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and then reproduces itself by gemmatioa The phenomenon
may be illustrated by the accompanying Fig. 10 which is based
upon observations made by Slator.H This figure represents
the offspring of a single cell up to and including the fourth
generation. The cycle was completed in 232 minutes from the
second generation, the average time for the interval between
one generation and the next, that is the generation time, being
seventy-six minutes.

Elaborating this general statement, the growth of the yeast
exhibits a sequence of phases the conspicuousness and dura-
tion of which varies with the conditions. The "seed" on
being sown in the wort may remain inactive for a time ; this
is the lag phase, the duration of which depends in the main
on the age of the seed, old plants showing a longer quiescent
period than young plants grown from spores which may show

* Robertson: "Arch. Entwicklungsmech. Org.," 1913, 37, 497.
fReed and Holland: " Proc. Nat. Acad. Sci.," 1919, 5, 135.
JReed: "Journ. Gen. Physiol.," 1920, 2, 545.

See Drabble, E. and H., and Scott : " Biochem. Journ.," 1907, 2, 221.
||Slator: Id., 1918, 12, 248.

8*



ii6 GROWTH

no lag phase. When once growth has started, it is unrestricted
and follows the logarithmic law, for which reason this phase
often is termed the logarithmic phase. This is followed by a




FIG. 10.



period of retardation due to the operation of factors such as
the accumulation of carbon dioxide and the lack of oxygen.
This is a normal sequence : but departures from the normal
may result on varying the conditions of growth ; thus if the



GROWTH OF THE YEAST PLANT 117

seeding be heavy, but not excessive, the logarithmic phase may
disappear and retardation set in immediately after the lag
phase. If the seeding be excessive, the retarding factors may
prevent any measurable growth.* Priestley and Pearsallf
look upon the logarithmic phase as the natural rate of increase
where increase in mass is an exponential function of time ; the
retardation phase, in which the growth rate is directly pro-
portional to time, is due to adverse operating factors, such as
the accumulation of end products, which are the outcome of
the earlier unrestricted growth. In addition to the obvious
products, carbon dioxide and alcohol, of the activity of yeast,
oxygen and sugar may also influence its growth.

The influence of oxygen is important, and this notwith-
standing the fact that yeast is capable of action under anaerobic
as well as aerobic conditions. The growth exhibited under
these different circumstances is not the same : when grown
anaerobically, yeast cells quickly acquire a static condition of
equilibrium with regard to the medium in which they are sus-
pended ; J the lack of oxygen, especially prior to the begin-
ning of gemmation, arresting the reproduction function. It is
the small amount of oxygen initially present in the wort
which is considered to explain the fact that under fixed con-
ditions the maximum cell increase is independent of the
number of cells of seed yeast per unit volume of yeast. Thus
Horace Brown found that up to 65 or 70 per cent of com-
plete aeration, the cell increase is a linear function of the avail-
able free oxygen at the commencement of growth. In other
words, during the period of active reproduction in a suitable
medium in which access to oxygen is limited to that initially
present in solution in the liquid and under conditions of culture
which eliminate the competition factor, the number of yeast
cells present at any moment is directly proportional to the time.
Reproduction can, however, take place under anaerobic con-
ditions to a small extent : Horace Brown finds the limit to be
6*5 cells for each initial cell of seed yeast. This is explained
by the fact that before reproduction takes place, the cells of
the seed yeast absorb and fix oxygen which renders possible

* Slator: " Biochem. Journ.," 1918, 12, 248.

t Priestley and Pearsall: "Ann. Bot.," 1922, 36, 239.

t Adrian Brown: "Journ. Chem. Soc.," Lond., 1905, 87, 1395.

Horace Brown : " Ann. Bot.," 1914, 28, 197.



ii8 GROWTH

this limited reproduction under anaerobic conditions. This
absorption of oxygen, which is a linear function of time, takes
place with great rapidity ; thus in one instance it was found
that "3 gram of pressed yeast per loo c.c. of liquid completely
absorbed the oxygen in two and a half hours. Horace Brown
concludes that the power of reproduction is impressed in the
cell at the very outset by the absorbed oxygen and that a
quantitative relation exists between this absorbed oxygen and
the number of units which the initial yeast cell can finally
gemmate. The action of the oxygen is one of induction and,
according to Horace Brown, all the known facts can be ex-
plained on the assumption that the available oxygen is equally
divided between the initial cells and the consequent variation
in the oxygen charge which these cells must receive when the
ratio of the seed yeast to the available oxygen varies.

