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Date of Gathering.

Per Cent

Per Cent

Per Cent

6 July .




i August




15 August .




i September




4 October .




Date of Gathering.

Per Cent

Per Cent

Per Cent

Per Cent
Starch and

9 June .





4 July .





i August





i September .





4 October




With respect to the carbohydrates, the weight of opinion
favours the view that in these substances is to be found the
origin of fats.

The conversion of carbohydrate to fat in plants, or parts
of plants, exposed to a low temperature ; the development of
fats in immature seed separated from the parent plant ; and
the fattening of animals on a carbohydrate diet indicate the
close physiological connection of fats with carbohydrates.

Quantitative results are not wanting. Numerous observa-
tions, amongst which those of Schmidt, Le Clerc du Sablon,
de Luca, Funaro, and Ivanow may be mentioned, show that
in the maturation of the seed, the increase in the amount of



fat and the decrease in the amount of carbohydrate are con-
current. This is illustrated in the above tables representing
the results of two experiments by Le Clerc du Sablon.

The carbohydrates which serve as raw material for the
elaboration of fats vary with the plant ; glucose, sucrose and
starch would appear to be most commonly employed in this
connection, the last two being initially hydrolysed.

It is a well-known fact that carbohydrates appear during
the germination of fat containing seeds. Thus Maz * found
that the isolated cotyledons of arachis seeds increased in their
dry weight from 2*2613 to 2-6153 grams, an increase due to
the fixation of oxygen, whilst their sugar content increased
from '3416 to -4684 grams.

The work of Le Clerc du Sablon, Ivanow, Miller and
others have thrown much light on the hydrolysis of fats in
their studies on the germination of various fat-containing
fruits and seeds. Lipase is the enzyme concerned and by its
activity the fat is hydrolysed into glycerol and fatty acid,
which products are used either immediately for the needs of
the seedling or are first converted into carbohydrate. Agree-
ment is general that as germination proceeds, the acid radicles
become saturated, as is indicated by the decrease in the iodine
value, and the rate of consumption of these acids is inversely
proportional to their degree of saturation.

As the fats decrease, the carbohydrates increase, a fact
brought out in the following analysis of arachis seedlings by

Age in Days.

Per Cent

Per Cent Carbo-
hydrate other than

Cellulose and
other Insoluble
























9- 4 8

The nature of the carbohydrate formed differs in different
plants ; glucose would appear to be the most common, but

* Maze : " Compt. rend.," 1900, 130, 424.
fMaquenne: " Compt. rend.," 1898, 127, 625.


saccharose, starch and dextrins also are described. It is not
unlikely, as Maquenne has pointed out, that the nature of the
carbohydrate depends upon the kind of fat and its degree of
saturation. He considers that the saturated fatty acids con-
tribute in a much lesser degree to the formation of sugars than
do the unsaturated acids, and that the saturated fatty acids
are principally used for respiratory purposes. Agreement,
however, between the authorities in respect to this aspect of
the subject does not obtain. Ivanow considers that there is no
real difference between the saturated and unsaturated fatty
acids in their power to give origin to carbohydrates. The
difference in their amounts is due to the more rapid conversion
of the unsaturated variety. However this may be, the salient
feature in the germination of a fat-containing seed is the con-
version of the fat into carbohydrate, the reverse to what
obtains during the maturation of the seed. The change is
effected by the activity of lipase which hydrolyses the fat into
glycerol and fatty acid.

The work of Ivanow has shown that lipase has a reversible
action, and the fact whether it hydrolyses or synthesizes fats
is merely a question of conditions, mainly the presence or
absence of water. The glycerol extract of a fat-containing
seed, which extract contains the lipase, mixed with oleic acid
will synthesize a fat : the addition of water will result in the
hydrolysis of this fat into glycerol and fatty acid.

In the synthesis of fats, Ivanow considers that higher
saturated acids of the fatty series are the first to be formed
from the sugar, and these are converted into unsaturated acids
which combine with the glycerol to form the fat.

With regard to the origin of glycerol, the chemical re-
lationship between this substance and glucose is so close as
to suggest at once the possible inter-relation of the two ;
further, glycerol may have an origin in respiratory processes
as is shown by the production of this substance during the
alcoholic fermentation of sugar.


