and
nature of the pit can also be seen in front of the pit border,
is
membrane Where
not limited to the central portion of the pit (Figs. 1-6). pit
membranes porose upper
are less Fig. right; Fig. top; Fig. right),
(e.g., 1, 4, 5,
may
membrane
the double thickness of the pit of the adjacent cells be
which membranes
represented on account of the varied degrees to pit are
shaved away in the sectioning process. The more porose pit membrane portion
membrane may parenchyma
nonporose be
that of a tracheid; the pit that of a
is
may
The more clearly reticulate patterns be the primary wall of only one
cell.
However, view
cell in face view. the oblique of Fig. 6 very valuable in
is
showing that porose membranes occur between adjacent tracheids, where pit
membranes must two seem
of both of the adjacent tracheids be present (the
The
and
fused together not as separate entities in Fig. fact that the
6).
sectioning process not highly destructive revealed by the presence of
is is
membranes
portions of pit so close to the cut surface (Fig. In tracheid pit
6).
membranes where no upper
porosities are visible (Fig. 3, left), the pit
end
represented very likely not from a tracheid wall.
is
—
The stem
Platycerium bifurcation (Figs. 7-11). longisections of tracheary
show membranes. The
elements variously porose or reticulate pit reticulate
membranes and some
nature of pit clearly evident in Figs. 10. In pit
is 7, 9,
The
membranes,
small holes are present reticulum, present,
(Fig.
8). if
membrane, shown Where
characterizes the entire pit as in Figs. 9-10). the
membrane
reticulate nature of a pit not visible over a pit border, as in Fig.
is 8,
membrane may
reticules in the pit not be observable because of lack of contrast
AMERICAN VOLUME NUMBER
PERN JOURNAL:
202 97 4 (2007)
SEM
Blachnum
Figs. 1—6. micrographs of longisections of tracheids of stems of brasiliense,
1)
Portions of two end-wall pits, showing reticulum that becomes finer toward the lateral ends (right).
2) Portions of two (Mid-wall pits, the reticulate pit membranes of which overlie both pit apertures
and pit cavities. 3) Portions of two facets; the facet at upper left shows nonporose pit membranes;
in the pit membrane at lower right, reticulate pit membranes are illustrated only in the two pits at
right, which have experienced more primary wall removal by the sectioning process. 4) Portions of
three pit membranes, illustrating a range in pore sizes. Portions of four pit membranes; tearing of
15)
pit membranes is distinguishable from the natural reticulum. 6) Oblique view of portions of pits
from a cul edge of two adjacent traeheids; the pit membranes between the two tracheids are porose.
=
Scale for figures 2 pin.
all
TRACHEARY ELEMENTS
CARLQUIST & SCHNEIDER: IN FERNS. 203
«H»
W
**>
*-!»
m
4
8
• I
»"".-*
i *
**, fft^gP
•
•,* i
r
i
.-
f^
-
10
12
SEM
rhizomes y cerium
7-12. micrographs of longisections of tracheids of of Plat bifurcation
Figs.
membranes which prominent
(Figs. 7-11) and Pellaea folcata (Fig. 12). 7) Portions of pit in
reticules are present, although a nonporose membrane is also seen (right). 8) Pit membranes with
rather large holes, some of which might represent a degree of artifact formation. 9) Portions of two
membranes, showing the nature of the reticulum, relatively free from artifacts. 10) Portion of
pit
membrane that shows a reticulate nature overlying both pit aperture and pit border. 11)
a pit
membranes
Nonporose pits of a lateral wall, as seen from the inside of a tracheid. 12) Portions of pit
=
that exhibit porose and nonporose areas. Scales 2 |im.
may
have
with the relatively bright underlying pit border, or else the reticule
collapsed onto the border and fused with as a result of processing. Drying
pit
it
memb
may
components account such
of gel of the cell wall for pit
placement
mb
,
AMERICAN VOLUME NUMBER
FERN
204 JOURNAL:
97
4 (2007)
^
WW* tj
i
*
*:*
l*
* --
•'
!
J*
S
|
p
~m
13
14
I—
J
M W
41 K Sm^L.
15
fc^^^T** .
IP
*
4
;?<•*#"
'""
^
PH(HPP^
><£
i*
.
:
SEM
Eics. 13-18. micrographs of longisections of tracheids of rhizomes of Woodsia obtusa
(Kigs. 13-17] and Cyatlwn coopari (Fig. 18). 13) A pit membrane apparently free From artifact
two membranes, showing
formation. 14) Portions of pit a range in size of pores. 15) Portion of a pit
A
membrane
with relatively large holes in the reticulum. IB) lateral wall between two adjacent
tracheids, cut ends of walls at left. 17) Portions of pit membranes, with a rip in one (center), the
membrane =
others apparently intact. 18) Pit portions showing a range in pore size. Scales 2 um.
The
(Figs. 7-10). lateral wall pitting of tracheary elements lacks any evidence
of porosities
(Fig. 11).
