3°°
EXPERIMENTS WITH THE STIPES OF FUCUS AND
LAMINARIA
BY
E. MARION DELF (MRS PERCY SMITH), D.SC. (LOND.), F.L.S.
(Received znd February, 1932.)
(With Two Text-figures.)
INTRODUCTION.
MARINE algae may be briefly characterised as either intertidal or totally submerged
forms.
The intertidal algae have a dual existence: they are land plants at low tide and
aquatics when covered by the sea. The proportion of time spent in the aerial or
aquatic medium must vary with the position on the shore and the factors influencing
tidal periodicity. The more exposed plants must have a tolerance for various light
intensities and for a wide range of conditions affecting water loss; and they must also,
if fixed to the substrate, be able to withstand more or less violent movements of the
water. These movements may be partly due to currents and partly to waves, frequently reinforced by wind.
Currents are said to have little influence on littoral plants, but wave action is
certainly an important factor, influencing the mechanical stresses and strains to
which the plants are subjected. These stresses differ from those to which land plants
are exposed in being predominantly a series of intermittent pulls or jerks, applied to
plants with little initial rigidity and great flexibility.
In the experiments about to be described, an attempt is made to determine the
" ultimate strength " (p. 302) and the elastic properties of the stipes of several of the
larger brown algae, differing in habit, habitat and construction. These may be
briefly indicated as follows: (a) two plants of closely similar structure but different
positions on the shore, ex. Fucus serratus L., Fucus vesiculosus L.; (b) two plants of
somewhat similar structure and corresponding positions on the shore, ex. Fucus
serratus L. and Ascophyllum nodosum Le Jol.; (c) comparison of plants of dissimilar
habit and construction, ex. Laminaria digitata1 Lamour., Fucus, and HaUdrys siliquosa Lyngb.
The strength of a material is usually measured by testing the pull, the compression or the torsion which it can sustain. For the present purpose, the first was the
method selected.
When a steel wire is moderately loaded, a slight stretching results and on removing the load the wire will return to its original length. By increasing the load
1
Laminaria digitata appears to be a somewhat ill-defined species, and it is probable that two of the
stipes used were in reality hybrids between the typical L. digitata and L. Clomtord. I am indebted
to Dr V. M. Grubb for the identification of the doubtful cases.
Experiments with the Stipes of Fucus and Laminaria
301
progressively and removing it between each addition, a point is found at which the
length is permanently increased. The limit at which this occurs is the limit of perfect
elasticity (the R limit of engineers). If, instead, a wire is continuously loaded, for each
added weight there is a corresponding slight increment of length which is proportional to the increment of weight, until a point is reached where this proportion is
exceeded. This is the limit of linear elasticity (the so-called P limit), and is higher
than the R limit. Further increase in the load gives a point at which a slight added
load gives a sudden large extension—the yield point—and a further increase of load
will soon induce rupture.
There are thus two main phases in extension under pulling strain, first that which
is within the elastic limits (P and R), and secondly that in which the extension may
be relatively much greater, leading to the final rupture. The latter phase is well
marked in ductile metals, but is only slight in brittle materials. A plant tissue is far
from isotropic and is therefore not strictly comparable with a wire, but we may
expect to find a certain parallelism in its reaction to mechanically imposed strains.
When wires are stretched to breaking point, it is found that in hard non-ductile
metals such as steel the fracture is a clean break transverse to the length. In ductile
metals on the contrary, the substance yields by shearing on an inclined plane, often
with a ring-like crater on one side and a truncated cone projecting on the other side
of the fracture. It is interesting to note that in these experiments both types of
fracture have been found; the clean break with a straight pull on Laminaria where
the structure is typically parenchymatous, but the truncated cone and irregular surface of fracture in both the species of Fucus examined, where the central core of
filaments was the last to yield. It was at first thought that this would be correlated
with the difference in structure, but in AscophyUum a clean break was nearly always
found, and here the structure resembles that of Fucus. The difference may be in the
fundamental nature of the cell walls.
EXPERIMENTAL PROCEDURE.