THE term carbon assimilation, although unfortunate from
some points of view, is employed to designate all those
activities, in part physical, in part chemical, which play a
r61e in the anabolism of carbon dioxide by green tissues.
The conspicuous facts of the process are that active chloren-
chyma on exposure to light forms, by means of its chlorophyll,
carbohydrate from the initial substances carbon dioxide and
water; oxygen, in volume roughly equivalent to the volume
of carbon dioxide consumed, is evolved during the process.*
Carbohydrate is the obvious and chief end product, but protein
also may be so formed, and such diverse materials as fat,
tannin and various organic acids have been considered, prob-
ably on insufficient evidence, to be of direct photosynthetic
origin. The earlier phases in these synthetic processes are
photochemical, a mutation of radiant into chemical energy,
and it is during this phase that the oxygen, a waste product,
is evolved. The presence of oxygen in the air-space system
of the active chlorenchyma may thus be considerably greater
than in normal air, and since this gas is continually excreted
during the process, it is not surprising to find that the quantity
of oxygen in the surrounding atmosphere is immaterial to
the process and that it may be decreased to 2 per cent or
increased to 50 per cent without adverse effect : f but since

* Bonnier and Mangin (" Ann. Sci. Nat. Bot.," 1886, 3, i) found by various
experimental methods that the ratio O 2 /CO 2 was always greater than unity
for ordinary plants; the lilac gave the smallest value 1*05, and the holly the
largest 1-24. A similar range was found by Aubert (" Rev. gn. Bot.," 1892,
4, 203) to obtain in ordinary plants, but succulents, which have a peculiar
metabolism, gave generally a higher value ; as high as 7-59 in the instance of
Opuntia tomentosa. Maquenne and Demoussy (" Compt. rend.," 1913, 156,
506) conclude from a large number of observations that the assimilatory
quotient approximates to unity.

t Friedel: " U.S. Dept. Agric.," 1901, Bull. 28.



the formation of chlorophyll is dependent upon the presence
of oxygen, the prolonged maintenance of a low oxygen
pressure will inhibit the development of more chlorophyll
and this will in turn react on carbon assimilation.

Carbon assimilation is conditioned by various factors ;
wherefore the rate of the process will vary considerably ac-
cording to the inter-relationships of these factors. For this
reason it is hardly remarkable to find that different values
of the rate of carbon assimilation have been reported by
different investigators : differences in value due not only to
inappreciation of the conditioning factors, but also to dif-
ferent avenues of attack and to experimental error.

The accompanying table gives a selection of values of
the rate of carbon assimilation in the open air of detached
leaves of the sunflower, Helianthus annuus, expressed in terms
of grams of increase in dry weight per square decimeter per
hour, obtained by the authorities named :

Sachs* . . . '01882
Brown and Morris f . -00985
Brown and Escombe '00361 - '00551

Thoday . . . -0169 (the average for fully turgid leaves) -
0016 (the average for quite flaccid leaves).

The figures of Sachs and Thoday were obtained by the
direct determination of the increase in dry weight, the well-
known method of Sachs, whilst the others were calculated
from the amount of carbon dioxide absorbed and assuming
that carbohydrate only was ultimately formed. Since the
ultimate fate of the carbon dioxide is not entirely known,
the extent to which it is directly used in the elaboration of
fat or protein for example, the dry weight method would
appear to give the most accurate


It is obvious that little or no profit will accrue from the
contemplation of the above figures unless they be correlated
with the factors which determine and control the process.

* Sachs: "Arbeit. Bot. Inst.," Wurzburg, 1884, 3, 19.
+ Brown and Morris: " Journ. Chem. Soc.," Lond., 1893, 63, 604.
% Brown and Escombe: " Proc. Roy. Soc.," Lond., B. 1905, 76, 29.
Thoday : Id., 1910, 82, 421.

II For a critical review of Sachs's and Brown and Escombe's methods,
see Thoday: " Proc. Roy. Soc.," Lond., B. 1909, 82, i.


