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molecule of maltose yielding 42 7 kg. calories, taking Brown's f
value of the heat of fermentation of i gram of maltose as being
125 calories. The alcoholic fermentation of sugar is therefore a
wasteful process for obtaining energy when compared with
oxidation, since by its means nearly thirty-one times as much
sugar must be consumed to obtain as much energy as is yielded
by the direct oxidation of sugar. The significance of this
figure of the heat of fermentation of maltose may be realized
by Brown's observations that between the temperatures of 14
and 1 6 C. the time required by a yeast cell to ferment its own
weight of sugar varies from eighteen and a half to nineteen
and a half hours and that the heat generated during one hour
is sufficient to raise the temperature of the cell by 15 or 16 C.
From such observations Brown estimates that at 30 C. yeast
can ferment its own weight of maltose in 2 '2 hours and the
potential rise in temperature in the cell in one hour will be
75'5 C. under adiabatic conditions, figures indicative of an in-
tense metabolism and, apparently, a great waste of energy.
Brown suggests that the explanation for this waste is to be
found in the fact that brewers' yeast is a cultivated plant grown
under unnatural conditions. The wild yeasts lead a quiet life
on the skin of, say, the grape : rupture of the skin of the fruit
provides a nutrient medium eminently suitable for growth and
reproduction accompanied by a free access of oxygen. A
period of intense activity immediately supervenes and budding
takes place, under the continued action of the oxygen, with

* Vol. I., p. 377. f Brown: "Ann. Bot.," 1914, 28, 197.


remarkable rapidity : this activity necessitates a continuous
supply of energy, which is provided by fermentation. Repro-
duction and fermentation thus are correlated. " That we can
by means more or less artificial keep the reproductive power of a
yeast in abeyance, whilst still availing ourselves of its fermenta-
tive power, has hitherto obscured the relation of the two
functions, and hence has given rise to the somewhat exagger-
ated idea of the purposeless and prodigal waste of the yeast
cell regarded as a living unit."

In view of the definition given above, it is obvious that
any process, oxidative or reductive, which liberates energy
available for use by the plant is to be included amongst
respiratory processes, irrespective of the initial products con-
sumed and the final products evolved. Thus, in addition to
the oxidative processes of the higher plants in which fats,
carbohydrates, proteins, and protoplasm may be physiologic-
ally consumed, the diverse metabolic processes of bacteria and
comparable organisms in reducing sugar to alcohol, sulphate
to sulphide, or oxidizing alcohol to acetic acid, lactose to lactic
acid, ammonium salts to nitrites, nitrites to nitrates, and so on,
are all processes of respiration, notwithstanding the fact that
many of these activities may be extra-cellular. For green plants,
oxygen is a common essential, although, as is well known,
certain organisms, such as the lactic bacteria, can only flourish
in the absence of oxygen whilst others, although oxygen is
essential normally, have the faculty of tiding over a period of
its absence. Hence respiration may be distinguished as
aerobic, anaerobic,* and facultative anaerobic. The final waste
products are diverse and depend upon the chemical nature of
the material consumed and the method of its physiological
combustion, whether by aerobic or anaerobic means.

The ordinary green plant, and certain non-green plants,
in their respiration absorb oxygen and ultimately give off
carbon dioxide and water. This respiration is unceasing and
is continued in all living members whether active or passive
until death ensues. If oxygen be entirely withheld, growth,
movement, irritability and activity in general ultimately

* The use of the term, " intra-molecular respiration " for anaerobic respiration
is wrong since all forms of respiration are essentially intra-molecular.


will come to an end.* Between the oxygen absorbed
and the carbon dioxide evolved there is maintained a cor-
relation sometimes so close as to provide a well-known
avenue for investigating certain aspects of the respiratory
processes.f This correlation, first appreciated by de Saussure,
is commonly called the respiratory quotient in the considera-
tion of which it is well to realize that it has but little value
in indicating the essential parts of the process, correlating
as it does merely a final product, carbon dioxide, of a long
series of changes with the initial oxygen, the two being but
remotely related. The ratio CO 2 /O 2 is variable not only in
different plants but also in the same plant at different phases
of its existence; in other words, the value of the ratio is
subject to the conditioning factors. Puriewicz,J for instance,
found that the amount of carbon dioxide evolved showed a
much greater range in variation than did the absorbed oxygen
in the same plant under different conditions, the figures ob-
tained in a series of experiments showing a variation of - 14
to 1 20 per cent of the average for carbon dioxide, whilst the
oxygen varied from o to 48 per cent of the average. Ruby
observed in the olive that the ratio remained practically
constant and was but little affected by the age of the plant
or of the organ examined ; generally it is rather higher in
leaves from fruiting branches, a fact possibly connected with
the greater abundance of available food, and in the early
season of the year it is less than unity but later rises to unity.
As will be seen later, the carbon dioxide evolved may have
different origins ; in those instances in which it is due to the
immediate physiological combustion of available substances,
it will be apparent that the respiratory quotient will vary
according to the nature of this food.

