(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "Experimental Plant Physiology"

PAGES MISSING 
WITHIN THE 
BOOK ONLY 

(25TO28) , (55TO58 ), 
(71TO74) 



00 

162962 



OUP 730-28-4-81 10,000. 

OSMANIA UNIVERSITY LIBRARY 

Call No. * ' Accession No. 

. 
Author 




This book should be returned on or before the date last marked 



EXPERIMENTAL 
PLANT PHYSIOLOGY 



EXPERIMENTAL 
PLANT PHYSIOLOGY 



FOR BEGINNERS 



BY 



LUCY E. COX; B.Sc. 



LECTURER IN BOTANY AND NTfUKE STUDY AT QFAYSTOKE 
PLACE TRAINING COLLEGE, LONDON 



WITH ILLUSTRATIONS 



LONGMANS, GREEN AND CO. 
30 PATERNOSTER ROW, LONDON 

FOURTH AVENUE & 30TH STREET, NEW YORK 
BOMBAY, CALCUTTA, AND MADRAS 



AU rights reserved 



PREFACE 

IN this book the main physiological facts gf plant-life 
ire connected together by a series of simple experi- 
nents, all of which can be carried out without the use 
of any elaborate apparatus. As far as possible the 
inferences obtained in the working of one experiment 
form the starting-point for the next. 

The book i$ intended essentially as a guide to practical 
work, and every experiment should therefore be care- 
fully worked through, not merely reaci. The best plan 
for a teacher to follow, in all physiological work, is to 
present a problem to the children, and encourage them 
to suggest the method by which it may be solved and 
to draw their own conclusions from the observations 
made. 

By a Standing Order of the London County Council, 
it must be stated that the Council accepts no responsi- 
bility for the opinions or conclusions given in this book. 

L. E. C. 



CONTENTS 

SECTION I 



PAGE 

THE FOOD OF THE PLANT . . . . . . 1 



CHAPTER 1 

I. THE COMPOSITION OK THE SOIL .... 1 
II. THE ABSORPTION OF FOOD FROM THE SOIL . . 2 

CHAPTER II 

I. THE COMPOSITION OF THE AIR .... 32 
II. THE ABSORPTION OF FOOD FROM THE AIR . . 43 

SECTION II 
THE BREATHING OF THE PLANT ..... 70 

SECTION III 
THE GROWTH OF THE PLANT 81 

INDEX 109 



vii 



EXPERIMENTAL 
PLANT PHYSIOLOGY 

SECTION I 
THE FOOD OP THE PLANT 

CHAPTER I 

T. THE COMPOSITION OF THE SOIL 

Introductory, The fact that as a plant develops it 
increases in weight, is one that needs no demonstration. 
The oak tree is obviously heavier than the acorn from 
which it grows. This increase in weight must have 
been caused by absorption of food-material on the part 
of the plant. Now the only available sources from 
which a plant can absorb food are the soil and the air. 
It is necessary to determine, therefore, whether the 
plant takes in food from one or both of these sources, 
and, further, what is the nature of the food taken in. 

The Composition of the Soil. The soil is composed 
of grains formed from the breaking down of rocks, to- 
getner with a varying amount of humus or decaying 
animal and vegetable matter. The particles are loosely 
held together, and the spaces between are filled with 
air and water. 

As a great many of the substances contained in the 
soil are soluble in water they pass into solution ; thus 
the water ceases to be pure and becomes a solution of 
various salts. 

An analysis of this solution is beyond the scope of 
this book, and we must be content to use the results 
given to us by chemists. 

A 



2 EXPERIMENTAL PLANT PHYSIOLOGY 

.The following is a list of the more common salts 
dissolved in the water of the soil : 

made up of sodium and chlorine. 
calcium, sulphur, and 

oxygen. 
magnesium, sulphur, 

and oxygen. 
,, sodium, sulphur, and 

oxygen, 

chlorine, nitrogen, 
oxygen, phosphorus, 
1 calcium, magnesium, 
and potassium. 



Common table salt, 
Gypsum 

Epsom salts 
Glauber's salts 

Traces of chlorides, 
nitrates, and phos- 
phates of calcium, 
magnesium, and 
potassium 



From this list it appears that the substances present 
(called by the chemist (< elements") are: sodium, 
chlorine, calcium, magnesium, sulphur, oxygen, nitro- 
gen, phosphorus, and potassium. 

It must be remembered that these are the more 
common elements present in solution in the soil, but 
they are not the only ones that may be present ; for 
instance, the important element, carbon, is often there. 
It is a compound of carbon, calcium bi-carbonate, that 
causes the " hardness" of water in limestone districts. 
But, as it will be shown later, carbon compounds in the 
soil are not necessary for plant-life. A plant thrives 
just as well irk. a soil that contains no carbon as in one 
in which this element is present. 

In the soil, then, certain substances are present that 
are available as food. By a series of simple experiments 
it is possible to find out how far these substances are 
made use of in the feeding-processes of the plant. 



II. THE ABSORPTION OF FOOD FROM THE SOIL 

In the working of any experiment there are four 
distinct steps that have to be considered. In the first 
place it is necessary to formulate a definite aim. After 



THE FOOD OF THE PLANT 3 

this some method of tackling the problem must be 
evolved. -Next, all observations made should be care- 
fully recorded. Lastly, the facts gathered from the 
observations must be set down in order. 
These steps may be written thus : 

Step 1. What it was desired to find out. 

Step 2. What was done. 

Step 3. What was seen. 

Step 4. What was learnt. 

Or, expressed more briefly : 

Step 1. Aim. 

Step 2. Method. 

Step 3. Observations. 

Step 4. Inferences. 

EXPERIMENT 1 

Aim. To determine whether a plant absorbs water. 

