J. Bryol. (1986) 14, 269-280
Ecological factors relating to morphological variation in the
aquatic moss Rhynchostegium riparioides (Hedw.) C. Jens.
JOHN
D. WEHR
and BRIAN
A. WHITTON
University of Durham
Published by Maney Publishing (c) British Bryological Society
INTRODUCTION
Many species of aquatic moss are morphologically very variable (Warnstorf et al. ,
1914; Watson, 1919; Lodge, 1959) and recent taxonomic works (Smith, 1978, Crum
& Anderson, 1981; Ireland, 1982) give examples where confusion may result from
such variability. During the course of a study of metal accumulation by field populations of Rhynchostegium
riparioides (Wehr & Whitton, 1983a, b) it became evident that this species exhibits a wide range of morphologies in different types of
lotic habitat. The present study was planned to run in parallel with that on metal
accumulation because, at least in Northern Europe, R. riparioides is perhaps the
most useful of all species for monitoring heavy metal pollution in rivers. The principal objectives were to reveal the extent of morphological
variation, to identify
those characters which were stable over a broad range of ecological conditions and
to determine which environmental
factors might be related to the variability.
R. riparioides is one of the most common aquatic mosses in rivers in Europe
(Nyholm, 1954-1969; Smith, 1978; Watson, 1981) and perhaps also some other
temperate regions (Smith, 1978). It tolerates a wide variety of ecological conditions
(Watson, 1919; Empain, 1978) and was present in 68.1 % of 1055 one kilometre
lengths of river surveyed in England, Scotland and Wales by N. T. H. Holmes
(personal communication).
The ecology of the species is reviewed in more detail by
Wehr & Whitton (1983a).
MATERIALS
AND METHODS
Materials for the present study were collected at the same time as the samples of
water and moss taken for chemical analysis. (Wehr & Whitton, 1983a). Details of
location, environment, sampling and analytical methods appear elsewhere and are
given here only briefly.
Water and R. riparioides populations were sampled from 105 reaches (each
10 m in length) on 71 different streams and rivers in a broad region of northern
England (Fig. 1); all samples were collected between May and September 1981.
The following variables were measured for the water at each site: pH, total alkalinity,
Na, K, Mg, Ca, Mn, Fe, Zn, Cd, Ba, Pb, NH4~N, N03- N, reactive P04-P,
F, Cl, S04-S, All elements were measured in the filtrable fraction, a 0.2 fJ-m
Nucleopore filter being used for metals and a No. 2 porosity sintered glass funnel
for anions. Samples of the moss were taken from at least five separate boulders or
other microhabitats within a reach, sampling being restricted to submerged plants
269
270
Fig.
JOHN D. WEHR AND BRIAN A. WHITION
1.
Location
Rhynchostegium
of
riparioides
sampling
in northern
I Alston Moor
10 sites
II West Allendale
5 sites
III East Allendale
4 sites
IV Derwent
4 sites
Valley
V Weardale
VI Durham
VIII
for
17 sites
Coalfield
VII Teesdale
Lower Tees
IX Arkengarthdale
X Swaledale
XI Holme Valley
Published by Maney Publishing (c) British Bryological Society
sites
England.
XII Mersey Catchment
9 sites
8 sites
2 sites
12 sites
13 sites
3 sites
8 sites
XIII
Ribble Catchment
3 sites
XIV
Lake Disrict
7 sites
in areas with the greatest current velocity. Herbarium specimens for the present
study were amalgamated from the entire collection of each reach.
Gametophyte
characters were used exclusively for analysis, since sporophytes
are usually present only during late autumn and winter (Wehr & Whitton, 1983b).
Furthermore,
preliminary observations had shown that many characteristics of the
sporophyte (colour, length, texture of seta, capsule and operculum shape) were
essentially uniform. Fifteen gametophyte characters were scored for each of the 105
populations (Table 1). The variables chosen were all essentially continuous, but
because of difficulties in quantification,
some (e.g. robustness) were ranked and
allocated
to a particular
category subjectively.
Quantifiable
variables were
measured on a minimum of 20 leaves or whole plants (depending on the character)
and means calculated where needed.
