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enced by temperature needs no elaboration : in the majority of
instances an increased temperature accelerates a reaction,
examples in which the contrary obtains are very few. Van't
Hoffs Law states that for every rise in temperature of 10 C.
the reaction is increased at a definite rate, in general terms
doubled or trebled, the precise value of which is specific to the
reaction.* The plant, however, is not a test-tube but a very
complex system of reacting substances, wherefore it is only in
experiments most carefully controlled and skilfully conducted
that approaches to the mathematical preciseness of well
ascertained physico-chemical laws will obtain.

Long has it been known that an increased temperature
results in an increased carbon assimilation, but it was not
before F. F. Blackman's experimental researches on vegetable
assimilation and respiration that the subject was really criti-
cally examined. Matthaei f at the outset of her work on
the effect of temperature on carbon assimilation found that
in addition to the influential external conditions there is an
important internal condition of a plant or plant member, the
result of previous treatment such as excess of food, starvation,
and change in temperature. This has a most important
bearing on experimental results, a fact previously overlooked,
so that in order to have comparable results it is essential that
the previous history of all the material employed should be the

In the case of Prunus laurocerasus it was found that for
each temperature to which the leaves were subjected there is
a definite amount of carbon assimilation, the maximal assimil-
ation for that temperature, which cannot be exceeded and

* This factor is termed the temperature coefficient and is represented by the
symbol K with a number attached indicating the number of degrees concerned,
e.g. K 10 .

f Matthaei : " Phil. Trans. Roy. Soc.," Lond., B. 1904, 197, 47.


cannot be attained unless the illumination be of sufficient
intensity and the carbon dioxide be adequate in amount.





|o 70




-10 +10 20 30 40

FIG. 5.

These maximal amounts of assimilation increase rapidly with
rising temperatures but at the higher temperatures the initial
rate for that temperature cannot be maintained for long but


falls off regularly at a rate dependent on the temperature ; the
higher the temperature the quicker the fall, which rapidity,
however, is not maintained. Thus there is a time factor for
the higher temperatures. Fig. 5 summarizes the results ob-
tained by Matthaei ; it will be seen that the turning point is
37 '5 C. which was found to be within a few degrees of the
temperature fatal to the leaf.

The results obtained for temperatures below 25 C. con-
formed to Van't Hoff s Law, the coefficient of increase in the
rate of carbon assimilation for a rise of 10 C. being 2-1. In
subsequent investigations Blackman and Matthaei found the
assimilation coefficient for the leaves of Helianthus tuber osus to
be K IO =2*3, whilst for Elodea, K IO = 2-05.* The culminative
point of the assimilatory curve (Fig. 5) in respect to increas-
ing temperature is important and is paralleled in enzyme
action ; in view of the results obtained by Willstatter and Stoll,
it is not unlikely that the inhibition or destruction of an
enzyme at these higher temperatures may be a limiting factor.

With regard to the internal temperature of assimilating
leaves, this will vary considerably in accordance with such
conditions as the intensity and quality of the light, the
character of the leaf surface and so on ; Blackman and
Matthaei f demonstrated, by thermoelectric means, an excess
of 7 C. to 1 6 C. in the leaves of Prunus laurocerasus in bright
sunlight above the adjacent shade temperature.


Of the light falling on a leaf, a portion only is available
for the chloroplasts since varying amounts, according to the
characteristics of the leaf surface, such as the presence of cuticle
and of hairs, the thickness of the leaf and so on, will be lost by
reflection, absorption or transmission. Of the energy absorbed
by the leaf, many have shown that a small proportion only,
and this in varying quantity, is used in carbon assimilation.
Owing to ignorance of certain factors and the degree of their
significance in the sequence of carbon assimilation, it is not
possible to give a satisfactory account of the energy relation-
ships of the plant. It may, however, be mentioned that Brown

* Blackman and Smith : " Proc. Roy. Soc." Lond., B. 1911, 83, 389.
f Blackman and Matthaei : id., 1905, 76, 402.


and Escombe,* the first to attempt the drawing of an energy
balance sheet of the leaf, concluded that Polygonum Weyrichii
used from -42 to I -66 per cent of the available radiant energy
for carbon assimilation, figures based partly on observation
and partly on calculation : Puriewitsch,-|- on the other hand,
found that Polygonum Sacchalinense similarly employed from
2*5 to 7*7 per cent of the radiant energy.