Method. A bean seedling or a daffodil bulb is grown 
in a jar of water. The surface of the water is covered 
with greased cardboard or tin-foil, to prevent loss 
through evaporation. The level of the water is marked. 

Observation. As the plant grows the water gradually 
decreases. 

Inference. Water must have been taken up by the 
roots that are in it. Thus : The root of a plant absorbs 
water. 

EXPERIMENT 2 

Aim. To determine whether parts of the plant, 
other than the roots, can absorb water. 

Method. Three well-grown bean seedlings or other 
small potted plants are used. The first is watered 
regularly in the usual way. The leaves and stem only 
of the second plant are watered ; to this end the whole 
of the pot as well as the soil should be carefully covered 
with tin-foil during the process of watering ; the tin-foil 
must be removed between the periods of watering, 



4 EXPERIMENTAL PLANT PHYSIOLOGY 

otherwise air also will be prevented from reaching the 
roots. The third plant is left unwatered. 

Observations. The first seedling thrives. The 
second and third die. 

Inference. The life of a plant cannot be maintained 
unless water is supplied to the roots. Thus : The root 
is the absorbing organ of the plant. 

(Note. This experiment applies only to land plants ; 
in submerged aquatic plants absorption takes place 
over the entire surface. It must also be remembered 
that a cut stem is able to absorb.) 

EXPERIMENT 3 

Aim. To determine the ratg, at which a plant is 

absorbing water and 
the external condi- 
tions by whiclrlffie 
rate is affected. 

Method. The 
pieces of apparatus 
^' ^ {'' | 'j= needed in this ex- 
periment are a glass 
flask or gas-jar with 
cork to tit, a thistle- 
funnel with a tap, a 
piece of very narrow 
glass-tubing, a ther- 
mometer, and a 
well -grown potted 
plant with a woody 
stem that will not 
easily be injured. 
FIG. i The glass-tubing 

is bent at right 

angles so that the long arm measures at least 15 inches. 

The length is marked off in inches by means of strips 

of gummed paper or a cardboard scale. 
The thistle-funnel, the glass-tubing and the plant 

are then fitted into the cork. 




THE FOOD OF THE PLANT 5 

To insert the plant it is well to cut a wedge-shaped 
piece out of the cork, fit the stem into the gap, cut 
off the angular end of the wedge and replace the 
remainder (Fig. 1). 

The glass vessel is then 'filled with water and the 
cork inserted. 

After this the thistle-funnel also is filled with water, 
and the tap is then carefully turned until water fills 
the whole apparatus, including the bent tube. .Qi&at 
care must JtejbaJken that no air is left L in ^the^ apparatus. 
TEe expulsion of air is facilitated if the short arm of 
the narrow glass-tubing does not project beyond the 
bottom of the cork. 

All connections must now be made air-tight. To 
this end a little paraffin-wax is melted in a porcelain 
evaporating dish. The paraffin should be allowed to 
cool until it is just setting, then it must be put over 
all the fittings of the apparatus and worked in well 
with the fingers. This method will be found cleaner 
and more effective than the pouring of melted paraffin 
over the joins. It will also prevent possible injury to 
the plant from the use of paraffin that is too hot. The 
beginner will often find it difficult to get a piece of 
apparatus air-tight ; if all the corks are soaked for a 
little while in melted paraffin before being used this 
difficulty is greatly lessened. Plasticine, instead of or 
in addition to paraffin, can often be used with advantage 
in making difficult fittings air-tight. 

Observations. Mark the Jime taken. for the water 
to be absorbedlthrough a faeasurea portion of the tube. 
Then refill the tube by means of the tap. Take at 
least two more readings and record the time taken in 
each case. If the observer wishes, the actual volume 
of water absorbed in a given time can be calculated 
from the formula irr^h (where 7r = -V 2 -, r= radius of tube, 
h~ length of tube). 

It is more important, however, to compare the rates 
of absorption under different conditions. Thus : 

1. Remove the apparatus to the coldest place 



6 EXPERIMENTAL PLANT PHYSIOLOGY 

pgssible. If ice is available the flask should be packed 
round with it. Take readings. 

2. Take readings when the apparatus is warmed. 

3. Compare the amount of absorption when the 
apparatus is in a windy place where the atmosphere is 
consequently drier. 

4. Take readings when solutions of gradually in- 
creased strength are substituted for water. 

The following is an account of an actual experiment 
made with the apparatus shown in the photograph 




FIG. 2 

(Fig. 2). A fuchsia plant was used and this proved 
very satisfactory, as it was hardy enough to withstand 
the various changes that it had to undergo during the 
course of the experiment. To make successful records 
a great deal of time and patience are necessary. One 
constant source of difficulty is the recurring leakage 
of the apparatus caused by varying contractions and 
expansions when the temperature is changed. This 
cannot be avoided, and therefore, as soon as a leak 
occurs, the fittings must be waxed again. The time 



THE FOOD OF THE PLANT 7 

given in every case is that taken by the water to move 
along one inch of the measured glass-tube. 

Thursday, May 1st. Dry, windy day. Temp. 13 G. 

Readings. Intervals. 
A.M. 
10.57 

11.10 13 min. 

11.25 15 

11.38 13 

11.50 12 

Average, 13'25 min. 

Ice was then packed ' round the jar as shown in the 
photograph. It was at once seen that the contraction 
of the water due to cooling would upset any readings 
made for a considerable time. The apparatus was 
therefore left until the following morning. Most of 
the ice had then melted and the temperature of the 
water was kept at C. by the addition of lumps of ice 
at intervals. It was assumed that by this time the 
temperature of the water inside the jar was constant. 
(More accurate readings could perhaps have been 
obtained by placing the thermometer inside the gas-jar, 
but this would have necessitated still another fitting 
to be kept air-tight.) 