Specimens were sent fresh from Plymouth with its sheltered harbour and from
Aberystwyth where the coast is more exposed. Precautions were taken to keep the
material moist during handling, and in the more protracted determinations (e.g. of
elasticity) the stretching strip was moistened between the readings. It was found that
by keeping the material on ice covered with wet cotton-wool kept cool by ice, the
plants could be kept fresh for several days. For the most part the plants were wellgrown adults, and all the plants of a consignment were used excepting any obviously
diseased or injured. As far as possible the whole stipe was used—ije. the region
between the hapteron and the lowest node or dichotomy. In Fucus vesiculosus the
stipe was sometimes too short for fixing, and then the lowest practicable region was
used, and the same was always true for AscophyUum andHalidrys: in these cases,
the lowest available part of the frond was pulled, but there is little differentiation
between frond and stipe in this region. The stipes of the larger specimens of
Laminaria were too strong for any weights at my command and had to be split
302
E. MARION D E L F
longitudinally into four or more parts. Each part was then broken separately and
the total load of all equated to the area of cross-section of the whole stipe. With the
stipes of Fucus, this method was unsatisfactory, for the split stipes gave values quite
incommensurate with their expected strength, probably owing to the inevitable
cutting of many of the medullary filaments. The stipes of the strongest specimens
of F. vesiculosus were thus too tough for estimation with the means at my disposal.
In order to estimate the strength of a stipe, a scale pan was attached by a strong
wire passing over a steel pulley to a grip which held one end of the stipe, the other
being firmly held in a vice screwed to the edge of a bench. Weights were gradually
added to the scale pan, with intervals of 30-60 sec. between each addition, until at
last a rupture was produced. The area of cross-section was estimated from previous
measurements, with a micrometer screw, of the diameters in two principal planes
taken at each end of the stipe within the region needed for fixing. The load per unit
of area of cross-section which just produces fracture with continuous loading may be
called the "breaking stress"; it corresponds with the "ultimate strength," a term
applied to certain materials used by engineers, and it just exceeds the "tenacity" or
greatest longitudinal stress which can be borne per unit of area icithout rupture. In
reckoning the load, the weight of scale pan, wire and grip was included.
On mechanical grounds one would expect that the breaking stress would vary
with the method of application. In my experiments, the initial load was small, usually
2 or 4 lb., in relation to the final load expected. Thereafter weights were added
cautiously, without jerking, in increments of 2 lb. or 1 lb. With slender stipes, such
as that of Halidrys, ounce weights were used and finally loose shot. Occasionally the
load would be jerked off by a sudden slip, and on re-setting a premature break
nearly always occurred. This is in accordance with the well-known mechanical
principle that a strain set up by a sudden application of force is much greater than
(and may be double) that of the same force applied gradually.
A point of practical difficulty was to get a firm enough grip of an end without
actually crushing the tissues. At first the more slender stipes of Fucus and Ascophyllum were set in cubes of plaster of Paris to avoid compression, but the plaster itself
broke with loads of 10-15 lb. and the vice was then used direct, the rough inner surface of its jaws being carefully protected with a layer of sheet rubber and the end
closely wrapped in cotton-wool before insertion. It became a matter of experience to
judge the pressure required to give a firm hold without crushing. In the latter case
a very slight load was sufficient to cause sudden rupture, and the experiment was
then ignored. With practice it was also possible to set the stipes so as to avoid torsion;
unless this was done, the resulting fracture was oblique and was apt to be premature
if the torsion was at all considerable.
The variability in area of section was a difficulty in calculating the mechanical
stresses involved. Mostly there was a more or less elliptical cross-section tapering
slightly in the upward direction (Fucus, Laminaria) or downwards (Ascophyllum).
The diameters in two planes at right angles were measured in advance, near the
upper and lower ends, where it would clear the supports, and when the fracture occurred midway between the two ends, the mean area was obviously a suitable basis;
Experiments with, the Stipes of Fucus and Laminaria
303
but when the break occurred definitely at either end, the area of section at that end
was used. The basis is thus the nearest approximation to the original cross-section
at the point of fracture; not to the sectional area after the strip has been stretched (as
was done by Willecs)). In Fucus and Laminaria, however, the diminution in crosssection due to stretching is so slight that it would make no appreciable difference to
the results.
Separate determinations of variability of area show that so long as the whole area
is not too far from the standard area (1 sq. cm.) the error involved lies within the
order of accuracy of the experiments. In the case of Halidrys, however, this cannot
be claimed, and the error may reach 30-50 per cent, on the load per unit of area.