Under no natural conditions is the full radiant energy made
use of by plants: the photosynthetic value of the noontide
sunshine at the summer solstice in these latitudes lies, according
to Blackman and MatthaeiJ between '04 and '05 grams of
carbon dioxide per 50 sq. cm. of leaf surface per hour ; the
highest assimilation actually measured by these workers was
0290 grams.

The general statement that carbon assimilation varies with
the intensity of the illumination is true only when light is the
limiting factor ; other factors, particularly temperature, are
intimately associated in the process in nature. For this reason
it is impossible to consider the effect of one condition to the
exclusion of the other factors, a fact well demonstrated by
Blackman and Matthaei. In one of their experiments, an
abstract of which is given below, the leaves of Helianthus
tuberosus were surrounded by an atmosphere containing on the
average 4 per cent carbon dioxide and the light throughout
was diffused and of varying intensity.


Temperature of
Leaf C.

Real Assimilation in
Grams per 50 Sq. Cm.
per Hour.

i. Very overcast .



2. Raining but lighter .



3. Raining but lighter still



4. Raining, heavy clouds



5. Raining



6. Much brighter .



7. Sunshine but leaf in shade



It will be seen from observations 2 to 4 and 6 and 7 that
the assimilation was remarkably uniform under conditions of

* Brown and Escombe: " Proc. Roy. Soc.," Lond., B. 1905, 76, 29.
t Puriewitsch : " Jahrb. wiss. Bot.," 1914, 53, 210.
% Blackman and Matthaei : " Proc. Roy. Soc.," Lond., B. 1905, 76, 402.


constant temperature and varying illumination ; this shows
that temperature was the limiting factor. The low value in
the first observation was due to the low light intensity. In
observation 5 in which the illumination was about the same as
in 4, the temperature was raised from 177 to 30*5 C. with
the result that the assimilation was about doubled : but since
at this higher temperature an assimilation of at least '0289
grams is possible, it follows that in this instance the illumina-
tion was the limiting factor.

An important generalization arrived at by F. F. Blackmail
and his collaborators is that equal intensities of light incident
on equal areas of leaf produce the same amount of assimilation,
provided that light is the limiting factor and that the tempera-
ture does not involve the so-called time factor ; agreement
within 5 per cent was found to obtain in such diverse instances
as Helianthus, Prunus, Bomarea, Aponogeton, Elod:a and
Fontinalis. Hence the conclusion is reached that " leaves in
general have the same coefficient of economy in the photo-
synthetic * economy."

The amount of light required by a leaf is a specific value
for a given temperature; thus in the examples studied by
Blackman and Matthaei, Helianthus and Prunus have at low
temperatures similar assimilatory maxima which diverge at
higher temperatures. At 29-5 C. Helianthus can assimilate
twice as much carbon dioxide as can Prunus, but in so doing
it requires twice the amount of illumination. The difference
in the two leaves lies in their having different coefficients of
acceleration of their assimilatory activity with increased tem-

With regard to the use made by the plant of specific parts
of the spectrum, it has hitherto been generally accepted that
those wave lengths associated with the prominent bands in the
red of the absorption spectrum of chlorophyll were the only
_^ ones concerned with carbon assimilation. The important
investigations of Ursprung f show that radiant energy of any
wave length to, and including the greater part of, the ultra-
violet is capable of inducing starch formation in green leaves.

* The term photosynthetic is used in its narrow sense to refer only to those
radiations specific to this process.

t Ursprung: " Ber. deut. hot. Gesells.," 1917, 35, 44; 1918, 36, 73. 86, in,



He even found starch to be formed on exposure to infra-red
rays ; it is, however, not clear whether this starch was a direct
product of carbon assimilation. Further investigation is
required on the relationship between carbon assimilation and
the specific wave lengths absorbed. Ursprung found in the
instance of Phalaris arundinacea var. picta^ which plant was
selected since its variegated leaves permit a comparison between
the green and non-green portions, that in the red region of the
spectrum the maximum absorption obtains between the B and
C lines, in the green the absorption is general, whilst in the
violet the absorption is greater than between the B and C lines
of the red, and beyond this there is a rapid fall. Subsequent
work on Phaseolus showed a sharp rise in the assimilation curve
from the outer limits of the red to a maximum situated near
the C line, from which point there is a gradual fall towards the
violet This curve does not correspond, more especially as
regards the region of shorter wave lengths, with the curve of
absorption which rises from E, in the green region, to the
violet. In view of the fact that carbon assimilation takes place
in this part of the spectrum, it is obvious that some new con-
ditioning factor is operative ; it is suggested that this is found
in the action of the 'violet rays causing the stomates partially
to close.

