*COMMENT ~ Character notes. 
 
*CHARACTER NOTES 
#11. The variations in growth form to which this alludes are often
mentioned in published descriptions and evidently have potential value
for identification, but there is a need for reliable comparative
observations in terms of better character and state definitions. 
#14. A feature quite widely mentioned in published descriptions, and
potentially a very useful character in keys, for which confirmatory
observations and more data are needed. 
#15. \i{}Circinnate (circinate)\i0{}: of a leaf which develops in a
coil, with the apex at its centre. Characteristic of most ferns and
cycads, but unusual in angiosperms. 
#16. \i{}Pinna\i0{}: a primary branch of a compound leaf. 
#18. \i{}Compound\i0{} here denotes leaves with distinct pinnae or
orders of pinnae, the pinnae being stalked (rachillate) or with much
narrowed bases on a non-laminate main axis (\i{}rachis\i0{}). Note that
\i{}simple\i0{} leaves may be variously lobed or dissected. For example,
the \i{}pinnately compound\i0{} leaf of \i{}Blechnum\i0{} (q.v.) is to
be distinguished from the \i{}simple, pinnately lobed (dissected)\i0{}
one of \i{}Polypodium\i0{} (q.v.). \par{}\sb50{}In attempting to
identify ferns, it is essential to avoid confusing the stem-like
petioles and pinnae of complexly compound leaves with stems. If
necessary, examine a plant of bracken to confirm that you understand its
structure in this context. \par{}\sb50{} 
#20-21:23-25. In complexly divided fern leaves, the degree of complexity
decreases towards the apex. \par{}\sb50{}In attempting to identify
ferns, it is essential to avoid confusing the stem-like petioles and
pinnae of complexly compound leaves with stems. If necessary, examine a
plant of bracken to confirm that you understand its structure in this
context. \par{}\sb50{}\i{}Pinna\i0{}: a primary (first order) branch of
a compound leaf, usually but not always laminate.
\par{}\sb50{}\i{}Pinnule\i0{}: a subdivision of a pinna of a bipinnate
leaf, or an ultimate unit of a complexly compound one (usually but not
always laminate). 
#32. \i{}Petiole\i0{}: the component of a leaf joining the stem (usually
a rhizome) and proximal to the lowest pinna or (in a simple leaf) to the
lamina. The leaf stalk, or stipe. 
#33. \i{}Petiole\i0{}: the component of a leaf joining the stem (usually
a rhizome) and proximal to the lowest pinna or (in a simple leaf) to the
lamina. The leaf stalk, or stipe. 
#34. \i{}Petiole\i0{}: the component of a leaf joining the stem (usually
a rhizome) and proximal to the lowest pinna or (in a simple leaf) to the
lamina. The leaf stalk, or stipe. 
#35. \i{}Petiole\i0{}: the component of a leaf joining the stem (usually
a rhizome) and proximal to the lowest pinna or (in a simple leaf) to the
lamina. The leaf stalk, or stipe. \par{}\sb50{}The extensive variations
in vascular patterns exhibited by fern leaves (cf. De Bary, 1884) have
obvious taxonomic potential, but there is a need for competent
comparative studies. Since descriptions in floras generally give no
information on the positions at which petiolar sections are to be made,
the compiled data are almost useless. 
#36. \i{}Petiole\i0{}: the component of a leaf joining the stem (usually
a rhizome) and proximal to the lowest pinna or (in a simple leaf) to the
lamina. The leaf stalk, or stipe. 
#37. \i{}Petiole\i0{}: the component of a leaf joining the stem (usually
a rhizome) and proximal to the lowest pinna or (in a simple leaf) to the
lamina. The leaf stalk, or stipe. \par{}\sb50{}\i{}Rachis\i0{}: the main
axis of a compound leaf above the lowest pinna. Not to be confused with
the \i{}petiole\i0{} (q.v.), which is the component joining the rhizome
and proximal to the lowest pinna or (in a simple leaf) to the lamina. 
#38. This aims to describe the outline of the laminate part of the leaf,
regardless of the degree of dissection. 
#43. \i{}Pinna\i0{}: a primary (first order) branch of a compound leaf. 
#44. \i{}Pinna\i0{}: a primary (first order) branch of a compound leaf. 
#45-46. \i{}Pinna\i0{}: a primary (first order) branch of a compound
leaf. 
#47-49. \i{}Rachis\i0{}: in this context, the main axis of a compound
leaf above the lowest pinna. Not to be confused with the
\i{}petiole\i0{} (q.v.), which is the component joining the rhizome and
proximal to the lowest pinna or (in a simple leaf) to the lamina.
\par{}\sb50{}\i{}Pinna\i0{}: a primary (first order) branch of a
compound leaf. 