In determinations of the elastic limits and total extensibility, a similar arrangement was used, but for this purpose the application of each load was followed by a
measurement of the distance between two marks; the strip was then released from
the load and after an interval of 30-60 sec. the relaxed length was again read. The
same time was allowed for the stretching after loading as for the relaxing after unloading. Ultimately by this intermittent method the breaking point is also reached,
but this is considerably below the value as found by the method of tension under
continuous loading, as is also the case for inert materials when loaded and unloaded
repeatedly at short intervals beyond their elastic limits.
Very few observations appear to have been made on the strength of the fronds
and the stipes of the marine algae. The work of Wille (5) in the Norwegian language
includes some observations of the kind1, but his experiments were mainly planned
for finding the elastic limits and he used almost entirely the intermittent method of
loading. Actually his figures are not very comparable with mine, for he used thin
strips of tissue with a sectional area of 2-3 sq. mm., and no details are given as to how
the strips were selected, for instance, from among the different tissues of the stipe of
Laminaria, or Fucus, where there is considerable differentiation in the inner and
outer regions. The well-known experiments of Schwendenertj) are not concerned
with the marine algae.
CALCULATION OF BREAKING STRESS.
Each type examined exhibited a considerable variation in the breaking stress. It
is probable that a part of this variability may be due to assignable causes, but in the
first instance an attempt has been made at comparison on general lines. For this
purpose the individual values were graded into strengths of 10-19, 20-29, 3°~39>
etc. lb. per sq. cm., the percentage number of cases in each group being then calculated (cp. Table I). By expressing these values as percentages of all the cases and
plotting graphs, using these percentages as ordinates and the corresponding strengths
as abscissae, a mean value wa3 found such that 50 per cent, of all cases examined are
stronger and 50 per cent, of all cases are weaker than this mean value. The correction
for the mean was found in the usual manner and the standard deviation {a) calculated
1
I have to thank Miss Astrid Karlsen of Bergen for help with the relevant matter when she was in
England.
304
E. MARION D E L F
as the square root of the variance (Fisher (o). The errors of the mean (cr/yn) and of
the standard deviation (cr/Vzn) were then calculated and are shown in their appropriate
columns (Table I).
Table I. Breaking stress o/Fucus serratus.
Breaking
strength
lb /sq. cm.
10-19
20-29
3O-39
40-49
SO-S9
60-69
70-79
80-89
90-99
100-109
no—119
120-129
130-139
140-149
Total number
of cases =
No. of
cases
( - firequency)
Total
Frequency
from
as % of
all cases beginning
0
0
1
0
4
4
4
1 x 63
4
8*
1 x
20
16
24
28
4
72
3
12
1
2
0
1
t
84
88
96
0
4
100
5
0
1
Variance
-FxD*-25
Frequency (F)
x deviation (D)
33
5 x 13
ix 3
6x7
=-63
3969
- 33
1089
-65
845
36
— 12
1 x 17
3 x 27
1 x 37
2 x 47
+
+
+
+
+
1 x 67
+ 67
4489
Algebraic
Sum
Total
- 16776
Variance
-671
25
42
17
81
37
94
I+338
= 165
204
289
2187
1369
2209
Correction for mean
> •&
= 6-6.
25
Standard deviation (cr)
• V671 - 2 5 - 8 .
-8
Standard error on deviation
= £ = 3-6.
V2 x n T<yj
Standard error on mean
= -p.
5
Vn
This procedure was adopted for each type examined and the results are summarised in Table II. It will be seen that although the numbers examined are somewhat low for statistical purposes, there is sufficient uniformity to justify the treatment
and grouping1. A typical example from one set of experiments is seen in Table I.
The individual figures in the two species oiFucus did not warrant any separation
on grounds of locality (sheltered, Plymouth, exposed Aberystwyth), stronger and
weaker stipes being found equally in plants from both localities. For the most part
the relative strength of these plants seemed independent of diameter. Thus in F.
serratus of 25 experiments, 14 plants had areas of cross-section at the breaking region
varying from 10 to 25 sq. mm. and the range of strength within these was about the
same as among those with much stouter or with more slender habit. In the experiments with F. vesiculosus, the majority of the plants had sectional areas of 10-25 S1mm. and the range of strength was also evenly distributed as compared with those
1
Excepting in the case of Halidrys.
Experiments with the Stipes of Fucus and Laminaria
305
remaining. In these plants, thickness may be some indication of age, for the fronds
usually last more than one season and there is progressive increase in diameter.
The dichotomies of the thallus in Fucus frequently occur so low down that the
stipe may be only 2-3 cm. in length and in such cases it was found impossible to fix
in the apparatus; thus the lowest internode above it was used for the experiments.