The consideration of the absorption of energy leads to a
host of questions regarding the conversion of radiant into other
forms of energy, chemical and electrical. And here the realm
of hypothesis is reached, for there is no certain knowledge as to
the fate of the absorbed energy, the relative and absolute values
of specific radiations, and in what form it is dissipated.

With regard to the ultra-violet rays, the more injurious of
which are absorbed by the atmosphere, and which in the light
of Ursprung's work assume a greater importance, Stoklasa and
Zdobnicky * brought about the synthesis of carbohydrate in the
absence of chlorophyll by passing light from a quartz mercury
lamp through a mica window into a vessel containing a mixture
of carbon dioxide and nascent hydrogen.

Formaldehyde was slowly produced and this, in the presence
of caustic potash, was polymerized with the formation of a sugar
or a mixture of sugars which was optically inactive and not fer-

* Stoklasa and Zdobnicky : " Chem. Zeit.," 1910, 945.


mentable by yeast. The authors suggested that the chlorophyll
in plants acts as a means of absorbing ultra-voilet rays, a sug-
gestion which has since been found to be true. Bertholet and
Gaudechon * found that formaldehyde is produced by the action
of ultra-violet rays on carbon dioxide in the presence of a re-
ducing agent, and, with regard to the reverse process, that
carbohydrates are decomposed by sunlight and by ultra-violet
light from a mercury lamp. The products of decomposition
are carbon monoxide, carbon dioxide, methane and hydrogen ;
aldehydic sugars differ from ketonic sugars both in the readiness
with which they are decomposed and in the composition of the
gaseous mixtures produced.

Usher and Priestley f found that ultra-violet light can bring
about the decomposition of aqueous carbon dioxide without
the intervention of an optical or chemical sensitizer, a result
contrary to Stoklasa and Zdobnicky who found that formaldehyde
was not produced by the action of ultra-violet light on carbon
dioxide and water. In view of these contrary results, Spoehr j
tried the effect of ultra-violet radiations on carbonic acid and its
salts ; in no experiment was a sufficiency of formaldehyde pro-
duced to give a positive reaction with the reagents employed.
In all experiments Spoehr found that formic acid was the only
reduction product. Attempts to reduce formic acid to formalde-
hyde by sun or by ultra-violet light failed, but after ten to fifty
hours exposure there remained on evaporation a non-volatile
yellow syrup, of a composition not yet determined, which re-
duced Fehling solution.

The subject has received renewed attention from Baly,
Heilbron, and Barker who find that an aqueous solution of
carbon dioxide yields formaldehyde when exposed to light of
very short wave length, 200 jifi. Under the influence of light
of wave length 290 /*,//,, however, formaldehyde in water is
polymerized to reducing sugars, but if substances, sodium
phenoxide for example, which absorb this wave length and
which are ineffective in the chemical actions involved, are

Bertholet and Gaudechon: " Compt. rend.," 1910,150, 1690, 151, 395;
!9i2, 155, 401, 831.

f Usher and Priestley: " Proc. Roy. Soc.," Lond., B. 1911, 84, 101.

Spoehr : " Plant World," 1916, 19, i.

Baly, Heilbron, and Barker: ' Joum. Chem. Soc.," Lond., 1921, 119,


present, the amount of formaldehyde is increased in an aqueous
solution of carbon dioxide, since the action of such substances
is to protect the formaldehyde from polymerization. These
are test tube observations : the plant can flourish under con-
ditions which entirely withhold radiant energy of these short
wave lengths. In such cases a photocatalyst is required and
Baly and his collaborators find that in ordinary visible light
the photosynthesis of formaldehyde and its polymerization to
carbohydrate can be achieved in two separate stages : in the
first stage formaldehyde is produced from carbon dioxide and
water in the presence of coloured basic substances such as
methyl orange ; in the second stage formaldehyde is poly-
merized to a reducing sugar, without the aid of a photocatalyst,
when exposed to the rays from a quartz mercury lamp.