#50-51. \i{}Pinna\i0{}: a primary (first order) branch of a compound
leaf. 
#54. \i{}Homospory\i0{} versus \i{}heterospory\i0{}. These terms denote
major differences in life cycles (q.v.). Typical ferns are
\i{}homosporous\i0{}, their sporophytes producing very numerous, tiny,
air-borne spores which germinate to produce relatively inconspicuous but
free-living prothalli which are at least potentially bisexual (i.e.,
capable of developing both antheridia and archegonia). In
\i{}heterosporous\i0{} forms (represented in Britain only by
\i{}Azolla\i0{} and \i{}Pilularia\i0{}), however, the spores are of two
kinds; viz., relatively few and relatively very large
\i{}megaspores\i0{}, in which develop the female prothalli producing
only archegonia, and smaller \i{}microspores\i0{} in which develop male
prothalli producing only antheridia and motile antherozoids. Neither
kind of prothallus becomes free living, being retained instead within
the resistant wall of the spore; and the product of fertilization (the
sporophyte plant) begins its development within the wall of the
megaspore. \par{}\sb50{}The life cycle common to all ferns involves a
regular \i{}alternation of generations\i0{} between \i{}gametophyte\i0{}
(sexual, haploid) and \i{}sporophyte\i0{} (asexual, diploid) phases. The
gametophyte produces male gametes (swimming antherozoids) in antheridia,
and non-motile female gametes (egg cells) in flask-shaped archegonia.
Fusion of an egg cell and an antherozoid within the archegonium results
in a diploid zygote, which develops into the sporophyte. The sporophyte
(represented by the familiar fern plant) ultimately produces sporangia,
in which haploid, non-motile spores are produced. The life cycle is
completed when the spores germinate and develop into haploid
gametophytes. The fern gametophyte, termed the \i{}prothallus\i0{}, is
inconspicuous relative to the sporophyte, which constitutes the familiar
fern plant. 
#55. \i{}Eusporangiate\i0{} versus \i{}leptosporangiate\i0{}. Terms
denoting differences in development and mature structure of the
sporangia. In eusporangiate forms, they are short-stalked or sessile,
relatively massive (generally well over 1 mm in diameter), the wall is
several cells thick, and many spores develop in each. In
leptosporangiate forms, on the other hand, they are borne on a slender
stalk, relatively very small and delicate (less than 0.5 mm in
diameter), the wall is only one cell thick, and few spores develop in
each. Nearly all the ferns of Britain and Ireland are leptosporangiate,
representation of the eusporoangiate type beibg restricted to
\i{}Botrychium\i0{}, \i{}Ophioglossum\i0{} and \i{}Osmunda\i0{}.
\par{}\sb50{}\i{}Sporangium\i0{}: a structure borne on the fern
sporophyte, in which haploid spores are produced by meiotic cell
division. 
#56. \i{}Sporangia(-ium)\i0{}: organs within which haploid, dispersible
spores are produced, borne on the (diploid) sporophyte. Commonly on the
underside of the leaf lamina, but sometimes on specialized leaves or
pinnae. 
#57. \i{}Sporangia(-ium)\i0{}: organs within which haploid, dispersible
spores are produced, borne on the (diploid) sporophyte. Commonly on the
underside of the leaf lamina, but sometimes on specialized leaves or
pinnae. 
#59. \i{}Sporangia(-ium)\i0{}: organs within which haploid, dispersible
spores are produced, borne on the (diploid) sporophyte. Commonly on the
underside of the leaf lamina, but sometimes on specialized leaves or
pinnae. \par{}\sb50{}\i{}Sorus\i0{}: a circumscribed group of sporangia
(q.v.). 
#60-63. \i{}Sorus\i0{}: a circumscribed group of sporangia (q.v.). 
#64. \i{}Indusium\i0{}: a structure more or less covering or enclosing a
sorus, in the form of a flap or a pocket, which protects the developing
sporangia. Indusia vary in derivation, as well as in shape. Hypotheses
concerning evolution of ferns have emphasized the difference between
true indusia, representing outgrowths from the underside of the lamina
adjacent to the sporangia, and false indusia", representing
modification of the leaf margin. \par{}\sb50{}\i{}Sorus\i0{}: a
circumscribed group of sporangia (q.v.).
\par{}\sb50{}\i{}Sporangia(-ium)\i0{}: organs within which haploid,
dispersible spores are produced, borne on the (diploid) sporophyte.
Commonly on the underside of the leaf lamina, but sometimes on
specialized leaves or pinnae. 