Thus, in general terms, if sugar be the immediate respir-
able substance, the respiratory quotient will be in the neigh-

* The irritability of plants is outside our present consideration : an intro-
duction to the problems regarding the minimum pressure of oxygen necessary
to maintain movements, the streaming of protoplasm in chlorophyll-containing
cells in darkness and in an atmosphere free from oxygen, and similar subjects
will be found in the larger text books on general plant physiology.

fSee Bonnier and Mangin: "Ann. Sci. Nat. Bot.," 1884, vi., 19, 217;
1885, vii., 2, 315 ; 1886, vii., 3, 5.

JPuriewicz: "Jahrb. Wiss. Bot.," 1900, 35, 573.

Ruby: "Ann. Sci. Nat. Bot.," 1917, 20, i.


bourhood of unity ; on the other hand, if fat be so consumed,
the ratio will be less than unity. To take actual figures : the
value of CO 2 /O 2 for germinating oats, which are starch-con-
taining, and for germinating mustard, which contain fat, de-
termined by physical methods was found to be '99 and '92

Although carbon dioxide represents a final product of
the complete respiration of carbon compounds, other sub-
stances may be end products if physiological combustion be
incomplete. Succulent plants provide instances of this ;
members of the Crassulaceae do not give off carbon dioxide
when first placed in darkness although the absorption of
oxygen is active. There is, however, an accumulation of
organic acids, malic and oxalic, and it is not until these
have accumulated in relatively large quantities that carbon
dioxide is evolved as in a normal plant* The peculiar meta-
bolism of these plants is generally associated with and ex-
plained by their massive structure rendering the movement
of gases a relatively slow process. In other words, the
catabolism stops at an organic acid stage which forms a
reserve of carbon dioxide which is rendered available for
carbon assimilation when the acids are decomposed under
photosynthetic conditions. There is thus a conservation of
carbon dioxide. Comparable phenomena occur in bacteria
under certain conditions, the formation of acids being a well-
known occurrence in their oxidative activities.

Of other products of respiration, carbon monoxide has
been described by Langdon and Gailey in the pneumatocysts
of Nereocystis Luetkeana*[ a unique example as far as is
known. The bladders contain an atmosphere of nitrogen,
oxygen, carbon monoxide, but no carbon dioxide. An
analysis of the gaseous contents of over a thousand of these
floats showed the carbon monoxide to vary from I to 1 2 per
cent by volume, whilst the oxygen ranged from 15 to 25 per
cent. Only when oxygen is present does carbon monoxide
form in the bladders ; if the oxygen be replaced by nitrogen
or hydrogen, no carbon monoxide results. The gas is pro-
duced naturally both by day and by night but it is not formed

*See Nicolas: "Compt. rend.," 1918, 167, 131.

f Langdon: "Journ. Amer. Chem. Soc.," 1917, 39, 149; Langdon and
Gailey : " Bot. Gaz.," 1920, 70, 230.


when the plant is ground and allowed to undergo autolysis
or decay, under which circumstances carbon dioxide and
hydrogen are produced. From these facts it is concluded
that the gas is a respiratory product and has no connection
with carbon assimilation.