Data were compiled on computer (Northumbrian
Universities Multiple Access
Computer (IBM 360/370) running under the Michigan Terminal System) and solutions calculated using the MIDAS statistical package (Fox & Guire, 1976). Where
necessary, data were normalized using the most appropriate transformations
prior
to statistical treatment (Statistical Research Laboratory,
1976; Wehr & Whitton,
Table 1. Characters used in analysing variation among populations of Rhynchostegium
riparioides (details of character states
are given in Table 2)
Plants
Leaves
Maximum plant length
Maximum leafy axis length
Maximum unbranched length
Mean weight of 2-cm apical tip
Relative 'robustness'
Plant colour
Proportion of plant leafy
Branching pattern
Mean length
Mean mid-leaf width
Width/length ratio
Leaf shape
Margin denticulation
Nerve length (proportion)
Angular cell shape
VARIATION
IN RHYNCHOSTEGIUM
RIPARIOIDES
271
1983a). Populations were classified using a cluster analysis to determine whether
distinct morphologies exist using the Clustan II package (Wishart, 1978). Several
algorithms were tested. Most gave broadly similar solutions, but the one which
gave the most meaningful and distinct clusters was based on Ward's method with
standardized
measurements
in Euclidean distance. Standardized
scores enabled
both ranked and continuous data to be used in this solution. In order to examine
the relationship between ecological· characters and morphological variation, continuous characters of the moss populations were correlated against environmental
variables measured in the streams from which they were collected.
Published by Maney Publishing (c) British Bryological Society
RESULTS AND DISCUSSION
Variation in plants
The maximum length of a shoot from an individual population (Fig. 2) ranged from
5.0 to 22.0 cm, the upper values exceeding the maxima quoted in various floras (lOIS cm: Szafran, 1963; Lawton, 1971; Smith, 1978; Watson, 1981). However, very
few shoots were collected which exceeded the standard deviation of the mean
(10.3 ± 2.5 cm). This limited variation also characterized
the lengths of the leafy
portion of the plant, although more than half of the populations were devoid of
leaves in one half or more of their length (Table 2). Side branches were produced
within the first 5 cm of the apex in more than 75% of all populations.
In contrast, 'robustness',
a subjective ranking which includes the rigidity of the
stem and how strongly the leaves diverge, varied markedly. Some plants were rigid
when removed from the water, while others were flaccid. Such variation perhaps
causes much of the confusion mentioned by authors. This robustness index can be
related to plant weight, which is quantified more exactly. The mean dry weight of a
2-cm apical fraction of the moss ranged from 0.55 to 3.26 mg, a factor of nearly six.
Variation in leaves
Average leaf length (Fig. 2) varied by a factor of less than two, but the distribution
of this measure was strongly skewed above the mean. Most floras which include
leaf dimensions (Lawton, 1971; Iwatsuki & Mizutani, 1972; Gangulee, 1978; Crum
& Anderson, 1981; Watson, 1981; Ireland, 1982) indicate a leaf range (1.5-2.5 mm)
the maximum of which is actually less than the mode found here. Measurements
of
width and the width/length ratio were more normally distributed and were within
the ranges given in all floras examined. One character, the shape of angular cells,
was consistently rectangular (never inflated) in all populations.
Leaf shape was typically ovate (often broadly so), but a few populations had distinctly lanceolate leaves (Table 2). This was usually found in the less robust,
smaller-leaved plants. In 77% of the populations the leaves were denticulate almost
to the base and nearly all had nerves which extended up more than three-fourths of
the length of the leaf. There were exceptions, however, with consistently weak
nerves, narrower leaf shape and little or no indication of teeth at the margin.
Biometric study of variation
Measured variation was considered as a whole by use of cluster analysis. Two of the
original. 15 characters were excluded from analysis. Angular cell shape was constant
272
JOHN D. WEHR AND BRIAN A. WHITTON
n
30
max.
plant
max. node
length
length
max.
20
x=
x=10'3
leafy
axis
x=
4 ..1
length
6'4
10
Published by Maney Publishing (c) British Bryological Society
6 8 10 12 14 16 1820 22 em
~
c
mean weight
U
20
of apical
2 em tips
x =1·76
Q>
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2 4 6 8 10
mean
leaf
em
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2 4 6 8 10 12 14 16 em
mean
leaf
width
x=
x=2·7
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10
0·8
1·6
width/ length
20
3·6 m g
2'4
ratio
2·0
2·4
2·8
1-0
3'2 mm
1'4
1·8 mm
of
leaf
x =0'49
10-
Fig. 2. Frequency
riparioides. 'Node'
distributions
of continuous
characters
of leaves
refers to the maximum unbranched
length.
and
plants
of Rhynchostegium
throughout
all populations.