It was necessary to re-wax the apparatus. 

Friday, May 2nd. Dry, windy day. Temp. C. 

Readings. Intervals. 
P.M. 
1.57 

2.26 29 min. 

2.51 25 

3.20 29 

3.48 28 

Average, 27*75 min. 

The apparatus was then left with the surrounding 
water; so that it might, with as little harm to the plant 
as possible, very gradually regain the normal tempera- 
ture. 

Further readings were then taken. 



8 EXPERIMENTAL PLANT PHYSIOLOGY 

' Monday, May 5th. Wet, still day. Temp. 13 C. 

Readings. Intervals. 
A.M. 
9.57 

10.17 20 min. 

10.37 20 

10.56 19 

11.15 19 

Average, 19*5 min. 

The apparatus was then heated to 20 C. This was 
done by placing it in a 7 lb. jam-jar containing water 
at 30 C. ; the water was allowed to cool until it reached 
20 C. and was then kept at this latter temperature by 
the addition of small quantities of hot water. Readings 
could not be taken for some time owing to the ex- 
pansion of the water inside the gas-jar. As soon as 
the temperature inside was constant, records were 
made. 

Afterwards the apparatus was heated to 30 C., and 
then to 40 C. by the same method. 

The whole day continued still and wet. 

Readings. Intervals. 
P.M. 

At 20 C. 1.7 

1.23 16 min. 

1.38 15 

1.52 14 

2.8 16 

Average, 15*25 min. 
P.M. 

At 30 C. 4.0 

4.10 10 min. 

4.19 9 

4.30 11 

4.40 10 

Average, 10 min. 
P.M. 

At 40 C. 5.38 

5.45 7 min. 

5.50 5 

5.56 6 

6.2 6 

Average, 6 min. 



THE FOOD OF THE PLANT 



9 



It was not considered desirable to raise the tempera- 
ture further lest the roots should become injured. 
The apparatus was then left to regain, gradually, the 
normal temperature. 

On the following day differences in the rate of ab- 
sorption, due to varying strengths of the solution ab- 
sorbed, were tested. 

A strong solution of common salt was made up; 
50 c.c. of this solution was then added to the water 
through the thistle funnel. A reading was taken. A 
second 50 c.c. was then added, and another reading 
taken. By the continuation of this process the strength 
of the solution inside the jar was gradually increased. 

As it was necessary to make allowance for the de- 
creasing vitality of the plant due to the continued ex- 
periment, two readings under normal conditions were 
first taken. 

Tuesday, May 6th. Dry, not windy. Temp. 13 G. 



Water. 
Water. 

50 c.c. salt solution added. 
100 
150 
200 
250 
300 
350 
400 



Readings. 
10.20 A.M. 
10.52 
11.22 
11.58 
12.38 P 

1.22 

2.12 

3.8 

4.4 

5.8 

6.28 



M. 



Intervals. 

32 min. 

30 

36 

40 

44 

50 

56 

56 

64 

80 



Inferences. The rate of absorption is affected by 
external conditions. It increases with rise of tempera- 
ture ; it also increases as the dryness of the surround- 
ing air increases. It is decreased by fall of temperature, 
and by an increase in the strength of the solution ab- 
sorbed. 

(Note. Had the apparatus been still further heated 
it would have been found that, after a certain tempera- 
ture was attained, the rate of absorption again began to 
decrease.) 



10 EXPERIMENTAL PLANT PHYSIOLOGY 

, Having now proved that a root absorbs water, it 
must next be determined whether other substances are 
absorbed with the water ; and, if so, what kind of sub- 
stances can be absorbed. 



EXPERIMENT 4 

Aim. To determine the class of substances that a 
root can absorb. 

Method. Two coloured substances are chosen so 
that any absorption that takes place may readily be 
seen by the change of colour that the plant thereby 
undergoes. Powdered eosin and powdered carmine 
are suitable for the purpose, as they are both bright 
red in colour. A small quantity of these substances is 
put respectively into two test-tubes and well shaken up 
with water. The eosin dissolves. The carmine simply 
remains suspended in the water; it is a very finely divided 
powder and does not, therefore, quickly settle at the 
bottom. 

A broad-bean seedling is then placed in each test- 
tube so that its roots are in the liquid. 

Observations. The seedling whose roots are in eosin 
soon becomes red all over; the colour of the other 
seedling remains unaltered. 

Inferences. The root can absorb substances that are 
soluble, but it cannot absorb insoluble substances. 

(Note. This experiment can be amplified by the use 
of other coloured substances of each class. It should 
be noticed that no account is taken here of the absorp- 
tion of gases, which is reserved for another place.) 

The broad deductions drawn from Experiment 4 
appear, on further thought, to need some amplifica- 
tion. 

It has been proved that a plant cannot absorb a 
substance unless it is dissolved, yet it is well known 
that marble is often corroded by the growth of roots 
upon it, and marble is not soluble in water. Does the 
root exude something which dissolves the marble ? It 



THE FOOD OF THE PLANT 11 

seems that there is some solvent in a root which aqts 
on substances that are not soluble in water. 

If a little hydrochloric acid be poured over a piece 
of marble, effervescence will be noticed and the marble 
will slowly dissolve away, thus showing that some 
substances that are insoluble in water dissolve in the 
presence of an acid. 

It is possible, quite simply, to find out whether there 
is present in a root an acid by means of which an 
insoluble substance, such as marble, may be dissolved. 



EXPERIMENT 5 

Aim. To determine whether any acid is present in 
a root. 

Method. (a) Using litmus solution. 

Litmus solution is blue in colour, but turns pink on 
the addition of a drop of acid. 