The authors point out that if a photocatalyst capable of
bringing about both changes in the same vessel in the labora-
tory were known, then the separate existence of formaldehyde
would not be demonstrable since the formaldehyde pro-
duced from carbon dioxide and water would at once be poly-
merized into carbohydrate. Such a photocatalyst has yet to
be found, and if chlorophyll be one such, then the small amount
of formaldehyde in carbon-assimilating leaves is to be expected.
This investigation is still in progress : as yet Baly and his
fellow-workers have not found the desired photocatalyst and
so far have not ascertained to what degree chlorophyll meets
their requirements in this respect ; their results are not in ac-
cord with those of Osterhout * nor do they examine the con-
tention of Spoehr, outlined above, regarding formic acid.

Chemical change may be brought about by electrical
energy ; indeed, in connection with plants, the effect of elec-
trical currents on vegetable growth is a not unimportant
branch of applied botany.

Royer f brought about the electrolytic reduction of carbon
dioxide, and by similar means Coehn,J in 1904, produced
formic acid from this same compound. Brodie found that
by means of a silent discharge formaldehyde, together with

* See Vol. I., p. 61.

f Royer: "Compt. rend.," 1870, 70, 731.

JCoehn: " Ber. deut. chem. Gesells.," 1904, 34, 2836, 3593.

Brodie : " Proc. Roy. Soc.," Lond., 1874, 22, 171.


marsh gas, was produced from a mixture of hydrogen and
carbon dioxide ; and Lob, * in 1 906, found that formaldehyde
may be produced by the action of a silent discharge of elec-
tricity through a solution of carbon dioxide in water. Fenton f
also has pointed out that the synthetic action of light and of
the silent electrical discharge are practically identical. Thus
there is evidence which suggests that electric energy may
play a part in the earlier processes of photosynthesis ; a sug-
gestion which is supported by the fact that, according to
Polacci,J the formation of carbohydrates is promoted in
leaves by electrical energy, provided it be not too intense,
especially when a continuous current is made to pass directly
into the tissues.

As a result of a number of experiments, Gibson comes
to the conclusion that the light rays which are absorbed by
the chlorophyll are transformed into electrical energy, and it is
this transformed energy which brings about the decomposition,
of carbonic acid to formaldehyde and oxygen. This opinion
is based on evidence the complete details of which apparently
have not been published

With regard to other forms of energy, attention may be
drawn to the work of Kernbaum,|| who found that water ex-
posed to the influence of ft rays and of ultra-violet rays led to the
production of hydrogen and hydrogen peroxide. Usher and
Priestley also found that an aqueous solution of carbon dioxide
could be decomposed by the a and ft rays from radium emanatioa
The action of -oooi c.c. of radium emanation on 200 c.c. of water
saturated with carbon dioxide resulted in four weeks in the
production of hydrogen peroxide and formaldehyde. Most of
the latter was in a polymerized state, but the solution contained
no sugar.

Stoklasa 11 considers that the essentiality of potassium,
which is feebly radioactive, to the well-being of green plants
is in part due to this property which is associated in the

*L6b: "Zeit. Electrochem.," 1906, 12, 282.

f Fenton : " Journ. Chem. Soc.," Lond., 1907, 91, 687.

JPolacci: "Atti. Inst. Bot.," Pavia, 1905, II., u, 7.

Gibson : " Ann. Bot.," 1908, 22, 117.

|| Kernbaum : "Compt. rend.," 1909, 148, 755, 149, 273.

T Stoklasa : " Biochem. Zeitsch.," 1920, 108, 109.


transformation of energy in the photosynthetic phase of carbon


CHLOROPHYLL.* Plant physiologists for long have recog-
nized that the intensity of carbon assimilation must be de-
pendent on the chlorophyll and its amount ; it is, however,
but recently that the problems involved have been critically
examined. Irving,f who used the leaf's carbon dioxide of
respiration in her experiments, found by gasometric methods
that etiolated leaves, either when they are orange-yellow or
when they have attained a considerable degree of greenness,
do not possess any appreciable power of synthesizing carbon
dioxide. If there be any photosynthetic activity, it cannot
be greater than one-tenth part of respiration nor come within
I per cent of the activity subsequently developed. Carbon
assimilation begins when the leaves are fully green and
develops very quickly ; wherefore it follows that the first
origin of this function is not correlative to the amount of
chlorophyll produced, or, in other words, that the amount of
chlorophyll is not a conditioning factor in the early stages
of carbon assimilation.