#65. \i{}Indusium\i0{}: a structure more or less covering or enclosing a
sorus, usually in the form of a flap or a pocket, which protects the
developing sporangia. In some ferns the indusia have been shed by the
time the spores are being shed, and in others they become reflexed or
deformed. They vary in derivation, as well as in shape. Hypotheses
concerning evolution of ferns have emphasized the difference between
true indusia, representing outgrowths from the underside of the lamina
adjacent to the sporangia, and false indusia", representing
modification of the leaf margin. \par{}\sb50{}\i{}Sorus\i0{}: a
circumscribed group of sporangia (q.v.). 
#66. \i{}Indusium\i0{}: a structure more or less covering or enclosing a
sorus, usually in the form of a flap or a pocket, which protects the
developing sporangia. In some ferns the indusia have been shed by the
time the spores are being shed, and in others they become reflexed or
deformed. They vary in derivation, as well as in shape. Hypotheses
concerning evolution of ferns have emphasized the difference between
true indusia, representing outgrowths from the underside of the lamina
adjacent to the sporangiua, and false indusia", representing
modification of the leaf margin. 
#67-68. \i{}Indusium\i0{}: a structure more or less covering or
enclosing a sorus, usually in the form of a flap or a pocket, which
protects the developing sporangia. In some ferns the indusia have been
shed by the time the spores are being shed, and in others they become
reflexed or deformed. They vary in derivation, as well as in shape.
Hypotheses concerning evolution of ferns have emphasized the difference
between true indusia, representing outgrowths from the underside of
the lamina adjacent to the sporangia, and false indusia", representing
modification of the leaf margin. 
#69. \i{}Indusium\i0{}: a structure more or less covering or enclosing a
sorus, usually in the form of a flap or a pocket, which protects the
developing sporangia. In some ferns the indusia have been shed by the
time the spores are being shed, and in others they become reflexed or
deformed. They vary in derivation, as well as in shape. Hypotheses
concerning evolution of ferns have emphasized the difference between
true indusia, representing outgrowths from the underside of the lamina
adjacent to the sporangia, and false indusia", representing
modification of the leaf margin. 
#71. \i{}Paraphysis\i0{}: in this context, filamentous structures
representing sterile sporangia. \par{}\sb50{}\i{}Sporangia(-ium)\i0{}:
organs within which haploid, dispersible spores are produced, borne on
the (diploid) sporophyte. Commonly on the underside of the leaf lamina,
but sometimes on specialized leaves or pinnae. 
#72. \i{}Sporangia(-ium)\i0{}: organs within which haploid, dispersible
spores are produced, borne on the (diploid) sporophyte. Commonly on the
underside of the leaf lamina, but sometimes on specialized leaves or
pinnae. 
#73. \i{}Sporangia(-ium)\i0{}: organs within which haploid, dispersible
spores are produced, borne on the (diploid) sporophyte. Commonly on the
underside of the leaf lamina, but sometimes on specialized leaves or
pinnae. 
#74. \i{}Annulus\i0{}: an arrangement of thick-walled cells, conspicuous
under a dissecting microscope, associated with the dehiscence mechanism
of a fern sporangium. Among ferns of Britain and Ireland, only the very
peculiar and
un-fernlike \i{}Azolla\i0{}, \i{}Pilularia\i0{}, \i{}Botrychium\i0{} and
\i{}Ophioglossum\i0{} exhibit sporangia without annuli. An illustration
accompanying the description of \i{}Dryopteris filix-mas\i0{}
exemplifies sporangia with the commonest type of annulus.
\par{}\sb50{}\i{}Sporangia(-ium)\i0{}: organs within which haploid,
dispersible spores are produced, borne on the (diploid) sporophyte.
Commonly on the underside of the leaf lamina, but sometimes on
specialized leaves or pinnae. 
#75. State 1 is associated with the relatively massive sporangia of some
eusprangiate ferns (q.v.), and is exhibited here only by
\i{}Osmunda\i0{}. States 2 and 3 distinguish among those of the
relatively tiny leptosporangiate forms.
\par{}\sb50{}\i{}Sporangia(-ium)\i0{}: organs within which haploid,
dispersible spores are produced, borne on the (diploid) sporophyte.
Commonly on the underside of the leaf lamina, but sometimes on
specialized leaves or pinnae. \par{}\sb50{}\i{}Annulus\i0{}: an
arrangement of thick-walled cells, conspicuous under a dissecting
microscope, associated with the dehiscence mechanism of a fern
sporangium. Among ferns of Britain and Ireland, only the very peculiar
and un-fernlike
\i{}Azolla\i0{}, \i{}Pilularia\i0{}, \i{}Botrychium\i0{} and
\i{}Ophioglossum\i0{} exhibit sporangia without annuli. An illustration
accompanying the description of \i{}Dryopteris filix-mas\i0{}
exemplifies sporangia with the commonest type of annulus. 