This occurred in F. serratus (Nos. I, XXI and XXII) and in 9 plants of F. vesiculosus, always in old bushy plants. Several attempts were made to compare the
breaking stress at the two places, but mostly the plants were unsuitable, the lengths
of each segment being so short that in using either, the other was crushed too much
in the vice or clamp for subsequent testing. In the two examples of F. vesiculosus,
where the comparison was made with apparent success, the breaking stress of the
stipe was much less than that of the next internode above it (No. XXIII, stipe 59,
internode 115; No. XXVI, stipe 43 and internode 120, lb. per sq. cm.). This was at
least in part due to the tearing away of the cortex in the region of the disk so that
the medullary filaments were torn in part from thefirmlyheld disk like the wick from
a short end of candle; and it seems that unless the vice can be fixed well above the
expanded part of the disk there is no indication of the full mechanical strength1.
Most of the plants of Laminaria were in the adult stage and fertile, with the
stipes about 40 sq. mm. or more in cross-section. If we exclude three cases of young
plants, there was no apparent correlation between area of section and ultimate
strength. For instance, the largest examined, of sectional area 207 sq. mm., had the
same strength (102 lb. per sq. cm.) as another of only 75 sq. mm. in sectional area.
The three young plants which seemed exceptional had sectional areas of 30 sq. mm.
or less, and their breaking stress was between 17 and 25 lb. per sq. cm. Two of
these plants were collected from Seaford in April, where the conditions are so unfavourable that although young plants were numerous, few were found of full size.
They were kept for 3 days moistened with sea water before use and were among the
first used in this series of experiments. They were so different from subsequent
plants received from Aberystwyth that they were disregarded, until much later
another young plant of similar dimensions was sent which with careful handling gave
the same low figure of 25 lb. to the sq. cm. This suggests that the young plants of
L. digitata are for a time at least much less mechanically efficient than at a later stage,
but more evidence is clearly needed.
The two plants of HaUdrys gave ten experiments with the slender much-branched
thallus.
Ignoring all but those experiments in which the breaking stress was determined
to within an ounce, the two plants of Halidrys gave eight values varying from 127 to
179 lb. wt. for the breaking stress, taken at the lowest region of the slender branches,
near the disk. At first sight this might appear to have some significance, although
from the statistical point of view it can give little or no information, but on careful
1
Himanthalia lorea is peculiarly suitable for such a purpose owing to the long straight internodes,
and here a marked diminution has been demonstrated not only between successive internodes of an
old plant over 2 metres long, but between the upper and lower regions of the same internode; and this is
what must be expected to occur on general grounds elsewhere, though presumably to a less extent in
the much less elastic tissues of Fucus.
306
E. M A R I O N DELF
measurement the diameters along the short lengths used varied by as much as i-8 per
cent, in one and 2-7 per cent, in another, the latter being the least and the former the
most regular of the pieces used. Since the total area of section was of the order of
5 sq. mm. or less, the most favourable cases (129, 167 lb. wt. with sectional areas
5-78 and 4*78 sq. mm. respectively) when reduced to the common basis (129, 167 lb.
wt. per sq. cm.) have an error of at least 30 and possibly 50 per cent. Unless therefore material can be found with stipes of more uniform diameter, this plant can
hardly be compared on the present basis.
Table I I . Breaking stress calculated as lb. per sq. cm.
Plant
-F. vesiculosus
(a)
W
F. serratus
A. nodosum
L. digitata
(d)
to
H. siliquosa
No. of
plants
examined
30
39
25
25
8
20
17
2
Mean breaking stress
Unco rrected
Corrected
90
103
100-3
109
116
90
us
83
77
98
104
83
92-4
119
Standard
deviation
Error on
a
5-5
3S-2S
46
S-4
27-2
2 S -8
107
3-8
3-6
IO-I
28-3
41
57
4-4
25-2
18
4-0
31
Correction
for mean
94
i4S
F. vesiculosus. (a) Using stipes only. (A) As (a) but including also 9 plants where lowest internodea
only could be used, (c) As (6) but excluding all cases of slipping.