The amount of oxygen in aerated wort may be very small
but its effect may be very great : thus I c.c. of oxygen in aerated
wort brings about a growth sixty times greater than the same
amount of oxygen in nonaerated wort* Slator is impressed
by the importance of carbon dioxide as a conditioning factor
in the growth of yeast and he considers that the influence of
this gas is much greater than is generally supposed, in fact
that some of the observed effects generally ascribed to the direct
influence of oxygen may be due to its indirect action in
lessening the supersaturation of the wort with carbon dioxide.
For measurements of the rate of growth in wort and in wort
saturated with carbon dioxide show much retardation, possibly
due to the carbon dioxide rather than to the lack of oxygen.
These measurements are confirmed by controlled experiments
in which the carbon dioxide was the limiting factor. In fact,
a correlation can be made out between the crop of cells and
the concentration of carbon dioxide in the medium :

Proportionate Concentration Crop of Crop

of Carbon Dioxide = a. Cells, t a

1-08 3*0 2-8

i -08 3'o 2-8

1-46 4-1 2-8

1-48 4*5 3'i

2'i3 6'i 2*9

2-33 6-5 2-8

* Slator: " Journ. Chem. Soc.," Lond., 1921, 119, 115.
fUnit of crop of cells = 7*65 x io 6 cells per c.c.



GROWTH OF THE YEAST PLANT



119



With regard to the influence of sugar, there is, according
to Slator, a proportion between the size of the crop and the
initial concentration of glucose up to about I per cent. The
accompanying curve (Fig. 1 1) shows the retarding influence of
sugar in increasing concentration, when sugar is the limiting
factor. Its slope corresponds to 3900 x io 16 cells per gm.,
a figure in fair agreement with the calculated number.

With respect to alcohol, under ordinary conditions of
brewing its accumulation is never so great as to be a significant
factor in the growth of the yeast ; if, however, alcohol be present



=60

o

UD



03



5 1-0 1-5 2-0 25
Cms. Glucose per 100 cc.

FIG. ii.

in excessive amount, it has a limiting action ; thus the presence
of 8 per cent of alcohol in the fermenting liquor may inhibit
reproduction, especially if the supply of oxygen is limited.

This survey, incomplete though it be, shows the usefulness
of offspring as an index of growth * and emphasizes the im-
portance of various conditioning factors in the governance of
life processes.

The growth rate of a plant varies with the age of the
organism and also may show periodic and seasonal variation.

* A little thought will show that measurements of increase in length and girth
of those parts of plants exhibiting merismatic activity are in reality measurements
of offspring of the dividing elements.



120 GROWTH

Eucalyptus regnans shows the greatest rate of increase of
area between the age of forty and fifty years.* The measure-
ment of the diameter of the annual rings indicates that the
growth rate falls off with time. In many instances growth in
thickness is periodic through the seasons on account of climatic
factors, thus at Peradeniya Hevea brasiliensis shows no growth
in thickness during the dry season January to March. From
the end of March to the beginning of October, the wet season,
growth is uniform ; whilst during the dry season October to
December growth is considerably less and sometimes may
cease altogether.! In instances such as this it would appear
that climate is all-important, especially as regards the provision
of adequate supplies of soil water. Thus the erratic growth of
Hevea observed during the second dry season may be pre-
determined by the amount of rainfall during the previous wet
season.

In more temperate climates, soil temperature is a significant
factor; the observations of McDougallJ show that the root
growth of forest trees begins in the early part of the year when
the soil temperature reaches a degree sufficiently high for
absorption of water to take place, and stops in the autumn
when the soil becomes too cold. There is no inherent tendency
for periodicity, and when a resting period obtains during the
summer months, its cause may be found in the decreasing
water supply. There is, however, as Fetch has shown for Hevea,
a personal as well as a specific physiology of plants 3 an almost
untrodden field, and many of the results observed may be
ascribed to personal peculiarity rather than to this or to that
factor.

Allusion has been made to the importance of the previous
history of the conditions and of the plant in determining the
activity of the plant at the present moment. In the present
connexion, the important and extensive work of Balls on the
cotton plant must be mentioned. From a long series of
observations, Balls concludes that the different behaviour of the
plants, as indicated by the crop of cotton, are the inevitable

*Patton: " Proc. Roy. Soc. Victoria," 1917, 30, i.

f Fetch: "Ann. Roy. Bot. Gard. Peradeniya," 1916, 6, 77.