Colour was omitted because it was found that the
darker shades were related to (and probably caused by) bound Mn and Fe, and
hence were not a characteristic of the plants themselves.
The analysis revealed four groups of approximately equal dissimilarity (Fig. 3).
Between these four, the larger plants, classified into Groups A and B, were most
closely related, while Groups C and 0 were joined at greater dissimilarity level.
Plant size clearly played an important role in discriminating between morphological
types. Linked with this· analysis, an ordination identified several of the original
morphological variables which had the strongest loadings along the first three component axes (70% of total variance among populations).
These were axis length,
primary node length, leaf length and width; and also branching pattern, leaf shape
and margin denticulation.
VARIATION
IN RHYNCHOSTEGIUM
RIPARIOIDES
273
Table 2. Distribution of discrete characters in 105 populations
of Rhynchostegium
riparioides, expressed as percentage
frequency
Character
State
Percentage
Published by Maney Publishing (c) British Bryological Society
Plants
Robustness
Robust
Moderately robust
Intermediate
Flaccid
Moderately flaccid
21
34
29
8
8
Colour
Pale to bright green
Medium to dark green
Brown to black
33
51
16
< 1/3
1/3-1/2
>1/2
14
42
44
Parallel
Intermediate
Dendroid
11
92
1
Shape
Ovate
Elliptical
Lanceolate
14
81
5
Proportion
denticulate
< 10%
5
2
20
73
Proportion
axis leafy
of
Branching
type
Leaves
10-33%
>33-66%
>66%
Proportionate
nerve length
< 1/2
Angular
Rectangular
Rhombic
Oval/inflated
cell shape
1/2-3/4
>3/4
1
7
92
100
0
0
Diagnostics at the four cluster level indicated that Groups A and B contained
distinctly heavier and longer plants than those in C or D; A and B also had longer
and broader leaves. While C and D plants were smaller and less robust, these two
were fairly dissimilar from each other. Group C had long, but narrower leaves and
ranged from intermediate to moderately robust. These plants were typically more
densely branched than members of all other groups. Group D plants were small to
average-sized,
with a flaccid to intermediate
texture; leaves were smaller than
average, with a short nerve and weak denticulation:
Average (2 cm) tip weight of
plants in Group D (0.96 mg) was half that in Group A (1.98 mg) and only a third
the average weight of the highly robust plants in Group B (2.94 mg). Two extremes
of this gradation in robustness are shown in Fig. 4.
A· feature of this analysis is that while the larger clusters are fairly distinct,
members within a cluster have varying (and often weak) levels of internal similarity. This indicates that although morphological
'types' may. exist, individual
274
JOHN D. WEHR AND BRIAN A. WHITION
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Published by Maney Publishing (c) British Bryological Society
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Published by Maney Publishing (c) British Bryological Society
275
RIPARIOIDES
I em
Fig. 4. Examples of shoots from two extremes
Rh ynchostegium
riparioides.
of robust ("A")
and flaccid ("D")
morphologies
of
populations cannot be identified easily as a member of that group. These results
suggest that forms or varieties that have been recognized in this species are not
sufficiently distinct to be useful in practice. More than a half a century ago,
Warnstorf et ale (1914) complained that it was a ' ... thankless task ... ' to unravel
the numerous forms and varieties which have been included within this species.
However, a few later floras still make such separations (Szafran, 1963; Pavletic,
1968).
Environmental factors
The streams, rivers and other lotic habitats from which populations
of R.
riparioides were collected differed greatly in their chemical composition.