F. terrains. Including five cases where only internodes could be used and two cases of slipping.
A. nodosum. Twenty-four experiments were performed with these 8 plants.
L. digitata. (d) All cases, (e) As (d) but excluding three cases of doubtful significance.
H. siliquosa. Ten experiments were performed with these plants.
If we consider the figures in the table as approximately true for the samples of
plants concerned, the following conclusions may be drawn:
(1) The stipes of F. vesiculosus appear to be appreciably stronger than those
of F. serratus. This is probably underestimated, for the strongest plants were unbreakable with the resources available and so were omitted. It seems reasonable to
suppose that the lowest internodes were not very different in strength from the
stipes immediately below them, and if so, the inclusion of these should not seriously
affect the comparison.
(2) F. vesiculosus occurs higher on the shore than F. serratus, but the difference in
strength cannot be correlated with the difference in habitat, in the light of other
evidence.
(3) The strength of stipe of F. serratus is appreciably greater than that of the basal
region of the larger thongs of A. nodosum. Both are surf plants occurring in similar
positions on the shore, but Ascophyllum favours more sheltered positions than
F. serratus.
(4) The adult stipes of Laminaria are about as strong as those of F. vesiculosus.
Experiments with the Stipes of Fucus and Laminaria
307
The latter occurs at the upper tidal zone, the former at the lowest of the intertidal
stretches. The former is a prostrate plant mostly emergent, the latter has an erect
stipe even during its short periods of exposure.
(5) Halidrys has a tough thallus of considerable strength, but on the present
evidence it cannot be compared numerically with the other types. It occurs chiefly
in pools towards the low tide levels and is subjected to little exposure, often remaining covered (at least in the south) at low spring tides.
ELASTICITY AND EXTENSIBILITY.
When a wire is loaded beyond its elastic limits, the total extension per unit length
produced by the load gives a measure of its extensibility. For each total extensibility
there is also a small amount of permanent extension found by measuring the length
after release from the load.
Some measurements of the elastic limits and the extensibility of thin strips of
tissue were made by Wille(s) and are quoted for comparison.
Table III. Elasticity and extensibility according to Wille ((5), pp. 8, 9).
Plant
Laminaria saccharina
L. digitata
SarcopkylUt edulit
Porpkyra lacirdata
Aspidistra lurida
Region
tested
Middle
Lamina
Extension, cm. per ioo cm.
Temporary
3
S7
11
Petiole
Permanent
Total
9
127
7-1
3-i
71
26
28-5
33
25
In another place, Wille states that in Laminaria a load of 50 gm. per sq. mm.
(about 2-2 lb. per sq. cm.) exceeds the limits of elasticity, and involves an extension
of 3 per cent. Also that a strip of L. digitata can be stretched until it increases by
48 per cent, of its length with a permanent lengthening of 25 per cent. It is not clear
whether this refers to the stipe or to the lamina, but presumably it is to the latter.
In my earlier experiments on the ultimate strength of Fucus and AscophyUum a
marked difference was noted in the amount of stretching before fracture occurred
and a few measurements were made. The latter behaved somewhat like the Laminaria
described by Wille, having a high total extensibility (about 20 per cent.); the former
had a much lower extensibility, about 5 per cent, or in one young plant as much as
9 per cent.
The record of a typical experiment is appended (p. 308).
The method of intermittent loading has already been described (p. 303). In
F. serratus, No. XXVII, two white marks 4 cm. apart were made on the stipe such
that they could be measured with a small scale when in position for stretching. The
successive loads are shown in column 2; the corresponding increments in length
measured 1 min. after loading and calculated as a percentage of the original length
are in column 3. Weights, scale-pan, wire and grip were then removed, and the distance between the marks again read after a minute of relaxation. This was repeated
3 o8
E. MARION
DELF
until the stipe broke; which happened suddenly, the fracture occurring just beyond
one of the white marks, so that the final length could be read. The limit of tensile
strength in this plant is very near the limit of elasticity; in Ascophyllum, on the other
hand, there is possible a total extension of 2-3 times that at the elastic limit before
fracture occurs (Table V). Subsequently these observations were extended to other
plants of which the breaking stress was being determined.
Table IV. Extensibility of Fucus serratus. No. XXVII.
Bushy plant with short stipe; used lowest internode; sectional area at break 27-27 sq. mm.
Total
Load
Length
cm.
4
4
4-°5
cm./ioo cm.
lb /sq.cm.
lb.
0
5 49
3-5
5-5
410
5
T
8-5
4-15
4-17
4-18
4-20
4 20
5 0
5°
4-05
1-25
3'75
31-11
4-25
450
42-02
—
0
0
0
0
0
0
27-45
105
—
40
4-0
4-0
4 0
40
4-0
2'5
"•5
cm./ioo cm.