The accumulation of the products of physiological com-
bustion will bring about a modification, if not a complete ces-
sation, of the process which will lead to the termination of
other activities. Thus rotation of protoplasm in the cells of
Elodea will come to an end in the presence of an undue
amount of carbon dioxide. The germination of seeds is re-
tarded or inhibited by high partial pressures of carbon dioxide
in the atmosphere : this inhibition may remain in force only
so long as the seeds are exposed to the enriched atmosphere,
germination taking place after removal to a normal atmosphere
as in the bean, cabbage, barley, pea, and onion ; or, the in-
hibition may continue indefinitely after removal to normal
surroundings, germination only taking place after complete
drying and re-wetting or by the removal of the testa as in
Brassica alba. The degree of increase in the partial pressure
of carbon dioxide required to effect inhibition of germination
varies for different plants, and the retardation of germination
depends on the time of exposure and the character of the seed.
Similarly the sprouting of a potato is inhibited by an increase
of 20 per cent in the carbon dioxide of the atmosphere. A
higher concentration causes marked injury and ultimately

These facts are of considerable importance not only as re-
gards the economic aspect but also in their bearing on experi-
mental work : results obtained for subjects contained in closed
vessels, as is not infrequent in experiments on respiration,
in which the products of the oxidative processes accumulate,
may be an expression of the plant's activity not in a normal
but in a pathological condition and, therefore, may be value-


In general terms, the more active the body the more intense
the respiration, provided that the conditioning factors such as

* Kidd ; "-Proc. Roy. Soc.," Lond., B., 1914, 87, 408 ; ' New Phytol," 1919,
18, 248.


temperature, food, facility of gaseous exchange and circulation
in the plant, and so on, are favourable. Considering the plant
as a whole, Bonnier and Mangin * recognize two respiratory
maxima in its seasonal development, the first at germination,
or on the unfolding of the leaf buds, and the second at the
opening of the flower buds. Ruby f found that the intensity
of respiration was greater in leaves of young than of old
plants ; thus the amount of carbon dioxide evolved per hour
per gram of fresh weight of leaves from trees one year old, three
years old, and many years old was respectively '2OO c.c., "150
c.c., and *ico c.c. In all cases the growth period showed a one-
and-a-half to two-fold increase in respiration as compared with
the non-growing periods. Nicolas J compared the respiration
of the vegetative parts of annual, bienniel, and perenniel plants
and found that leaves and portions of stems of the same branch
varied according to their age, those from the apical regions
showing a three to seven-fold intensity of respiration as compared
with similar structures from the basal parts.

These observations have been confirmed and extended by
Kidd, West, and Briggs who have studied the respiration of
Helianthus annuus both in the laboratory and in the field. They
point out that the factors which may affect the rate of respira-
tion per unit of dry weight of tissue are the concentration of
the respirable material, the concentration of oxygen, the tem-
perature, and the effective amount of respiring cell matter per
unit of dry weight. This last is the " internal " factor, the re-
sultant of many factors, none of which as yet fully understood
and some of which probably not yet formulated. The internal
factor can only be accurately measured when the other factors
are not conditioning respiration. For purposes of measuring
its effect, Kidd, West, and Briggs employ a respiratory index
which is the respiration, measured by the rate of carbon
dioxide produced, per gram of dry weight at 10 C. when the
amount of respirable material is not limiting and when the
external concentration of oxygen is that of the atmosphere.
From a large number of observations they conclude that the

* Bonnier and Mangin : " Ann. Sci. Nat. Dot.," 1885, 5, 315.

fRuby: W., 1917, 20, r.

Nicolas: " Rev. g6n. Bot.," 1918, 30, 209.

Kidd, West, and Briggs: " Proc. Roy. Soc.," Lond., B, 1921, 92, 368.


respiratory index of the entire plant continuously declines with
increasing age. For example, entire plants 2 days from
germination gave 3 mg. carbon dioxide per gram of dry weight
per hour, whilst plants 136 days from germination yielded
but -39 mg. carbon dioxide per gram of dry weight per hour.
A similar decrease in the respiratory index is exhibited by the
stem, leaves and flowers. In the stem the value fell from '8
mg. on the thirty-sixth day from germination to *o8 mg. on the
1 36th day from germination; during the same period the
measure for the leaves decreased from I "56 to -44 mg. The
fact that the initial respiratory index of successive leaves de-
creases with the age of the plant indicates a respiratory decre-
ment of the merismatic tissue with age and from this it follows
that the fall in the respiratory index of the whole plant is not
due to the proportionate increase with age of dead tissue,
sclerenchyma and tracheae for instance.