Fill two test-tubes with a weak solution of litmus. 
Into the mouth of one put a broad-bean seedling with 
a radicle of about an inch in length. 

(b) Using litmus paper. 

Litmus paper also changes colour. Alkaline litmus 
paper is blue, and becomes red when 
brought into contact with an acid ; simi- 
larly, acid litmus paper will be changed 
from red to blue in presence of an alkali. 

A cylindrical gas-jar is lined with a 
piece of blotting-paper, and the paper 
is then made thoroughly damp. 

A soaked broad-bean and some strips 
of blue litmus paper are inserted between 
the glass and the blotting-paper. One 
strip of the litmus paper is arranged so 
that the growing radicle will come into 
contact with it. 

Observations.-(a) After a day or two 
the colour of the solution in which the 
bean root is growing has become pink ; 
the other solution remains unchanged in colour (Fig. 3). 



FIG. 3 

Blue colour is denoted 
by darker tint, pink 
by lighter tint. 



12 EXPERIMENTAL PLANT PHYSIOLOGY 

,(&) As the radicle grows the piece of litmus paper 
touched by it gradually turns red; the other strips 
remain unchanged. 

Inferences. The root contains an acid which, to 
some extent, passes from it into the surrounding 
water. 

As a result of this acid-exudation, the root can absorb, 
in addition to substances soluble in water, those sub- 
stances in the soil that will not dissolve in water alone, 
but are soluble in the acid that the root gives out. 

Growth in Culture Solutions. It has already been 
stated (page 1) that the soil is made up of solid particles 
loosely held together, the spaces between them con- 
taining air and mineral solutions. 

It is now seen that the plant can feed only on the 
solutions, since it cannot absorb solid particles however 
fine they may be. 

In order to determine whether all the soluble sub- 
stances present in the soil are really necessary to plant- 
life a series of experiments can be made by growing 
seedlings in various solutions. Such solutions are 
termed Culture Solutions; one that contains all the 
soluble salts commonly found in the soil, in the propor- 
tion in which they are present, is termed a Normal 
Culture Solution. Such a solution may be made up as 
follows : 

1000 c,c. Water made up of Hydrogen and oxygen. 

1 gramme Potassium nitrate Potassium, nitrogen, and 

oxygen. 
*5 ,, Calcium sulphate Calcium, sulphur, and 

oxygen. 
5 Magnesium sulphate Magnesium, sulphur, and 

oxygen. 
*6 Calcium phosphate Calcium, phosphorus, and 

oxygen. 

*5 Sodium chloride Sodium and chlorine. 

A trace Iron sulphate Iron, sulphur, and oxygen. 

If the elements in this list are compared with those 
given in the table of soluble salts in the soil (page 2) 
it will be seen that they are all included here. 



THE FOOD OF THE PLANT 13 

A series of seedlings should be grown, one in a 
normal culture solution, one in a culture solution 'with 
potassium omitted, another in a solution with calcium 
omitted, and so on : in every case one of the elements 
should be omitted and all the others included. By this 
means it is possible to see which of the elements are 
actually necessary for the full development of the 
plant. 

All this is simple enough in principle, but, unfortu- 
nately, the experiments are not easy to carry out. Very 
large vessels are needed for the solutions, and they 
are only with great difficulty kept free from fungus- 
growths. 

To avoid disappointment the beginner is advised to 
wait until experience has been gained in experimental 
methods before attempting to work with culture 
solutions. 

Some general results are therefore given here. 

For full development, all the elements present in the 
list are necessary. A seedling grown without nitrogen 
will be very small ; one without iron will not be of a 
normal colour; and, in general, the absence of any 
element will cause the seedling to suffer in some way. 
But, given all the elements present in the list, the 
seedling will thrive as well in the culture solution as in 
the sou, always provided that its roots will adapt 
themselves to the watery medium in which they are 
growing. It will be noticed that no carbon was included 
in the culture solutions ; it can thus be proved that the 
plant does not require any carbon from the soil, although, 
as seen before (page 2), it is often present there in the 
form of carbonates. 

The Force that produces Root-absorption. The 

next step is to determine what it is that causes the 
absorption of water by the root. It is not possible 
actually to see the process directly, but an experiment 
can be set up in which a similar phenomenon can be 
observed, and which will serve to make clear the force 
by which the absorption is brought about. 



14 EXPERIMENTAL PLANT PHYSIOLOGY 



EXPERIMENT 6 

Aim, To determine what takes place when a sugar 
solution and water are separated by a permeable mem- 
brane, such as pig's bladder or a piece of parchment. 

Method. A piece of pig's bladder or parchment is 
soaked in water, then tightly stretched over the mouth 
of a long-stemmed thistle-funnel and tied on with thin 
string. The stem of the thistle-funnel is then held 
obliquely while golden syrup or a strong sugar solution 
is poured into the funnel until the 
bulb is full. If syrup is used it 
must be warmed so that it can 
flow down the tube easily, but it 
must not be made hot enough to 
injure the bladder. Syrup is easier 
to use than a sugar solution, as 
the latter may crystallize out if it 
is too strong. 

When the bulb is full the thistle- 
funnel is fixed in an erect position 
by means of a clamp and retort- 
stand, and the level of the syrup is 
marked with a strip of gummed 
paper. The bulb is then suspended 
in a beaker of water in such a way 
that the level of the water in the 
beaker is the same as that of 
the solution in the thistle-funnel 
(Fig. 4). 

Observations. The liquid in the 

tube begins to rise and continues so to rise for some 
considerable time. Finally a maximum height is 
reached; and then the level of the liquid in the tube 
gets gradually lower. Meanwhile the water in the 
basin gets sweet. 