—
Pellaea falcata fFig. 12 J. Porose pit membranes were observed on some
The
tracheid walls pores probably occur throughout given
(Fig. 12). a pit
membrane, where
but obvious they
are less overlie a pit border.
—
Woodsia 13— membranes
obtusa end
(Fig. 17 Pit in tracheid walls contain
J.
notably large holes (Figs. 14, 15), although smaller pores are visible also
TRACHEARY ELEMENTS
CARLQUIST & SCHNEIDER: IN FERNS. 205
We
have no reason believe that the holes or pores result
to
(Figs. 13, 14, 17).
from formation, because there very indication of fracturing in
artifact is little
We
membranes.
nonhydrolyzed remnants (= reticulum) of the pit were,
the
membranes
however, unable to detect pores in marginal portions of pit
were
overlying borders 13-15, Lateral walls of tracheids clearly
pit (Figs. 17).
membranes
observed to have pit devoid of pores or holes (Fig. 16).
—
proved
Cyathea cooperi Although material of this species
(Fig. 18j.
because of the sinuous course of vascular bundles, porose pit
difficult
The
membranes were on pores occur
detected several tracheids
(Fig. 18).
membrane.
randomly
over the entire surface of a pit
—
some
Polypodium Despite fracturing of
californicum pit
/Tigs. 19-20J.
membranes
membrane strands, the reticulate nature of pit in this species is
There tendency toward alignment of primary wall strands
evident. a axial
is
The membranes 19 have pores generally smaller than those
(Fig. 20). pit of Fig.
shown
in Fig. 20.
—
Our proved
Cyrtomium falcatum 21 material of this species difficult
(Fig.
j.
have Porose
section because the stems a hard, fibrous texture. pit
to
membranes were few
evident in pits of a tracheids.
—
Small pores were observed be characteristic
Davallia fejeensis (Fig. 22j. to
of some tracheid pit membranes in this species, although fracturing ol the pit
common
membranes was occurrence
a (Fig. 22).
—
membranes
Angiopteris evecta 23 Tracheid pit in roots of this
(Fig.
).
random
species have a distribution of pores of various sizes.
—
nudum
Minute densely occur
Psilotum pores, distributed, in
(Fig. 24j.
some membranes examined.
of the tracheids
pit
Discussion
by
Our using with aqueous ethanol solutions, sectioning
studies fixation
hand with razor blades, and drying in air provided results different from those
we
The
obtained with macerations. appearances report as porosities, reticulate
membrane
membranes, and remnants here consistent
threadlike are
pit
pit
within the specimens studied. Those appearances coexist on specimens with
numbers membranes which no such structures occur, and the
large of pit in
membranes end
porose threadlike consistently occur in central areas of
to pit
show
upper and lower of end wall surfaces transitions to
walls: the tips
We show
know
membranes. no comparisons
nonporose of rigorous that
pit
methanol
that the above methods to be any less reliable than use of or critical
point drying in minimizing artifact formation in pit membranes. Cracking in
membranes, an which probably results from handling as well as to
pit artifact
some extent from drying, can easily be differentiated from porose to threadlike
We
membrane have observed time prolonged
appearances. in real that
pit
and
beam can
exposure current high magnification result in fracturing
to at
membrane
peeling away of pit membranes, and in production of corrugated pit
and
surfaces. Such appearances are easily recognizable as artifacts are
excluded from our descriptions.
AMERICAN VOLUME NUMBER
FERN JOURNAL:
206
4 (2007)
1)7
SEM
Figs. 10-24. micrographs of pit membranes from fern tracheids. Figs. 10-22. Longiscctions
from rhizomes. Figs. 10-20. Polypodium californicum. Portions of two adjacent membranes
19) pit
membrane
with different degrees of porousness. 20) Pit remnants tend to be strands oriented in an
Cyrtomium membranes
axial direction. 2 Portions of pits, falcatum; pit present above center, but
1 )
absent below, apparently because of sectioning. 22) Portion of pit membrane, Davalliu fojannsis,
showing membrane
a pit with small pores; the rather extensive tearing an artifact. 23)
is
membranes
Angiopteris evecta, pit from longisection of a root, showing reticula broken by some
nudum,
tearing. 24) Psilotum portions of two finely porose pit membranes from a longisection of
=
the snbaerial portion of an upright axis. Scales 2 jam.
Our
newer results have produced, in our opinion, reliable images of the
membranes we
nature of tracheary elements and
pit in in ferns, therefore are
new
offering conclusions here, together reassessments our
witli of earlier
studies. Broader issues, such as the terminology of tracheary elements the
in
TRACHEARY ELEMENTS
CARLQUIST & SCHNEIDER: IN FERNS. 207
funct
come
membranes,
—
membranes The present study
porose tracheids.