A
summary of the data of Wehr & Whitton (1983a), which is relevant to the present
paper, is given in Table 3. These data indicate the broad range of environments in
which this species occurs. The rivers range from sites with quite soft water (nine
sites < 10 mg 1- I Ca) to calcareous ones. The pH spectrum is also broad, although
few sites were found to have a pH < 7.0. Many of the soft water sites were located
in the Mersey Catchment (region XII, Fig. 1) and several streams in Weardale
(region V) and Allendale (regions II & III). As sites included small upland streams,
industrially polluted rivers and even a sewage effluent, it is not surprising that concentrations of reactive orthophosphate-P
and nitrate-N spanned more than three
orders of magnitude. A number of sites were also polluted by heavy metals, both
276
JOHN D. WEHR AND BRIAN A.WHITION
Table 3. Chemistry
Published by Maney Publishing (c) British Bryological Society
Variable
pH
Total alkalinity
Na
K
Mg
Ca
Mn
Fe
Zn
Cd
Ba
Pb
NH4-N
N03-N
P04-P
504-5
Si
F
CI
of 105 sites with Rhynchostgium
(For details, see Methods)
Units
meq I-I
mgl-I
mgl-I
mgl-I
mgl-I
mgl-I
mgl-I
mgl-I
lJ,.gl-1
mgl-I
mgl-1
lJ,.gl-1
lJ,.gl-1
lJ,.gl-1
mgl-1
mgl-1
mgl-1
mgl-I
Minimum
Mean
6.8
0.10
2.6
0.08
0.72
3.72
<0.004
<0.002
<0.006
0.06
<0.02
0.0010
<5.0
7.5
< 1.5
1.30
0.64
0.025
5.2
7.8
1.42
12.4
2.04
6.29
32.4
0.049
0.14
0.122
0.47
0.17
0.011
96.4
1360
72.9
12.1
2.42
0.26
18.1
riparioides
Maximum
8.7
7.12
87.8
14.8
60.0
90.4
0.75
0.58
1.62
3.32
0.74
0.178
1990
31900
3180
80
9.9
1.30
155
from mining and industrial sources. Further discussion of the ecology of this species
has been given previously (Wehr & Whitton, 1983a, b).
During the course of the study it became apparent that populations with certain
morphological features were characteristic of certain types of stream. Robust forms
with larger leaves and longer stems were observed most frequently in springs and
upland streams of calcareous
districts,
particularly
in Arkengarthdale
and
Swaledale (regions IX and X, Fig. 1). The opposite extreme, with smaller leaves
and a flaccid texture, was found mostly in larger rivers, especially those which
might be regarded as eutrophic (elevated Na, NH4-N, reactive P04-P). This environmental pattern was not, however, entirely consistent. For instance, a distinctly
non-robust population was collected from an unpolluted, soft water stream in the
Lake District. The groups identified by cluster analysis do have some evidence of
geographical trends. The smallest, non-robust Group D consists of populations of
which 800/0 are from streams and rivers west of the Pennines (i.e. regions XII-XIV,
Fig. 1). In contrast, only two populations from these western regions were grouped
in either clusters A or B (40/0 of total).
The significance of the relationship
between morphological
variation and
environment was considered further by correlating all continuous morphological
variables (plus proportionate
length of marginal denticulation of the leaf) with the
chemical data (Table 4). A derived variable, the ratio of aqueous Na:Ca, was included in the correlations.
Very few of the environmental
variables were significantly correlated with variations in shoot length, leafy length of the primary axis.
The characteristics
which correlated most strongly were the average weight of a
2 cm tip, leaf length and marginal denticulation.
All three characters were negatively correlated
(highly significantly) with aqueous Na, NH4-N, N03-N and
VARIATION
IN RHYNCHOSTEGIUM
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278
JOHN D. WEHR AND BRIAN A. WHITION
reactive P04-P. Although preliminary observations had noted robust, large-leaved
plants in many calcareous districts, correlation of morphological
characters with
aqueous Ca were mostly nonsignificant.
However, correlations with the ratio of
Na:Ca were all greater than for Na alone. The heavy metals Zn, Cd, Ba and Pb
apparently had little effect on plant morphology, even though several sites were
grossly polluted.
Published by Maney Publishing (c) British Bryological Society
General comments
It is hardly surprising that a species which is ecologically so widespread should be
morphologically variable. This has been observed widely among terrestrial mosses
(Longton, 1981; Schofield, 1981). It is of course impossible to decide whether the
variation is genotypic or ecotypic without experimental studies such as reciprocal
transplants. Many of the habitats where R. riparioides occurs have been profoundly
influenced by the activities of man. R. riparioides does, however, contrast with the
other species of moss which occurs most widely in European rivers, Fontinalis
antipyretica (Say & Whitton, 1983), for robust plants of the latter are common in
nutrient-rich waters.
Comparison with other species
In view of the importance of Rhynchostegium riparioides for monitoring purposes,
some practical comments are included to aid in identification.