Inferences. The sugar must, in some way, have 
attracted the water through the bladder, and this with 
so much force that the liquid was able to rise in the 
tube against the force of gravity. 




FIG. 4 



THE FOOD OF THE PLANT 15 

It was observed also that the water in the basin 
became sweet, therefore some of the syrup from the 
bulb of the funnel must have passed out into the water. 
This interchange of liquids is termed Osmosis. 

The above experiment illustrates very roughly the 
method of absorption in the root of a plant. The syrup 
is analogous to the cell-sap, the membrane represents 
the wall of a cell of the root, the beaker of water takes 
the place of the water in the soil. Further, the passage 
of some of the sugar solution out into the water is 
analogous to the outflow of acid proved in Experi- 
ment 5. 

A further experiment will determine whether sugar 
exerts a similar attraction when acting through the 
tissues of a plant instead of through pig's bladder. 



EXPERIMENT 7 

Aim. To find out whether sugar possesses the power 
of attracting water through the cells of a plant. 

Method. For this experiment three large potatoes 
are used. Young ones are the best to work with, and 
they should have sound skins. From the end of each 
a sufficiently large piece is cut to enable the potato to 
stand steadily. At the other end a pit is made. This 
is best done with a cork-borer. Each pit should have 
a diameter of about half an inch and a depth of one 
inch. A width of about half an inch at the cut end of 
the potatoes is carefully peeled, and they are then 
placed in separate glass crystallizing-dishes. 

A little crystallized sugar is put into two of the pits, 
and a few drops of water, just enough to moisten the 
sugar, are added. The pit in the third potato is left 
empty. Water is poured into two of the glass dishes, 
those which hold respectively the potato with the 
empty pit and one of the potatoes with sugar in 
the boring. Thus the second potato which has had 




16 EXPERIMENTAL PLANT PHYSIOLOGY 

sugar put into the cavity stands in an empty dish 
(Fig. 5). 

Observations. In about an hour's time the pits 
which contained 'the sugar are found 
to be half full of liquid. The 
empty pit still remains empty. Later 
the pits containing sugar become 
full of liquid and begin to overflow 
FIG 5 while the empty pit gets gradually 

drier. 

No difference is at first discernible between the 
potato surrounded by water and that in the empty 
dish, but, after a day or so, the one not surrounded by 
water begins to look shrivelled, especially at the base, 
whilst the potato in the water remains as turgid as it 
was at the beginning of the experiment. 

Inferences. Osmosis has taken place between the 
sugar solution in the pit and the sap in the cells of the 
potato : the sugar has attracted the weaker sap from 
the cells of the potato adjacent to the pit, at the same 
time a little of the sugar 'has passed from the pit into 
these adjacent cells, thus strengthening their cell-sap; 
as a result of this increase in density, these cells have 
been able, in their turn, to draw sap from the next 
layer; this process has been repeated through each 
succeeding layer until the outermost cells of the potato 
have been reached. Thus a flow of liqtlfcKo wards the 
pit has been set up. 

In the case of the potato surrounded by water the 
outermost cells of the potato draw the water from the 
dish and thus a continuous flow is set up from the dish 
to the pit. 

In Experiment 3 it was proved that absorption 
decreased as the strength of the solution absorbed in- 
creased, and that it ceased altogether after a certain 
strength of solution was reached. The reason for this 
is now clear. The solutions outside must be weaker 
than the cell-sap if these solutions are to be drawn 
into the root, and the greater the difference in the 



THE FOOD OF THE PLANT 17 

densities of the cell-sap and of the solutions the more 
rapid will be the flow from the weaker to the stronger/ 

The Work of Root-hairs. For the study of root- 
hairs, mustard or cress seeds may be germinated on 
damp flannel. An examination of the radicles of the 
seedlings shows them to be covered, at a little distance 
from the tip, by very fine hairs. These are the root- 
hairs and they last for a few days only. As the apex 
of the root grows, new root-hairs are constantly being 
formed just behind the tip, whilst those furthest from 
the tip die away as the new ones grow. 

It is chiefly by means of the root-hairs that absorp- 
tion takes place. 

It is not, however, easy to demonstrate the absorp- 
tive power of the root-hair by any simple experiment, 
since the young root is able to absorb even when 
deprived of root-hairs. That this is so may be shown 
by cutting off the radicle of a broad-bean seedling at a 
point above the root-hairs. The seedling is not neces- 
sarily killed by this treatment, but immediately puts 
out secondary roots above the cut. 

Moreover, when seedlings are grown with their 
radicles actually in water, root-hairs are usually not 
formed at all. 

At the same time, under normal conditions of growth, 
it is through- the very thin wall of the root-hair that 
osmosis takes place. 

The Ascent of Water in Plants. So far then it has 
been proved that a plant absorbs, by means of its roots, 
water from the soil and substances that are dissolved 
in the water. It has been seen that this solution 
passes up the stem and supplies the branches and 
leaves. Something of the nature of the force which 
causes the absorption of water and its passage through 
the plant has been learnt. Of this passage of the water 
there will be further discussion later, but it may be 
stated at once that the rise of water in tall stems is by 
no means fully understood. No adequate explanation 

B 



18 EXPERIMENTAL PLANT PHYSIOLOGY 

of this phenomenon has yet been advanced. The 
mighty force great enough to raise water to the top- 
most branches of the tallest trees, against the opposing 
force of gravity, is still a mystery. 

By means of red ink or other coloured solutions it 
has been demonstrated that the water taken in by the 
root passes up through the stem and reaches the 
leaves. But although the leaves have become red the 
colour of the stern externally is unaltered. The ink 
cannot, therefore, have passed through the bark; it 
must have gone up through the inner part of the stem. 

EXPERIMENT 8 

Aim, To find out where the water passes up the 
stem. 