#76. \i{}Annulus\i0{}: an arrangement of thick-walled cells, conspicuous
under a dissecting microscope, associated with the dehiscence mechanism
of a fern sporangium. An illustration accompanying the description of
\i{}Dryopteris filix-mas\i0{} exemplifies sporangia with the commonest
type of annulus. \par{}\sb50{}\i{}Sporangia(-ium)\i0{}: organs within
which haploid, dispersible spores are produced, borne on the (diploid)
sporophyte. Commonly on the underside of the leaf lamina, but sometimes
on specialized leaves or pinnae. 
#77. Potentially of wide application, but recorded only for
\i{}Polypodium\i0{} species. \par{}\sb50{}\i{}Annulus\i0{}: an
arrangement of thick-walled cells, conspicuous under a dissecting
microscope, associated with the dehiscence mechanism of a fern
sporangium. \par{}\sb50{}\i{}Sporangia(-ium)\i0{}: organs within which
haploid, dispersible spores are produced, borne on the (diploid)
sporophyte. Commonly on the underside of the leaf lamina, but sometimes
on specialized leaves or pinnae. An illustration accompanying the
description of \i{}Dryopteris filix-mas\i0{} exemplifies sporangia with
the commonest type of annulus. 
#78-79. \i{}Spores\i0{}: in this context, the small, dispersible,
unicellular products of meiotic cell divisions within the sporangia of
the fern sporophyte. Resembling pollen grains in the
sporopollenin-containing wall, which is very resistant to decay and
often ornamented in a manner characteristic of a species or genus. On
germination, the spore gives rise to the prothallus (q.v.). 
#80. \i{}Monolete\i0{} spores have one furrow, and are usually
bean-shaped. \i{}Trilete\i0{} spores have a tetrad scar comprising three
furrows forming a Y, and may be detectably tetrahedral, triangular or
more or less round. \par{}\sb50{}\i{}Spores\i0{}: in this context, the
small, dispersible, unicellular products of meiotic cell divisions
within the sporangia of the fern sporophyte. Resembling pollen grains in
the sporopollenin-containing wall, which is very resistant to decay and
often ornamented in a manner characteristic of a species or genus. On
germination, the spore gives rise to the prothallus (q.v.). 
#81. \i{}Perispore\i0{}: a membrane surrounding some fern spores, which
is differently patterned from the usual exine.
\par{}\sb50{}\i{}Spores\i0{}: in this context, the small, dispersible,
unicellular products of meiotic cell divisions within the sporangia of
the fern sporophyte. Resembling pollen grains in the
sporopollenin-containing wall, which is very resistant to decay and
often ornamented in a manner characteristic of a species or genus. On
germination, the spore gives rise to the prothallus (q.v.). 
#82-83. \i{}Prothallus\i0{}: the small plant formed by germination of a
spore, on which develop the male gametes (antherozoids) in antheridia
and/or the female gametes (egg cells) in archegonia. Comprising the
sexual, haploid component of the life-cycle, the prothalli of most ferns
are free-living, but in a few \i{}endosporic\i0{} forms they are
retained within the spores. \par{}\sb50{}The life cycle common to all
ferns involves a regular \i{}alternation of generations\i0{} between
\i{}gametophyte\i0{} (sexual, haploid) and \i{}sporophyte\i0{} (asexual,
diploid) phases. The gametophyte produces male gametes (swimming
antherozoids) in antheridia, and non-motile female gametes (egg cells)
in flask-shaped archegonia. Fusion of an egg cell and an antherozoid
within the archegonium results in a diploid zygote, which develops into
the sporophyte. The sporophyte (represented by the familiar fern plant)
ultimately produces sporangia, in which haploid, non-motile spores are
produced. The life cycle is completed when the spores germinate and
develop into haploid gametophytes. The fern gametophyte, termed the
\i{}prothallus\i0{}, is inconspicuous relative to the sporophyte, which
constitutes the familiar fern plant. 
#85. Note that some calcicole species are often found on mortared walls,
far from their original chalk or limestone habitats. 
#86. Note that some calcicole species are often found on mortared walls,
far from their original chalk or limestone habitats. 
#89. Mostly quoted from Stace's (1997) account. 
#90-92. Representing 2007 data, from records maintained by the Botanical
Society of Britain and Ireland. \par{}\sb50{}WARNING. Geographical
information can often facilitate identifications, but must be
used with caution. Recorded distributions may be incomplete, and plant
distributions change with time. Identifications depending on the
vice-county records may be wrong, and should be verified
with reference to the full descriptions. In case of doubt, the Intkey
'tolerance' facility (q.v.) should be invoked.
\par{}No identification can be relied upon until it has been confirmed
with reference to a detailed description. 
 