0
12-81
24-13
34-77
38-43
95
Permanent
Relaxed
length
1-25
Table V. Limits of elasticity and extensibility.
Plant
No.
Length
cm.
Extension at
elastic limit
(BX.) cm. %
Temporary
F. serratus
XXV
XXVI
XXVII
Ascopkyllum
Laminaria
Halidrys
Aucuba
Aspidistra
EXa
1X6
XVII
XIX
III
IV
I
II
Permanent
i-94
3-25
4-0
8-2
5-25
i-5
0
5°
1-25
3-1
806
10-5
4-8
664
161
2-8
10
6
4-8
4-0
i-45
0-65
4 0
i-8
Load
lb./cm.'
At
EX.
148
97-5
38-4
391
27-2
14-85
2665
5-2
6-25
—
0-15
i-66
1 04
2-5
—
56
78-1
107-3
i-o
0
221
i-o
0
221
Maximum extension (M.E.) cm. %
At
M.E.
158
97-5
42-9
75'9
40-0
55-5
51-2
144-9
122-8
107-3
154
551-2
Temporary
Permanent
93
i-5
0
125
5-25
50
216
27
—
13-12
426
5
4-16
162
129
18-75
138
231
i'37
—
8-75
10-3
I5-3
—
In order to compare the extensibility of the different types, a number of graphs
have been made in typical cases, plotting the extension per unit length against the
load per unit area. The course of these graphs reveals in a striking way the contrast
between the behaviour in the different plants (Fig. 1).
In Wille's account, Ambronn's figures are quoted for Aspidistra htrida (cp.
Table I) as being provided with collenchyma around the bundles which are the main
source of mechanical strength. I made a few observations on the common Aspidistra
80
100
120
140
160
180
200
<.
220
I
240
I
260
I
Graphs showing c o m e of extension under intermittent loading. Each curve repreaenta an actual experiment, excepting that
of Aucuba (Auc.)where it 13 the mean of two.The point of fracture is indicated thue
I.
60
I
Fig.
40
I
._-
ASP:
,
.- - - - --
ABSCISSE LBS PER SQ CM.
ORDINATES CMS PER 100CM.
20
I
,
0
ASC: IX b
ASC:IXa
b.4
P,'
5
E!
r
3
FL
Q
Q
0
F
%
9
2
9
C:
1
'-3
310
E.MARION DELF
cultivated in England as pot plants, but an adult leaf in the prime had bundles
heavily encased in lignified fibres. These strands were always the last to break. In
one petiole of sectional area i3-3i4 sq. mm., a load of 36-5 lb. gave an extension of
only 0-5 mm. on an initial length of 4 cm. and was then within the limits of perfect
elasticity. The breaking stress was found by cutting the petiole vertically into strips
and handling each strip separately. The total load thus carried was 73-5 lb., i.e.
about 552 lb. per sq. cm.1, a very different result from that of Ambronn for his
strips of Aspidistra lurida, and typical of lignified tissue; thus the tensile strength of
the oak is said to have a value varying from about half a ton to one and a half tons
per sq. cm. (Llandolt and Bornstein(a)).
A difficulty in these experiments is the lack of uniformity of the tissues over any
considerable length. In the stipes of Fucus, the behaviour seemed uniform over the
length, which was necessarily limited; but in Ascophyllum, the region below the
lowest node (often 5-10 cm.) exhibits a visible difference in elasticity, one end of
the strip stretching much more than the other if strips longer than 3-5 cm. are used.
The same applies to Laminaria (cp. No. XVII, Table V), where most of the elongation in the piece occurred in the morphologically upper 5 or 6 cm.; the percentage
is thus too low as calculated. On the other hand, with shorter lengths, any error in
reading the marks becomes correspondingly magnified in calculating percentages.
However, in spite of these difficulties, it is clear from the figures in Table V that
there are two types of behaviour represented, the one seen in Fucus which has a
slight total extensibility but generally a high tensile strength, the other seen in
Ascophyllum, and to a less extent in Laminaria and Halidrys, in which the maximum
extension is considerable, the load required to produce the extension being variable,
and least in Ascophyllum.
Within the limits of elasticity the extensibility is about the same (i.e. about
5 per cent.) in Fucus (stipe and lowest internode), Laminaria (stipe), and Halidrys
(base of thallus). In Ascophyllum, the extreme base of the thallus has an extension
within the elastic limits of 8-10 per cent., ije. nearly double that of the others.