Method. Three leafy twigs should be used, two 
being those of dicotyledons having respectively a 




B; ^noco'tyTdonous stem. ) Transverse and longitudinal 
C, herbaceous stem. ) sections. 

p, pith, w, wood. &+c, bast and cortex, bk, bark. 

rar, medullary ray. g, ground tissue. 
The distribution of the tissue which stains red is shown in each case. 

FIG. 6 

woody stem (e.g. beech, lime) and a herbaceous stem 
(e.g. sunflower, hogweed), and the third that of a 



THE FOOD OF THE PLANT 



19 



monocotyledon (e.g. Indian corn, palm). These should 
stand in some coloured liquid, such as red ink or. a 
solution of eosin, until the leaves are turning red. 
At this point the stems should be cut transversely and 
longitudinally. 

Observations. The parts of the stem which become 
red are shown in the following table (Fig. 6) : 





Woody Stem, 


Herbaceous 
Stem. 


Stem of 
Monocotyledon. 


Transverse 
section. 


A circular ring 
of tissue. 


Small isolated 
patches ar- 
ranged in a 
single ring. 


Small isolated 
patches ar- 
ranged ir- 
regularly. 


Longitudinal 
section. 


Two broad 
vertical bands. 


Two narrow 
vertical bands. 


Two or more 
narrow verti- 
cal bands on 
either side. 




Thus the red 
portion forms 
a hollow cyl- 
inder. 


The red portion 
forms a ring 
of separate 
strands. 


The red portion 
is in strands 
irregularly ar- 
ranged. 



When similar branches which have not been in red 
ink are examined, the same differentiation of parts 
can clearly be distinguished even in the uncoloured 
stem. 

If the various parts are pricked with a needle, the 
red-staining portions will usually be found to be the 
hardest. 

Inferences, The red ink, or, more generally, the 
solution from the soil, passes up the stem through the 
hard part only. This part is the wood : it is arranged 
either in a complete cylinder or in isolated strands 
which may be regularly or irregularly disposed. 

The Structure of the Stem of the Plant. It will be 



20 EXPERIMENTAL PLANT PHYSIOLOGY 

well at this point to examine the stems rather more 
closely and to learn the names of the various parts 
noticed. 

When looked at in transverse section the woody 
stem shows three concentric zones. The centre is 
made up of a white substance and is soft. This is 
the pith. The middle zone is not quite so white ; it 
is harder ; and faint radiating lines can be seen running 
through it. This part is the wood, through which the 
solution from the soil has been found to travel. The 
outermost layer is a very narrow one. If the stem is 
" peeled " the whole of this band comes off. The inner 
part of this is the bast and the cortex ; the outermost 
part is the bark. 

In the herbaceous stem the same parts are recog- 
nizable. In this case the wood and bast form isolated 
"bundles" arranged in a ring. It can easily be seen 
that there is a small part of each bundle lying towards 
the outside of the stem which does not stain when the 
twig is put into the coloured solution ; this is the bast. 
Here too the pith and cortex are connected by strands 
of tissue ; these are termed medullary rays. 

In the monocotyledonous stem " bundles " are again 
present, but there is no differentiation between pith and 
cortex; for this reason all the tissue in which the 
bundles lie is termed ground tissue. 

The Cause of the Ascent of Water in the Stem. In 

Experiment 6 it was seen that the liquid in the thistle- 
funnel rose to a considerable height, but it did not 
continue to rise indefinitely, and the highest level 
reached was but a little way when compared with the 
height of a tall tree. 

Now it has already been stated that no full explana- 
tion of the cause of the ascent of water through the 
stem has yet been given, but for that very reason it 
will be specially interesting to investigate it as far as 
possible. 

If stem-sections are examined with a microscope, 
the wood, through which the water travels, is seen to 



THE FOOD OF THE PLANT 21 

be made up of a number of little parts like elongated 
boxes in shape. 

These are the " vessels " of the wood; they do not 
contain any cell-sap or any solution similar to the 
sugar solution experimented with in the thistle-funnel, 
or the acid cell-sap of the root-tip ; that is, in the 
vessels of the wood, there is no osmotically active 
substance. 

How then can the water pass upwards through the 
wood of the stem against the force of gravity, which 
must always be pulling it back again ? The following 
questions naturally present themselves: Can it be 
pulled up from above, or forced up from below ? 

EXPERIMENT 9 

Aim. To find out if the leaves exert any pull which 
helps the water to rise in the stem. 

Method. For this experiment two leafy twigs are 
required. The twigs must be as nearly alike as possible 
so that fairly comparable results may 
be obtained. To this end it is well 
to cut them at the same time and 
from the same branch. 

Two test-tubes are filled with water 
to the same level and the level in 
each case is carefully marked. A 
small quantity of some light oil such 
as cedar-wood oil is then poured over 
the water so that none of it may be 
lost by evaporation. 

All the leaves are stripped from 
one of the branches, and the branches 
are then put into the test - tubes 
(Fig. 7). 

Observations. The water m the 
test-tube which contains the leafy 
branch diminishes at a much greater 
rate than that in the test-tube in which the leafless 
branch is standing. 




FIG. 7 



22 EXPERIMENTAL PLANT PHYSIOLOGY 

'Inferences. The presence of leaves on a twig is not 
altogether responsible for the passage of food solutions 
up a stem, but their presence greatly accelerates the 
rate at which the solution rises. 



EXPERIMENT 10 

Aim. To find out if the root exerts any pressure 
which forces the water up the stem. 

Method. A small potted fuchsia plant is used for 
this experiment. The whole of the shoot is cut off at a 
distance of from one to two inches above 
the level of the soil. A long piece of 
narrow glass-tubing is then attached to 
the cut end and fixed uprightly by means 
of a clamp. Rubber-tubing is used to 
make the attachment, and is firmly fixed 
at both ends with wire or thin twine 
(Fig. 8). The plant is watered in the 
usual way. 