McDougall : " Amer. Journ. Bot.," 1916, 3, 384.

Balls: " Phil. Trans. Roy. Soc./' Lond., B., 1917, 208, 157.



EFFECT OF TEMPERATURE 121

consequences of the known environmental conditions, provided
that proper regard is paid to the distinction in time between
the incidence of the conditioning factor and its manifestation in
the crop. Thus the daily fluctuations in the flowering curve
are predetermined and controlled by the weather conditions
which obtained a month before the flower opens. This is the
principle of predetermination : its importance is obvious ; an
accurate knowledge of predetermining factors will amongst
other things give to certain aspects of physiology, ecology and
agriculture a precision which now is sometimes lacking.

THE CONDITIONING FACTORS.

TEMPERATURE. The factors which condition growth and
its rate are precisely those which influence anabolic and
catabolic activities, and the effect of any one such factor on
growth is the resultant of its action on the opposing com-
ponents of growth. Thus temperature accelerates respiration
and also carbon assimilation, but since in a vigorous green
plant the products of one hour's carbon assimilation is a
sufficient provision for many hours' respiration,* there will be
a balance on the credit side ; wherefore an increased temperature
will result in an increased rate of growth, other things being
equal. But there is a limit to the height of the temperature;
if a certain degree be exceeded, growth will be retarded, come
to a halt, and ultimately a decrement will obtain. In illus-
tration, it is a common laboratory experience that the increase
in length of the radicle of a seedling is a linear function of time
and that there is a gradual increase in the rate of growth up to
about 28 C. Talma f observed the growth of the radicle of
Lepidium sativum from o to 40 C. Under conditions of
constancy of temperature for at least three and a half hours,
it was found that measurable growth obtained at o C. and that
the greatest rate occurred at 28 C. Van't Hoff's law is appli-
cable only for small ranges of temperature and, in general
terms, the temperature coefficient decreases with a rising
temperature.

*See Boysen-Jensen : " Bot. Tidsskr.," 1918, 36, 219.
t Talma: " Koninkl. Akad. Wetensch.," 1916, 24, 1840.



122 GROWTH

The same general observations have been recorded for
lower plants. Thus Fawcett * found that the radial growth
rate of cultures of the fungus Pythiacystes atrophthora remained
constant as long as the environmental conditions did not
change. With increasing temperature a greater rate of
growth is exhibited up to a certain degree; when this is
passed, the rate of growth falls off. Thus :

At 10 C., radial growth rate = 2-5 mm. per 24 hours.
"2oC., =6-0

,, 28 C., ,, ,, = 7'5 ,, ,, ,, ,,
33 C., = 2-6 ,,

Balls f measured the growth in the length of the hyphae
of the fungus causing " sore skin " on the cotton plant. The
growth rate at different temperatures is what might be ex-
pected from van't HofFs law; but at higher temperatures,
38 C., there is a decrease in growth followed by complete
cessation probably due to the accumulation within the cell
of certain products of catabolism ; these deleterious substances
presumably are formed at lower temperatures but at a much
slower rate; they diffuse out into the surrounding medium,
especially at higher temperatures, possibly on account of altera-
tions in the permeability of the protoplasm at these higher
temperatures.^ In the case of higher plants, the outward
diffusion of these harmful bodies, provided they be formed,
must be slower on account of the more massive nature of
the structures, or they are oxidized within the tissues them-
selves.

As Balls points out, since the conditions under which
this decomposition takes place must be fairly uniform in a
higher plant, growth optima are shown which are the ex-
pressions of the internal struggle between the increasing
rapidity of chemical change with the rise in temperature and
the inhibitory action of the accumulating products of cata-
bolism.

The investigations of Leitch on the influence of tempera-
ture on the rate of growth in the roots of Pisum sativum,
show that the relationship can be expressed as a uniform

* Fawcett : "Johns Hopkins Univ. Circ.," 1917, 193.

f Balls : " Ann. Bot.," 1908, 22, 557.

JSee Eckerson: " Bot. Gaz.," 1914, 58, 254.

Leitch: "Ann. Bot.," 1916, 30, 25.