Nicolas* also found that the presence of anthocyanins
was a factor, or rather the symptom of an internal factor, of
importance. Leaves containing a red pigment either as a
youth form or as a permanent character absorbed more oxygen
and exhibited an increased respiration as compared with green
leaves, whilst leaves turned red either by accident or by stress
of conditions! showed a decreased respiration as compared
with green leaves, the amount of carbon dioxide being smaller.

The intensity of respiration commonly is measured by the
amount of an end product given off in unit time : in aerobic
respiration the final product measured is carbon dioxide or, in
special cases, temperature is measured.

With regard to the measurement of carbon dioxide, it is
clear that the results obtained may not be a true expression of
the respiratory activity since the exhalation of the gas may be
greatly hindered by various circumstances. The surface of
the respiring organ is one such : Hoffmann J found that the
amount of carbon dioxide evolved in twenty-four hours per
kilogram of large, medium, and small-sized potatoes was 259,
314, and 326 mg. respectively; barley gave confirmatory
results. He also found that the nitrogen content was im-

* Nicolas: "Compt. rend.," 1918, 167, 131.

t See Vol. I., p. 251.

J Hoffmann : " Journ. f. Landw.," 1916, 64, 289.


portant. Further, a depression of the carbon dioxide output
is associated with succulent plants, hence in order to obtain a
figure representing a true measure of the activity of respiration,
the increase in organic acid content in addition to the gaseous
carbon dioxide should be determined. This aspect is con-
sidered by Maige and Nicolas,* who point out that in the
flowers they examined, the respiration intensity increased with
age when stated in terms of gaseous exchange, but showed a
decrease with age when measured in terms of wet and dry
weight. In the case of the gynaecium, that shows, as might be
expected from the activity of the contained structures, a real
increase with age, whilst the other parts of the flower exhibit
a decreasing respiration with age.

With regard to the heat of respiration, it is easy to demon-
strate by relatively simple means, due precautions being taken
against loss, that the evolution of heat is a concomitant of
respiration. The temperature attained is cumulative and a
remarkable rise may result which may be realized by thrusting
the naked arm into a barrow load of fresh lawn cuttings, especi-
ally if there is a good admixture of clover. Exact measurements
have been made by various investigators : Molisch f found that
the bulked leaves of Carpinus betulus reached a temperature
of 51 C. in fifteen hours, a fall then took place so that at the
end of forty-eight hours the temperature was 34 C. After
the lapse of one hundred and four hours a secondary maximum
at 4 7 C. was attained, the temperature again showed a fall to
31 C. after one hundred and eighty hours. Of these two
maxima, the first is an expression of the true respiration
intensity of the leaves, whilst the cause for the second is to be
found in bacterial activity. Pierce J found that in germinating
peas the greatest average gain in heat was 923-9 calories
accumulated in 23-5 hours, which is about equivalent to 8-55
calories per minute per kilogram of peas, a measure roughly
one quarter less than the amount of heat given off by a mouse
under similar experimental conditions. It was further found

* Maige : " Rev. ge"n. Bot.," 1907, 19, i ; " Ann. Sci. Nat. Bot.," 1911, 14, i.
Maige and Nicolas : " Rev. ge"n. Bot.," 1910, 22, 409.

t Molisch: "Bot. Ztg.," 1908, 66, 211.

Pierce : " Bot. Gaz.," 1912, 53, 89 ; see also Bonnier : " Ann. Sci. Nat. Bot.,"
1893, 18, 12.


that the amount of heat liberated by germinating peas decreased
with age.

Since respiration is a means of obtaining energy for the
needs of the plant, the evolution of heat represents excess of
energy and is a waste product, for which reason the temperature
of a normally respiring plant is not by itself a sure guide to the
amount or intensity of physiological combustion but rather a
measure of the inefficiency of the organism.