This experiment should be set up 
several times at different seasons of the 
year. 
Observations. Water rises in the yer- 

7tical glass-tube. The rate at which it rises 
is increased by warming the roots and de- 
creased by cooling them. In spring and 
early summer the rise is rapid. It is 
autumn, and often ceases entirely in the 




FIG. 8 



slow in 
winter. 

Inferences. The root does exert a pressure which 
forces water upwards. 

The push of the root, or " root- pressure/' is regulated 
by the amount of water absorbed by the root, since the 
conditions that affect the rate of absorption (Experi- 
ment 3) similarly influence the rate of rise in the tube. 
Thus : Root-pressure exists sometimes but cannot 
always be demonstrated. 

As seen in Experiment 10, root-pressure cannot 



THE FOOD OF THE PLANT 23 

always be demonstrated. It can be observed only in 
certain plants at certain seasons of the year. 

No trace of root-pressure can ever be detected on 
cutting the stem of a plant that has plenty of leaves 
and is growing in a dry, sunny atmosphere. 

Of course this does not necessarily snow that the force 
has ceased ; it is merely negative evidence. At the same 
time it prevents our regarding root-pressure as an all- 
sufficient cause of the ascent of water through the stem. 

Thus after making full allowance for the work done 
by the pressure from the roots and the pull from the 
leaves we are still left face to face with one of the big 
mysteries of plant-life. 

Transpiration. If the amount of water absorbed by 
a healthy growing plant be observed, and compared 
with the growth and increase in weight of the plant, 
it at once becomes evident that all the water absorbed 
cannot be retained. The method by which a plant 
gets rid of its surplus water must now be 
determined and some reason found for the 
absorption of so much more than seems to 
be necessary. 

EXPERIMENT 11 

Aim. To determine how a plant gets rid 
of its surplus water. 

Method. A gas-jar is filled with water and 
fitted with a cork " through which has been 
passed the stem of a leafy branch. The cork 
itself and all the connections are made air- 
tight so that there may be no possibility of 
evaporation from the water in the jar. A 
second cylinder is then made perfectly dry. FIG. 9 
The ground glass edges of both cylinders are 
well vaselinea and trie dry one is inverted over the 
other (Fig 9). 

Observation. Very soon drops of water appear on 
the inner surface of the upper cylinder. 



24 EXPERIMENTAL PLANT PHYSIOLOGY 

Inferences. The water that has appeared in the 
upper cylinder must have been given oft from the leafy 
branch in the form of vapour and then have condensed 
on the sides of the glass. Thus : The leaves of a plant 
give off water-vapour. This process is termed trans- 
piration. 

If the above experiment be kept under observation a 
little longer it is observed that the amount of water on 
the upper cylinder does not go on increasing. 

This result seems at first a little contrary to expecta- 
tion. It will be realised, however, by this time, that 
no experiment can give a false result; whatever the 
result, it must be true for the conditions set up. 

The giving off of water from the leaves into the air 
is somewhat analogous to the drying of clothes on a 
clothes-line. The arier the air is the more quickly do 
the clothes give up the water contained in them. If 
this is also the case with the leaves it is seen at once 
that they have been put into an atmosphere which 
very soon becomes charged with the moisture that they 
themselves are giving off, and so the process under 
observation is checked and finally ceases entirely. 

An experiment can now be set up to find out whether 
the rate at which water is given off by the plant really 

is affected by the dryness 
or dampness of the air 
surrounding it. 

EXPERIMENT 12 

A i m. T o fi n d out 
whether the rate of trans- 
piration is affected by the 
dampness or dryness of the 

| \ surrounding air. 

FIG. 10 M e t h o d. C a 1 c i u m 

chloride must be used in 

this case. This substance has the power of absorbing 
moisture from the air around it. 




THE FOOD OF THE PLANT 29 

In some plants, which grow in dry situations where 
little water is available, no transpiration at all is pos- 
sible from the upper surface of the leaves. On the 
other hand when leaves present both surfaces equally 
to the sun, transpiration takes place equally from both 
sides. 

When looked at with the naked eye a leaf appears to 
be covered by a complete skin, and it is difficult to see 




A, privet leaf, lower surface. C, maize leaf, lower surface. 

B, ,, upper surface. D, ,, upper surface, 
c, cell. s, stoma. h, hair, gr, guard-cell. 0, opening. 

FIG. 13 

how transpiration can be effected at all. If, however, 
a piece of this " skin" is examined under a microscope 
it is seen to be made up of a number of tiny parts re- 
sembling somewhat the cells of a honeycomb (Fig. 13), 
and for this reason termed " cells " by the botanist. 
Scattered among the cells are little pores (s), and it is 
through these pores that the water- vapour escapes. 
They are called "stomata" or "mouths." Sometimes 
the stomata are to be found on both surfaces of the 



30 EXPERIMENTAL PLANT PHYSIOLOGY 

leaf as in the maize (Fig. 13) ; sometimes only on the 
lower surface as in the privet (Fig. 13) ; but always 
the amount of transpiration will be proportional to the 
number of mouths. 

The stomata are bounded by two lips or " cells" 
called " guard-cells/' These guard-cells have the power 
of opening and closing the pore, and so resemble the 
lips of our mouths. When they come together the 
stoma is closed and transpiration ceases; when they 
separate the stoma is open and transpiration takes 
place. In this way a plant is able to regulate the 
amount of water given off. 



EXPERIMENT 16 

Aim. To find out whether the transpired liquid is 
pure water or whether any of the substances absorbed 
in solution by the root are lost at the same time. 