As the extension beyond the elastic limits is such a marked feature of the tissues,
a further comparison has been made, namely between the extensibilities at hah0 the
total maximum extension, the values being read from the graphs of extension in
Fig. 1. These values are given in Table VI.
If we compare these algae with some inert substance such as steel, we find a great
contrast in strength, but a curious resemblance in their elastic properties. According
to the literature, the tensile strength of steel is very variable, mild steel having an
ultimate strength of about 3 tons to the sq. cm., steel wire 11-16 tons per sq. cm. and
piano wire as much as 23 tons to the sq. cm. The range of extensibility is much less,
apparently varying from 15 to 30 per cent, on the original length, when measured
at the limit of perfect elasticity. This is much nearer to that of the algae at the same
elastic limits than would be expected from the excessive difference in their tensile
strength. Moreover, the algae examined (excepting Fucus) have a considerable
1
This is rather more than the value given for cast lead, viz. 484 lb. per sq. cm., according to Sir
William Thomson (4).
Experiments with the Stipes of Fucus and Laminaria
311
capacity for extension beyond the elastic limits. In inert material, it is the elastic limits
which largely determine how much load can be borne without permanent strain, but
in marine algae, capacity for extension without injury beyond these limits may also
have a biological importance.
Table VI. Linear extensibility (at half maximum extension) in relation to load
producing it.
Plant
Load (L)
lb./an.1
Extensions
(E) = cm./ioo
cm.
Ratio
E/L
Mean
• F . serrattu
XXV
XXVI
XXVII
A. nodosum
105
78
2I-S
4-65
0-044!
0-033)
o-n
0-038
68
36
16-25
131
0-300
31
8-66
0-239}
0-361/
0-28
76
7-25
10-25
0-625
0-095)
0-093)
0-003
0-091
0-094
DCfl
DC*
L. digitata
Aucuba
I
II
Atpidittra
H. siliquosa
no
207
76
2-61
2-S
69
Ratio of
means
13
37
100
93
3i
1
3°
• Nos. XXV, XXVI, stipes used; No. XXVII, lowest internode at base of thallus.
INFLUENCE OF TIME.
When a load is stretching a wire within its limits of elasticity, the resulting stress
can be borne for an indefinite period, but when these elastic limits are sufficiently
exceeded, the time factor becomes of importance and a slow gradual extension may
result, leading to ultimate fracture.
Ambronn states that he took a "strip of collenchyma" (Wille(5), p. 10) and hung
on a weight which he estimated exceeded the limits of elasticity for 2-3 days and the
extension at the end was not greater than in the first hour. Wille, performing similar
experiments with Laminaria digitata and other seaweeds, says: " I hardly believed
my own eyes when I found that a weight large enough to procure permanent
lengthening did not cause further extension if left on for a long time."
So far as I have been able to look into this question I can only conclude that the
length of time which an overstretched wire or tissue can hold a weight depends greatly
on the extent by which the elastic limit is exceeded, but that this extent is an illdefined quantity to be determined within wide limits only by practical experience.
The graphs in Fig. 1 show something of the kind for the thallus of Ascophyllum
nodosum where in the first case (IXa) the load held safely for an hour and in No. 1X6
where a relatively smaller load (but also beyond the elastic limits) held for 45 hours
and then suddenly yielded, giving a clean transverse fracture. In Fig. 2 the slow
extension which occurred—mostly in the first few hours—is plotted against time.
From the biological point of view (as was suggested by Wille), this capacity to
312
E. M A R I O N
DELF
endure excessive strains in nature allows time for the recovery, e.g. after a storm, or
for the strengthening of the stretched walls by the deposition or intussusception of
new particles. The same would doubtless be true in the intertidal periods between
exposure to direct wave action, and it may be that we have here the biological equivalent of the well-known effects of " tempering" in metals by repeated blows.
On the whole, the elastic properties oiFucus are characterised by tensile strength
rather than by extensibility, in this resembling the lignined tissues of the higher
plants. Ascophyllum and Laminaria exceed in extensibility the petiole of Aucuba
which has collenchymatous mechanical tissue. Halidrys resembles Laminaria in
elastic properties rather than Fucus which it resembles in structure. It thus 1 seems
that the strength and elastic properties of these marine algae reside in the nature of
their cell walls rather than in the nature or arrangement of their tissues—possibly
in the proportions of mucilaginous material in their composition.