Willstatter and Stoll J were the first to make quantitative
estimations of the amount of chlorophyll in leaves, by the
methods already outlined. Also they measured the amount
of carbon assimilation of the leaves of different plants and
of the same plant in different conditions normal, etiolated,
autumnal, and so on and thus arrived at the assimilation
number which is the ratio between the amount of carbon
dioxide assimilated per hour and the chlorophyll content
both expressed in milligrams. A selection of the values ob-
tained are tabulated below.

Willstatter and Stoll, whose experimental methods were
similar to Irving' s, with the chief exception that they used
a 5 per cent concentration of carbon dioxide, found that

* A general account of chlorophyll, its chemistry and constitution, will
be found in Vol. I.

t Irving : " Ann. Bot.," 1910, 24, 805.

I Willstatter and Stoll : " Ber. deut. chem. Gesells.," 1915, 48, 1540.

See Vol. I., Section on Pigments.



leaves with but a small portion of their full chlorophyll
content developed can assimilate to a measurable degree.



Kind of Leaf.

Content in

CO 2 Assimilated
per Hour


Primula .



J 05







Tilia |





Populus <

Dark green autumn leaves
Yellow-green autumn leaves





Elm [

Yellow variety
Green variety



This conclusion is contrary to that of Irving, a difference
probably due to the fact that Irving used young leaves whilst
Willstatter and Stoll employed older and sometimes much
older material. This explanation is due to Briggs,* in whose
memoir a critical examination of the work of the above-
mentioned authors will be found ; this author demonstrates that
the age of a leaf and the lapse of time from the greening to
the measurement of photosynthetic activity are all important.
If a leaf is cut from a seedling in the dark at an early stage
in its development and partly greened by exposure to light,
its photosynthetic activity will be zero or very small ; if, on
the other hand, the same procedure is repeated with a similar
leaf from the same plant after an interval of a few days, the
photosynthetic activity will be strongly marked. Briggs
confirms Irving's main conclusions : a young green leaf may
show no or very little carbon assimilation and the power
of photosynthesis lags behind the development of chloro-
phyll. This power increases with age whether the leaf be
in the dark or in the light even though there be no concurrent
increase in the chlorophyll content.

THE UNKNOWN FACTOR. The fact that the temperature
relations of carbon assimilation are those of a chemical rather
than a photochemical reaction indicates the presence of an
internal factor, independent of the chlorophyll and associated
rather with the protoplasm, which controls the rate of carbon

* Briggs : " Proc. Roy. Soc.," Lond., B. 1920, 91, 249.
VOL. II. 3


assimilation. Irving concludes that this factor controls the
beginning of the process since it is not developed so quickly
as the chlorophyll, wherefore the rest of the mechanism must
await its appearance.

The assimilation numbers arrived at by Willstatter and
Stoll * are inconstant, which is indicative of there being some
other operating factor : if they were constants, strong evidence
that chlorophyll was the all-important conditioning factor
would be provided. According to Willstatter and Stoll f it
is an enzyme which thus limits carbon assimilation. They
find that in leaves rich in chlorophyll, increased illumination
has but little effect upon assimilation nor is it diminished
if the illumination is decreased to one quarter. This indicates
that the chlorophyll is present in excess compared with the
assimilatory enzyme. The increase in carbon assimilation
following an increase in temperature they consider to be
due to the stimulation of the enzymatic process (cf. p. 48).

In leaves containing little chlorophyll and in yellow varie-
ties, the conditions are reversed; the enzyme here being in
excess, increased temperature has little effect in stimulating
assimilation. On the other hand, increased illumination has
a very marked effect.

The remarkable phenomena accompanying autumnal
changes in leaves are due to the fact that either the chloro-
phyll suffers more than the enzyme, resulting in increase of
assimilation number, or conversely the enzyme suffers most,
in which case the assimilation number falls.

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