The stimulation of the plant accelerates respiration, which
acceleration is marked by a rise in the output of carbon
dioxide and generally also by a rise in temperature. This is
particularly true for the stimulus provided by wounding ; as
F. F. Blackman said on a certain occasion, " Precisely the
same effect is produced by peeling a potato as by flaying
a saint." This intensification of respiration resulting from
traumatic stimulation long has been known and now is not an
uncommon laboratory exercise. Richards* found a gradual
rise in temperature following the stimulus, attaining its
maximum about twenty-four hours after the infliction of the
injury. In massive tissues the effect is local, but in less
compact structures, an onion bulb for example, the rise in
temperature may be demonstrated over a more extensive
area. The rise may be two or three times as large as the
difference between the normal temperature of a potato and that
of the surrounding air. The precise increase in respiration
depends on the extent of the injury and the nature of the tissue
operated on. With regard to the output of carbon dioxide,
there is an initial outburst followed by a fall, a feature not at
all uncommon under drastic stimulation, and the absorption of
oxygen is rather greater than the amount theoretically required
for the quantity of carbon dioxide evolved. Richards con-
siders that the initial outburst of carbon dioxide in part is due
to the release of the gas normally enclosed within or absorbed
by the tissues, the " residual " carbon dioxide which normally
is not exhaled.

With regard to other forms of stimulation, White f found
that pollination produces a rapid increase in respiratory activity

* Richards : " Ann. Bot.," 1897, II, 29. f White : Id., 1907, 21, 487.


and affects the CO 2 /O 2 ratio which generally is greater in
pollinated carpels as compared with unpollinated gynaecia.
The most striking instance was afforded by the Pelargonium,
the pollinated carpels of which evolved from five to eight times
as much carbon dioxide as the unpollinated.

Mention also may be made of Schley's * observations who
found that the respiration of a geotropically stimulated root is
greater than that of an unstimulated root, the respiration rate
of the convex side being greater than that of the concave side
during the period of perception and response.


The action of anaesthetics on the output of carbon dioxide
is partly stimulatory and partly narcotic. Irving f found that
the effect of a single dose depended on its strength : a dose of
I c.c. of chloroform in 970 c.c. of air brings about an immediate
rise in the output of carbon dioxide, this effect subsequently dis-
appears and the leaves then evolve as much carbon dioxide as
in normal respiration. In medium doses, -2 c.c. of chloroform in
970 c.c. of air, there obtains the same initial outburst of carbon
dioxide which falls away more quickly than in the case of a
small single dose and for a time remains below normal ; after
about six hours recovery is complete and the evolution of carbon
dioxide is normal. With a dose of I c.c. of chloroform in 970
c.c. of air, the initial outburst of carbon dioxide is earlier and
its curve is steeper, the production of carbon dioxide slowly
diminishes and there is no recovery. After a strong dose of
chloroform, 10 c.c. in 970 c.c. of air, there is no detectable
initial outburst and the carbon dioxide output quickly falls to
zero. The administration of a continuous dose of chloroform
produces the same effect as a single dose two or three times as
strong. ThodayJ likewise found that in the instances of
Prunus laurocerasus, Helianthus tuberosus and Trop&olum
ma/us that a small dose of chloroform leads to an immediate
stimulation of respiration, the evolution of carbon dioxide and
the absorption of oxygen increasing in like proportion, which
indicates that the two are co-ordinated. If, however, the dose

* Schley : " Bot. Gaz.," 1920, 70, 69.
f Irving: "Ann. Bot.," 1911, 25, 1077.
JThoday : Id., 1913, 27, 697.


is sufficiently large to effect a visible disorganization, such as
change in colour,* there is an initial outburst of carbon dioxide
then a fall to a very low level and the absorption of oxygen
no longer shows any co-ordination with the amount of carbon
dioxide evolved. The absorption of oxygen in some way is
connected with the cell contents, especially tannin. In the
Tropceolum leaf, which is free from tannin, the absorption of
oxygen is much lower than the output of carbon dioxide, where-
as in the cherry laurel and the artichoke, both of which contain
tannin, the initial up-take of oxygen is very rapid, then it
declines but remains at a level much higher than the output
of carbon dioxide.

Similar observations have been made by others, thus
Thomas f found that wheat subjected to the action of ether
shows an increase of respiration followed by a decrease.
Exposure to 7-3 per cent of ether is only stimulatory provided
it be of short duration, an exposure of more than thirty
minutes resulting in death. Similarly the main observations
of Thomas have been corroborated and extended by Smith J
who used the hydrogen ion concentration method in finding
the rate of production of carbon dioxide in wheat seedlings.
It was found that the first effect of ether, used in concentrations
of I per cent, 3*65 per cent and 7*3 per cent, was to depress
the rate of respiration ; this was followed by a rapid increase
to above the normal rate, which was in turn followed by a
decline to much the same level in all concentrations of anaes-
thetic in times varying with the dose employed ; the stronger

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