INFLUENCE OF TEMPERATURE 123

curve for the range of temperature - 2 C. to 29 C. and
resembles those of Kuijper for respiration. Above 29 C. there
is so much fluctuation that relationship cannot be expressed
in a single curve, wherefore for each higher temperature a
different curve must be made to express the rate of growth
in successive periods of time. This is owing to the operation
of one or more of those imperfectly known factors termed by
F. F. Blackman the time factor. For instance, at 30 and 3 5 C.
the rate of growth in the first ten minutes is the highest at-
tained, in the first half hour there is a rapid fall followed by a
recovery marked by a rise to a second maximum, after which
there is a gradual fall. At 40 C., the decrease in growth
rate is uniform and rapid, there being no recovery.* As in
the cress, observed by Talma, so in the pea ; the coefficient
for a rise in the temperature of ten degrees shows a distinct
falling off as the temperature rises, and, according to the
observations of Leitch, it is only between 10 and 22 that
the coefficient value lies between 2 and 3, for which reason
the complete curve is not regarded as a van't Hoff curve.
The extremes of measurable growth was observed at - 2
and 44*5 C., the highest rate being at 30-3 C. With re-
ference to these observations, Leitch distinguishes four cardinal
points : the minimum temperature, the maximum temperature,
the optimum temperature and the maximum rate temperature.
The minimum temperature for any physiological process is
the lowest temperature at which the process takes place;
the maximum temperature is the highest temperature at which
the process takes place ; the optimum temperature is the highest
temperature at which there is no time factor operating; and
the maximum rate temperature is that temperature at which
the process attains its highest intensity.

The fluctuations observed at higher temperatures may be
due to several factors ; under such conditions some bio-
chemical reactions may be intensified but not necessarily at
the same rate ; and some may be inhibited in varying degrees.
The operations of enzymes, for example, are not affected in
precisely the same way by the same high degree of tempera-
ture. Such a contingency may interfere with the supply of

* See also Sierp : " Ber. deut. hot. Gesells.," 1918, 35, 3 ; *' Bot. Zentrbl.,"
1920,40, 433-



124 GROWTH

soluble food, and, concurrently, the rapid accumulation of the
products of respiratory processes, which also are accelerated by
an increased temperature, may have a toxic action. In a few
words, up to a certain degree a rise in temperature accelerates
physiological actions and here a Van't Hoff curve may be
expected : beyond this degree, co-ordination becomes less and
less, wherefore metabolic derangement obtains. Priestley and
Pearsall,* in their examination of Leitch's results, point out
that the growth rate of radicles is dependent on the chemical
reactions involved in the merismatic activity of the growing
point, and on the hydrolysis of the reserve food in the seed and
the translocation of the products to the growing parts.

The situation therefore is this : the increase in temperature
accelerates both growth and hydrolysis, but the merismatic
tissue quickly uses up the food immediately available, wherefore
a decrease in the rate of growth must ensue for that period of
time taken by the food materials in their translocation from the
seed to the apical regions of the root. The arrival of this food
accounts for the second maximum in Leitch's curves. Finally
the growth rate is diminished by the dislocation of the meta-
bolic processes. If this contention be correct, a close correla-
tion between the length of the root and the time of the incidence
of the second maximum should obtain.

LIGHT. The influence of light on growth is a subject of
considerable magnitude, especially when the term growth is used
in its general sense : the directive action of light in tropistic
and kindred phenomena, its influence in the determination of
growth form and the facies of a flora are aspects of the subject
outside the scope of the present consideration.

The action of light on growth is both direct and indirect,
and its action is most marked in those members which use
light as a source of energy. Thus for the ordinary green
plant, increase without light is an impossibility since light is
the source of energy for the making of food; in non-green
members of plants, in total parasites, and saprophytes, on the
other hand, light, for obvious reasons, is not a factor of
consequence. It is a laboratory commonplace to find that
for the subjects generally used for experimental purposes, light

* Priestley and Pearsall : " Ann. Bot.," 1922, 36, 239.



INFLUENCE OF LIGHT 125

influences growth, as indicated by its external expression of
increase in length and area, in various ways. Thus internodes
in darkness attain a much greater length than in light ; leaves
may develop hardly at all in darkness, as in the instance of
the pea, whilst in other cases, the wheat for example, the
leaves attain a size more or less equal to that in light. This
difference in behaviour is apparently due to the amount of
carbohydrate, relatively large in the wheat and relatively small
in the pea, available for structural purposes. In passing,
attention may be recalled to the fact that the humidity of the
atmosphere is an important factor in the configuration of a
plant, wherefore in experiments on living plants in closed
chambers, allowance must be made for the humidity conditions.