30
25.
ABSCISS/E_HRS
ORDI NATES _ CM S/lOOCM
15
10
20
30
40
50
Fig. 2. Graph showing slow extension in Atcopkyllum nodosum, No. TXb loaded beyond the
elastic limits (40 lb. per sq. cm.) for 45 hours. Compare dotted vertical line in Fig. 1.
CONSIDERATIONS OF FORM AND STRUCTURE.
Externally Fucus, Ascophyllum and Halidrys are similar in possessing a wellmarked discoid hapteron and a much divided, more or less strap-shaped frond, so
pliant as to remain prostrate when exposed by the tide. In all three the disk is
composed of branching filaments interwoven together, some in the vertical, others
in the transverse direction. In Fucus and Halidrys, the disk is compact, but in
Ascophyllum the disk is irregular in shape and splits easily in the vertical direction,
between the bases of the individual thalli.
The structure of the thallus is characterised by a well-developed filamentous
medullary tissue; it has been likened to a hempen rope, but it differs from a rope in
having numerous filaments interweaving between the vertical strands, the whole
being intricate but loosely woven together, giving the great flexibility of the typical
surf plant. When one considers how the surf plant is thrown about, twisted and
tossed by the waves, it is evident that there must be a strong shearing force developed
between the cortex and medulla, and it is here that we may expect the transverse
filaments to play an important part. This shearing force tends to increase with
greater development of the lamina. It may become very considerable in Fucus, for in
1
The suggestion that the elastic properties arise from the nature of the cell walls was made by
Prof. T. G. Hill when the substance of this paper was read to the Society of Experimental Biologists,
December, 1931.
Experiments with the Stipes of Fucus and Laminaria
313
many of the well-grown specimens used the number of dichotomies varied from 300
to 500, without counting tufts of small outgrowths due to secondary proliferations.
Ascophyllum and Halidrys differ from Fucus in combining flexibility with a much
greater extensibility.
The adult plant of Laminaria differs from most other marine algae in possessing
a stipe sufficiently rigid to stand erect when emergent at low water. The stipe is
supported at the base by a series of outgrowths or crampons, and these bear slender
finger-like extremities which find their way in all directions, fitting accurately into
crevices by the unequal growth of the surfaces at points of contact with some
substrate. Both crampons and their finger-like extremities become firmly cemented
to the substrate by the outgrowth of short hairs with mucilaginous cell walls from
all the superficial cells at a surface of contact. These crampons form at successively
higher levels after the manner of the successive prop roots which support the stem
of Zea mais.
Both crampons and stipes are almost entirely parenchymatous in nature, and
the filamentous central region of the so-called "trumpet hyphae" forms an insignificant part of the adult whole, mechanically speaking. Therigidityappears to be
maintained by the thick-walled cells of the cortex, and is peripheral rather than
central in disposition. The stipes thus exhibit a mechanical structure suited to
resist bending strains, and the development of the characteristic features of the
crampons point in the same direction.
SUMMARY.
1. The breaking stress or ultimate strength of a number of algae has been determined by the method of fracture by tension. The magnitude of the breaking stress
was not correlated with exposure due to the locality or to the position on the shore.
2. The elastic limits and the extensibility have been determined for Fucus,
Laminaria, Ascophyllum and Halidrys in a few cases; and, for comparison, the petiole
of Aucuba with collenchymatous and of Aspidistra with lignified tissue.
3. Fucus has relatively the least power of extension and the greatest resistance
to stretching of the algae examined.
4. Ascophyllum and Laminaria have much greater power of extension and less
power of resistance to it.
5. Attention has been drawn to the long endurance of the base of the lamina
of Ascophyllum to a load which caused stretching well beyond the elastic limits.
6. Some quality in the cell wall is invoked to account for (a) the form of break
on rupture by tension, (b) the breaking stress, and (c) the characteristics of the
elasticity and extensibility.
REFERENCES.
(1) FISHER, R. A. (1930). Statistical Methods for Research Workers.
(2) LLANDOLT and B6RNSTEIN (1923). Physikalische Chemische Tabellen.
(3) SCHWENDENER, S. (1874). Das Mechamsche Princip.
(4) THOMSON, SIR WILLIAM (1890). Mathematical and Physical Tables, 3.
(5) WIIXB, N. (1884). Bidrag til Algernes Physiologiske Anatomi. Kongl. Svensk Vet. Akad., 21,
12,490.
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20