Distribution pit in fern
of
more images porose
could have been extended indefinitely to obtain of or
m
was
in of the ferns studied. The selection not biased in favor of particular
all
no rhizome
habit types or habitat preferences, so there appears relationship, in
mb
rhizomatous
me
ma
m
SEM
macerated
studies of
and Wood,
Marsilea, Pteridium,
work showed
Discounting reports of vessels in ferns other than these, our
number and
membranes
surprising of ferns (Carlquist
porose in a
finely pit
and
Schneider, 2001). Our present study confirms that porose reticulate pit
mbran
much
seem more
we
macerating our studies, believe, did not create the
the fluid in earlier
me
now porose
evident. The picture that emerges that reticulate to pit
is
membranes on end walls of fern tracheids. Variations in
are characteristic
networks occur, and need further exploration.
these
by having
defined
Def In dicotyledons, vessels are
more
one
and Schneider, 2002a): there are or
four features (Carlquist
(1)
end
membranes) on the end wall; the wall
perforations (free of pit (2)
architecture (perforations) different from that of the lateral wall (pits); (3)
is
elements they
vessel elements are shorter than the imperforate tracheary
accompany; vessel elements are wider than the imperforate tracheary
(4)
elements they accompany. The two features represent a division of labor
last
with vascular
between two conductive possible in vascular plants
cells
Gnetum
Ephedra and possess
cambia, and aside from dicotyledons, only this
and
two applicable ferns
Thus, only the criteria are to
feature. first
Pteridium,
monocotyledons. only the genera Astrolepis, Marsilea,
In ferns,
second by having
and Woodsia Introduction) satisfy the criterion
(see
secondary wall architecture of end walls different from that on lateral walls.
may
Such end walls possibly occur in a few other genera.
distinctive
me
ements
micro
When SEM
m
and dicotyledons, but in some putatively primitive families of both, pit
and
membrane remnants do occur Carlquist, 1992; Carlquist Schneider,
(e.g.,
genus
2002b, Schneider and Carlquist, 2003). In fact, within the Illicium,
2"H AMERICAN KERN JOURNAL: VOLUME NUMBER
97
4 (2007)
membrane
various degrees
of pit occurrence occur. anisatum and
In
L.
/. /.
floridanum one
could designate
Ellis, vessels present or absent in terms of
J.
membrane
presence end membranes may
pit in walls: pit be nonporose,
and
porose, strandlike,
reticulate. In Illicium, the three; criteria of vesselhood
membrane
other than absence end
pit in walls are
satisfied.
In ferns other than Astrolepis, Marsilea, Pteridium, and Woodsia,
degree
of
membrane
pit presence is the only criterion by which vessel presence might be
claimed. one
says that possession of membranes,
If reticulate
pil as in
Blechnum and
Platycerium, could
constitute one could
a criterion, conceiv-
ably develop more 50%
a criterion that hydrolysis of than of the membrane
pit
would
result in formation of vessels, and thus Blechnum and Platycerium
might be have
said
to vessels. Clearly, ultrastructure erases any usable
boundary between
tracheids and vessel elements number The
in a of cases.
most
sensible solution seems be
to to call attention intermediate conditions
to
when
The
they
occur. inherent
interest is not in the definition, but the
in
evolutionary and physiological phenomenon.
significance of
this
Possible functional —Given
significance tracheid
of ultrastructure the
widespread
occurrence of weblike or porose membranes membrane
pit or
pit
remnants ends
walls
in of fern tracheids, the probability
exists that this
mode
structural represents an adaptive
system. Reticulate or porose
pit
membranes
would
represent an intermediate on way
stage the attainment
to of
vessels. Intermediate stages, however, are often whereas
labile, the tracheid
end-wall membranes One
pit of ferns are not. can compare the condition
in
ferns to the similar differentiation end walls Telracentron
in of tracheids
(see
Carlquist, 1992). Porosities in the margos of coniferous tracheid membranes
pit
also bear comparison. The margo
pores illustrated Podocarpaceae by
for
Meylan
and
same
Butterfield (1978) are in the size range as the pores or
we
spaces
reticular
figure for ferns. In coniferous tracheids, the margo pores
may
increase conductive (Zimmermann, The
rate
1983). larger pores of
margo
a coniferous can allow passage under some
air conditions, process
a
down
shut by
aspiration of the pit as the torus forced against the border.
is pit
The
nonhydrolyzed
cellulosic strands of the conifer margo permit
pit the torus
function among
to in pit aspiration; the openings the strands are relatively
The
margo
large. pores in conifers that are likely to transmit air are often about
diameter (Zimmermann,
1 urn in 1983). whereas the pores in fern tracheid
pit
membranes much
art; smaller, as our illustrations show. Fern tracheids do not
have and
tori, thus pores as large as those of conifer margo woidd
a pit risk
embolism The
passage. adaptive
significance of the porose or
reticular pit
membranes
end
in walls of fern tracheids thus appears be enhancement
to of
conductive
without
rates increase in the risk of embolism. Understanding
of
the physiological
action of fern tracheids has lagged behind understanding
of
the relationship between and
ultrastructure physiology
in the case of conifer
woods,
very because economic
likely of the value woods.
of conifer Porose
pit
membranes
certainly do occur between shown
adjacent tracheids of
ferns, as in
The
Pig. possibility remains where nonporose membranes
that on
6.
exist the