The doctrine of limiting factors, now well known, is due
to F. F. Black man * who enunciated the axiom that when
a process is conditioned as to its rapidity by a number of
separate factors, the rate of the process is limited by the pace
of the slowest factor. The limiting factor in any definite
instance may be identified by the experimental application
of the principle that " when the magnitude of a function is
limited by one of a set of possible factors, increase of that
factor, and of that one alone, will be found to bring about
an increase of the function." j-

The principle may be illustrated by one of Matthaei's J
many experiments on the effect of temperature on carbon
assimilation under conditions constant except for temperature
and illumination. In the case of Prunus laurocerasus there was
a gradual increase in the assimilation as the temperature was
raised; at about 11 C. a maximal assimilation of 22 mg.
of carbon dioxide per 50 square cm. per hour obtained and
was not increased even by raising the temperature to 25 C.
By doubling the light intensity, however, the maximal as-
similation was equivalent to 37-5 mg. of carbon dioxide
per 50 square cm. per hour and again there was no increase
on raising the temperature. This means that light intensity
was a limiting factor and only by its increase could a greater
carbon assimilation be obtained.

The factors which condition carbon assimilation are ex-
ternal and internal : the external factors are capable of
control whilst the internal are much less amenable to experi-
ment and thus are less understood. Of the external factors,
the raw materials, temperature, and illumination are the
most conspicuous ; and of the internal factors, chlorophyll
and the products of carbon assimilation are the best under-

*Blackman: "Ann. Bot.," 1905, 19, 281.

4-Blackman and Smith: " Proc. Roy. Soc.," Lond., B. 1911, 83, 389.

Matthaei: " Phil. Trans. Roy. Soc.," Lond., B. 1904, 197, 47. For the
application of the principle to the growth of field crops see Balls and Holton :
"Phil. Trans. Roy. Soc.," Lond., B. 1915, 206, 103, 403; and Balls: Id.,
1917, 208, 157.



Water and carbon dioxide are the requisite raw materials
for carbon assimilation.

WATER. Water is essential not only as such for the fabrica-
tion of food, but also to keep the leaf tissues in a condition
mechanically fit for the processes to take place. Thoday found
that the rate of carbon assimilation lessened as the leaves of
Helianthus annuus lost their turgidity ; in an extreme instance,
when the leaves were very flaccid, the stomates were all but
closed and the increase in dry weight was very small indeed.
Some determinations by Thoday of the average increase in
dry weight of leaves in different conditions of turgidity have
been mentioned. If cells become plasmolysed, constructive
activities must cease ; if in such cells the turgid condition be
not recovered, death supervenes. With regard to the water
supply, the transpiration current is the immediate source ; it
is, however, not convenient on the present occasion to con-
sider the problems presented by this phenomenon.

CARBON DIOXIDE. Under normal conditions, the carbon
dioxide for carbon assimilation is derived from the atmosphere
and to a lesser extent from the products of respiration.* The
amount of respiratory carbon dioxide is conditioned mainly by
the temperature and may be equal to half the possible inflow
from the atmosphere at the higher temperatures possible in
laboratory experiments. The entry of atmospheric carbon
dioxide into the plant is either through the intact epidermis,
as in those plants which like certain aquatics lack stomates, or
mainly through the stomates and, to a much lesser and negli-
gible extent, provided the amount of carbon dioxide is not
unduly increased, through the unbroken epidermis. This cuti-
cular path of gaseous interchange once was thought to be the

* Under abnormal conditions it appears that plants can make use of carbon
dioxide from the soil. Pollacci ("Atti. Inst. Bot. Univ. Pavia," 1917, 17,3)
found that plants grown in soil rich in humus or in water culture enriched with
carbon dioxide could form starch and increase in dry weight notwithstanding the
fact that their aerial parts were in an atmosphere freed from carbon dioxide.
The assimilation, however, was not sufficient for normal growth.

VOL. II. 2


main route, but the work of F. F. Blackman * and of Brown
and Escombe f has shown that the stomates are the important
paths. F. F. Blackman found by direct measurement that the
degree of gaseous interchange was proportionate to the distri-
bution of the stomates, results which Brown and Escombe con-
firmed in respect to plants with stomates on but one surface
of the leaf, but with regard to instances in which stomates
occur on both surfaces of the leaf, they found that in bright
sunlight the intake of carbon dioxide into the upper surface
is greater than would be expected from the ratio of distribu-
tion of the stomates on the two sides ; in light of a lesser
intensity, however, there is a closer, but not very close, corre-
spondence between the intake of carbon dioxide and the pro-
portional distribution of the stomates. It is suggested J that
the greater infusion found to obtain into the upper side of
amphistomatous leaves may be accounted for in that partial
opening of the stomates is likely when the incidence of illumi-
nation is on that side, and that since the palisade parenchyma
is the more active part of the mesophyll, there will be a steeper
diffusion gradient in the upper side which will promote a more
rapid flow of carbon dioxide through the stomates of the upper