With regard to the different qualities of light, it is commonly
accepted that growth is promoted by the blue and violet rays
whilst those less refrangible retard. Here again care must be
taken to ensure a just comparison : to judge the effect of, say,
blue light and red light, the intensities of each must be the
same or, at any rate, known ; further, in such an experiment
due allowance for the different heating effects of light of dif-
ferent wave lengths must be made, and in some cases, if not in
all, the internal temperature must be observed rather than the
temperature of the surrounding medium.

The problem presents three aspects : intensity, duration,
and quality. Sierp * found that the general effect of an increase
in light intensity is to accelerate the rate of growth of the
coleoptile of Avena sativa and to shorten the time within which
the maximum rate occurs and that the incidence of this maxi-
mum is earlier as the light intensity increases. The net result
is that the total growth is reduced as the intensity of light
increases.

With regard to the effect of the duration of light on plant
growth, Garner and Allard f conclude from their observations
on the tobacco, soy, and other plants that the amount of
vegetative growth is proportional to the duration of exposure
to daylight, short exposures resulting in small slender plants
exhibiting a slower rate of growth. They also find that the

* Sierp: " Ber. deut. hot. Gesells.," 1918, 35, 8 ; "Bot. Zentrbl.," 1920, 40,

433-

f Garner and Allard: " Journ. Agric. Res.," 1920, 18, 553.



126 GROWTH

duration of light is an important factor in instituting the repro-
ductive phase ; by modifying the periods of exposure, bienniels
may be made to complete their life histories in a few months,
and, on the other hand, annuals may be converted into bienniels.

In this connexion the results obtained by Blaauw * and of
Vogt f from their studies on the growth of the sporangiophore of
Phy corny ces and on the coleoptile of Avena respectively are
important. According to Blaauw, the effect of light on growth
is an acceleration followed by a retardation to a rate lesser than
the normal followed by a gradual increase to the normal rate.
The time of incidence of the initial acceleration varies with the
intensity of the illumination ; thus on exposure to a light of
intensity of one unit, the acceleration begins in about eight
minutes, but in an intensity of fifteen units, the acceleration
begins in about three and a half minutes. The amount and
duration of the reactions vary with the degree of illumination :
for the lower light intensities the total acceleration of growth
exceeds the total retardation ; and for the higher light intensities
the total retardation exceeds the total acceleration. For the
former, Blaauw finds that the increased growth is proportional
to the cube root of the amount of light. These results are
extended by Vogt, who not only finds the same acceleration
and retardation in the growth of the coleoptile of Avena sativa,
but also a considerable initial decrease in growth rate on tempor-
ary exposure to a sufficiently strong illumination. In this
instance, therefore, there is first a rapid decrease, immediately
followed by an acceleration, which is in turn followed, especially
under increased or more protracted illumination, by a second
inhibition phase which exceeds the previous acceleration ; hence
the total effect may be a considerable fall in the rate and in
the amount of growth. For a given reduction in growth, the
product of the light intensity and its duration is a constant.
Gregory J also found that under comparable conditions the
average leaf area of the cucumber is determined by the product
of the intensity and the duration of the light radiatioa

The initial decrease in the growth rate is considered by
Vogt to be due to the action of light alone, not to the combined

* Blaauw: "Zeitsch. Bot.," 1914, 6, 641.

fVogt: Id., 1915,7, 193-

I Gregory: "Ann. Bot.," 1914, 35, 97.



INFLUENCE OF LIGHT 127

effect of alternating light and darkness nor to increased
transpiration.

The following observations of Vogt illustrate the photo-
tonic reaction : when the coleoptile of the oat was alternately
illuminated by light of the same intensity, and darkened for
periods of fifteen, thirty, and sixty minutes, lesser growth was
found to obtain only in the two latter periods of illumination.
In these instances the greater growth in darkness is considered
to be due to the stimulation of the previous exposures to light.
The slower growth in periods of illumination is merely a part


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Online LibraryPaul HaasAn introduction to the chemistry of plant products (Volume 2) → online text (page 11 of 13)