Several characters
were found to be relatively stable and here are apparently independent of a broad
range of ecological conditions. In our experience, taxonomic confusion is most frequent between R. riparioides and either Amblystegium riparium or Hygrohypnum
ochraceum. Table 5 gives a summary of 'typical' characters based on this study,
supplemented by several floras. Among gametophytic characters in Rhynchostegium
riparioides the most constant is the presence of rectangular chlorophyllose cells in
Table
5. Comparison of 'typical' characteristics of gametophytes and sporophytes of Amblystegium
riparium, Hygrohypnum
ochraceum and Rhynchostegium
riparioides
Character
Amblystegium
riparium
Hygrohypnum
ochraceum
flaccid-intermediate
leafy throughout
divergent ± complanate
flaccid
leafy throughout
imbricate
robust or rigid
denuded below
imbricate
lanceolate or ovate lanceolate
entire
ovate - oblong
ovate to broadly
ovate
strongly denticulate
Rhynchostegium
riparioides
Plants
Texture
'Leafiness'
Leaf arrangement
Leaves
Shape
Margin
Costa
Angular
cells
single;
pellucid
112-3/4
length
entire or minutely
denticulate
single or double; ~
inflated, auriculate
112 length
single; >3J4Iength
chlorophyllose
Sporophytes
Season
Operculum
spring - summer
conical
?; very rare
conical
autumn - winter
rostrate
Published by Maney Publishing (c) British Bryological Society
VARIATION
IN RHYNCHOSTEGIUM
279
RIPARIOIDES
the angles of the leaves. These are unlike those of either of the other mosses
mentioned above. Another constant character (not part of the analysis) is the leaf
arrangement
in Rhynchostegium riparioides which is radially symmetrical even
when the plants are flaccid, while in Amblystegium riparium it is sub-complanate
to
complanate.
A useful character of Hygrohypnumochraceum
which removes any
possibility of confusion with flaccid plants of Rhynchostegium riparioides is the presence (in the former) of inflated hyaline cells in the epidermis of the stem (viewed
in cross-section);
this character is considered
diagnostic by North American
taxonomists (e.g. Crum & Anderson, 1981; Ireland, 1982).
Some characters,
even when variable, may still be considered
diagnostic.
Narrower leaves which lack denticulation
cannot be regarded as a fully reliable
taxonomic character in Amblystegium riparium, since these occur occasionally in
Rhynchostegium riparioides. However, leaves which are broadly ovate and have
strong marginal denticulation never occur in the former, so are positive characters
for R. riparioides. Finally, when present the sporophyte, with its rostrate operculum,
is also distinct from other similar aquatic species.
SUMMARY
A study was made of morphological variation in Rhynchostegium riparioides from 105
sites in 71 different streams and rivers; 15 gametophyte characters were scored for each
population. Nineteen water-chemistry
variables were also measured· at each site.
Variation was observed in the size and robustness of the plants, dimensions and shape
of the leaves, the degree of denticulation and relative length of the nerve. Cluster
analysis revealed broad morphological groups, with low internal similarity. These results do not support the recognition of subspecific taxa. Correlations with environmental variables showed that plants were significantly less robust, smaller-leaved and with
weaker denticulation when collected from nutrient-rich water. In view of the wide
morphological range shown by this species and its importance for monitoring heavy
metals, a comparison with other widespread aquatic mosses is given.
ACKNOWLEDGMENTS
This research was carried out under contract DG R/480/571 (Bryophytes
for
monitoring river water quality) from the Department
of the Environment,
U.K.
We are grateful for assistance in the field from I. G. Burrows and D. A. Donaldson
and for stimulating discussion with Dr A. Empain (formerly University of Liege,
now Jardin Botanique de I'Etat Meise, Belgium) and Dr P. J. Say (formerly University of Durham,
now Northern Environmental
Consultants
Ltd, Consett).
Thanks also go to R. C. Bailey (University of Western Ontario, London, Canada)
who provided considerable
assistance with statistical analysis. Professor W. B.
Schofield (University
of British Columbia,
Canada)
provided
many useful
comments on the manuscript.
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J. D. WEHR (Present
address.)
B. A. WHllTON (To whom
Department
of Biological Sciences, Fordham
Bronx, New York, 10458, U.S.A ..
reprint requests should be sent.) Department
Durham, South Road, Durham DHI 3LE.
University,
of Botany,
Larkin
Hall,
University
of