The movements of the carbon dioxide are in accordance
with the laws of gaseous diffusion; the pressure of carbon
dioxide in the active chlorenchyma will be very low, whilst in
the atmosphere surrounding the leaf it will correspond to, say,
three parts in 1 0,000. Thus there are set up diffusion cur-
rents the gradients of which vary according to the conditions,
rate of use and degree of atmospheric motion for example,
obtaining. The problem of interchange between the gases
contained in the leaf and in the surrounding atmosphere is
not, however, so simple as may appear from this statement.
Brown and Escombe, experimenting with leaves of Catalpa
bignonioides^ found that the rate of absorption of carbon
dioxide at normal temperature and pressure was about 0*07
c.c. per sq. cm. per hour; since the total area of the stomates
was but 0*9 per cent of the total leaf surface, it follows that

* Blackman : " Phil. Trans. Roy. Soc.," Lond., B. 1895, 186, 485, 503.

f Brown and Escombe : Id., 1900, 193, 223.

II bid. : " Proc. Roy. Soc.," Lond., B. 1905, 76, 29.


carbon dioxide must pass through the openings at the rate of
777 c.c. per sq. cm. per hour, an amount so considerable
when regard is had to the stomatal area and to the fact that
this rate of absorption is about fifty times greater than the
absorption of atmospheric carbon dioxide by a normal solution
of caustic potash, that it is hardly surprising that earlier physi-
ologists laid much stress on cuticular gaseous interchange.

The conditions obtaining in an active green leaf are briefly
these : the active chlorenchyma is absorbing carbon dioxide,
which must enter the cell in a state of solution, from the air-
space system on which it abuts ; hence the pressure of carbon
dioxide in the immediate neighbourhood of these surfaces of
absorption will have a very low value, possibly approximating
to zero under ideal conditions. Diffusion currents are thus
set up, a falling gradient of carbon dioxide density from the
" respiratory chamber " of the stomatal apparatus obtaining.
Renewal of carbon dioxide is from the external air through
the pore of the stomate, a cylinder of a certain length and,
in the simplest cases, of approximately uniform diameter
when the guard cells are fully turgid. Clearly various tensions
of carbon dioxide occur : the maximum in the atmosphere,
theoretically at an infinite distance from the leaf but practically
at a distance equivalent to five or six times the diameter of
the stomate, and the minimum at the absorbing surfaces of
chlorenchyma ; thus a gradient of density of carbon dioxide
is formed so that a drift of this gas from the outer atmosphere
to the chlorenchyma obtains. The path followed by the
carbon dioxide is obstructed by the stomates and may be
divided into three sections : from the remote atmosphere
where the pressure of carbon dioxide is greatest, /o, to the
outer opening of the stomate; through the shaft of the
stomate when the pressure of carbon dioxide is less, p ; and
from the inner opening of the stomatal shaft to the surfaces
of absorption where the pressure of carbon dioxide is, ideally,
zero. The interposition of the stomata exercises a pro-
found influence on this diffusion. Brown and Escombe*
demonstrated that the rate of diffusion through such an ab-
sorbing disc as is represented by a stomate is proportional to

* Loc. cit.


the diameter of the opening ; * they visualize zones of equal
density above the stomate varying from the atmospheric
density, /?, to a lower density, p', at the stomate cut perpen-
dicularly by the lines of flow of carbon dioxide converging
to the opening of the stomate f (Fig. i). Thus it is that the
increased flow of gas through the stomate is possible.

The second section in the route of the carbon dioxide is
through the tube formed by the guard cells. Through this
tube the flow is inversely proportional to the length of the
tube, but the system of external shells increases the resistance
to the flow (Fig. 2). Finally, in the third section the tube

FIG. 2.

opens into the air space system of the mesophyll bounded by
the absorbing surfaces of the chlorenchyma ; here the con-
verse of the first part of the path obtains, diffusion shells over
the lower opening of the stomate, where the density of the
carbon dioxide is p' y being formed (Fig. 3). Thus in the
whole system there is a gradient of density from p to, say, o,
with a set of shells at either end of the stomatal tube (Fig. 4).
The obstruction to gaseous diffusion inseparable from a
multiperforate septum such as the stomatal epidermis of a
leaf, varies according to the distance apart of the perforations :
if they are placed at distances roughly equal to ten times the

* Larmor's Law of Diameters : Q = zkpD where Q = amount of CO 2
absorbed in a given time ; k = coefficient of CO 2 in air ; p = density of at-
mospheric CO 2 at a point far removed from the absorbing disc ; D = diameter
of disc.

fQ = 2k(p-p')D.



diameter of a perforation, each will act independently without
interference by its neighbours and conform to the law of
diameters. When situated more closely together, it was
found that the obstacle to diffusion was much less than the
actual obstruction of area by the solid portions of the septum.
To quote concrete examples, Brown and Escombe calculated
that in Helianthus the leaves could absorb 2*578 c.c. of carbon
dioxide per sq. cm. per hour in moving air and 2-095 c - c - P er
sq. cm. per hour in still air, assuming that the stomates are

circular in shape. By actual measurement it was found that
in this same plant * in diffused light at a temperature of 19 C,
0*434 c.c. of carbon dioxide, per sq. cm. per hour was absorbed,
an amount much less than the capacity of the stomates to
supply. Those conditions, humidity of atmosphere, tem-
perature and illumination, which affect the size of the stomates
obviously will affect the infusion rate of carbon dioxide, f
Under natural conditions carbon assimilation is limited by the

* Brown and Escombe : " Proc. Roy. Soc.," Lond., B. 1905, 76, 29.
fSee Darwin: "Phil. Trans. Roy. Soc.," Lond., B. 1898, 190, 531; 1916*
207, 413. Knight: " Ann. Bot.," 1916, 30, 57-


low pressure of carbon dioxide in the atmosphere, the high
values obtained in experiments under conditions involving an
increased supply of the gas never being attained.

In experimental work with land plants a limit is set to the
increase of carbon dioxide supply by the narcotic effect of the
gas when in excess,* 25 per cent generally will inhibit growth ;
aquatic plants, on the other hand, are able to withstand a
relatively high concentration. Blackman and Smith t found
that Fontinalis and Elodea in water with a carbon dioxide
concentration of 33*92 and 35'82 per cent of saturation, and
under identical intensity of illumination and at temperatures of
23 C. and 28 C. respectively, assimilated -0223 and -0249
grams of carbon dioxide per hour per standard area of 137 sq.
cm. Fontinalis is less efficient, for reasons not finally determined,
than aquatic angiosperms such as Elode.i, Potamogeton and
Ceratophyllum in utilizing carbon dioxide. Blackman and
Smith, experimenting with Elodea and Fontinalis under
constant conditions of light and temperature and with a carbon
dioxide supply ranging from -0025 to '0540 gms. per 100 c.c.
of water, found that the carbon assimilation increases steadily
in proportion to the increase in the supply of carbon dioxide.
When the assimilation reaches about -023 grams of carbon
dioxide per hour, however, there is no further increase with
an augmented carbon dioxide unless the light intensity be
increased. In other words, light intensity in this connection
is a limiting factor.

The difficulties inseparable from critical investigations in
the field are obvious : of recent work, that of McLean J on the
carbon dioxide absorption of coconut leaves under natural
conditions may be mentioned. He found that the rate of
absorption is at a maximum in the morning, a depression
obtains at mid-day followed by an increase in the afternoon
and then a final decline towards sunset. Similar values were
obtained for detached leaves, but the curve showed a single
maximum at about noon instead of two maxima which
normally obtain with attached leaves, for which difference
there is no adequate explanation. Clearly some internal

* See Chapin : " Flora," 1902, 91, 348.

f Blackman and Smith: " Proc. Roy. Soc." Lond. B. 1911, 83, 389.

I McLean: "Ann. Bot." 1920, 34, 367.


limiting factor is operating, possibly connected with the
accumulation of the products of carbon assimilation (p. 35).
It was also found that immature and old leaves absorbed
carbon dioxide at a lesser rate than leaves of an intermediate


The statement that chemical change is profoundly influ-

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