Method. Set up a leafy branch as in Experiment 11. 
Put some coloured substance (e.g. eosin) into the 
water in the lower iar. 

Observation. The water that collects on the upper 
jar is not coloured. 

Inference. The plant has retained the dissolved 
substance and the vapour transpired is pure water. 

All the experiments that have now been worked 
have served to show the close link that exists between 
the giving off of water-vapour from the leaves and 
the absorption of solutions by the roots. The conditions 
that aft'ect one affect the other similarly. If a plant 
is to grow and thrive, absorption must be somewhat in 
excess of transpiration. If transpiration exceeds ab- 
sorption the plant withers. For this reason it is well 
to protect from the heat of the sun's rays transplanted 
seedlings whose young absorbing roots must have been 
injured during the removal. 

The Value of Transpiration to the Plant. Some 



THE FOOD OF THE PLANT 31 

reason for the absorption of such an excess of water 
must now be looked for. 

It has already been seen that roots can absorb dilute 
solutions only, that the rate of absorption is checked 
and finally stopped altogether by a gradual increase 
in the strength of the solution in which the plant is 
growing (Experiment 3). 

The cause of this gradual decrease in absorbing 
power is obvious on referring back to Experiments 6 
and 7, where it is seen that, if the solutions in the soil 
are of a higher density than the cell-sap, then the 
former cannot be osmotically attracted by the latter ; 
that is, root-absorption cannot take place. 

It therefore follows that the plant must take in an 
excess of water in order to get enough of the mineral 
foods which it obtains from the soil, as the mineral 
solutions are necessarily weak. 

Other advantages are also derived by the plant from 
the transpiration current, as the rapid flow of water 
through the plant is termed. One important gain is 
the lowering of the temperature of the leaf during hot 
weather ; such reduction of temperature being brought 
about by the evaporation of water from its surface. 

Leaf-fall. The connection between absorption and 
transpiration at once throws light on the question of 
the falling of leaves in autumn. The coming of winter 
means a decrease in the power of absorption by the 
roots, since it has been proved that absorption decreases 
as the cold increases. Transpiration decreases also, 
but, as has been seen, the power of a plant to prevent 
loss of water is only a limited one. The tree must 
therefore husband the water that it can obtain until 
warmer conditions bring back a renewed power of ab- 
sorption. This it does by dropping its leaves, since it 
is through the leaves that the water would otherwise 
be lost. 

Summary. Something has now been learnt of the 
food that a plant gets from the soil. 



32 EXPERIMENTAL PLANT PHYSIOLOGY 

It has been found to consist of water in which are 
dissolved those mineral salts that are capable of solution 
in the water as well as those that can be dissolved by 
the acid of the root. 

The method of absorption, the passage of the food 
solutions up the stem, and the transpiration of the 
surplus water have each been studied in turn. 

We are now ready to turn to the second source of 
food for the plant, namely, the air. 



CHAPTER II 

I. THE COMPOSITION OF THE AIR 

Before an attempt is made to solve any of the prob- 
lems relating to absorption from the air it is necessary 
to know something of its composition. The air is 
gaseous and is without colour, taste, or smell. Whether 
it consists of one or of several gases must be determined. 
If there are several the special properties of each one 
must be investigated. 

To find out whether two gases are alike or different 
may not seem at first a very easy matter. Gases 
cannot, as a rule, be seen,, nor can they be handled, 
and therefore tahey are not capable of comparison by 
the methods applied in dealing with solids or with 
liquid bodies. Tney have nevertheless very character- 
istic properties, and each gas should be submitted to 
the following tests : 

1. Its general appearance should be noted, its colour, 
and its smell, if any. 

2. Its solubility in water must be determined. It is 
not always easy to determine the degree to which a 
gas is soluble. Various methods by which a rough 
estimate of the solubility can be obtained will be con- 
sidered when dealing with individual gases. 



THE FOOD OF THE PLANT 



33 



3. By means of litmus-paper the acidity, alkalinity^ 
or neutrality of the gas should be tested. 

A strip of damp blue litmus-paper and one of red 
are put into a jar of the gas in question. If the gas is 
alkaline the red paper will be changed to blue ; if acid, 
the blue will turn red ; if neutral, the papers will remain 
unchanged. 

4. A lighted splint should be passed into a jar of the 
gas to find out whether the gas burns or whether it is 
able to support combustion. Note the brightness of 
the combustion. 

(This test might in some cases be a dangerous one 
to apply, but it is safe for all the gases dealt with in 
this book.) 

5. The weight of the gas in comparison with the 
weight of air should then be tested. 

This may be done by placing, mouth to mouth, two 
gas-jars, one containing air and the other the gas under 
investigation. Two sets of jars are arranged 
as in Fig. 14, in which A represents air and 
x the unknown gas. After a minute has 
passed the contents of all the jars should 
be tested. If x is lighter than A it will 
then be found in both the upper jars; if 
heavier, in both the lower ones. 

6. Finally it must be determined whether 
or not lime-water is turned milky when 
the gas is passed into it. 



x 



If two unknown gases are compared and 
are found to respond in the same way to FJG 14 
all these tests, they may then be regarded 
as one and the same gas; but if different results are 
obtained in any of these tests, then the gases must 
necessarily be different. 

We are now in a position to return to the study of 
the nature and composition of air. 

If a bell-jar is placed over a lighted candle, the 
candle very soon goes out. It seems, therefore, that 
the air must have been altered in some way by the 

c 



34 EXPERIMENTAL PLANT PHYSIOLOGY 

burning of the candle in it, since it is no longer able 
to support combustion. 

Again, a fire will not burn without a " draught " ; 
that is, a constant supply of fresh air must be allowed 
to pass through it. 

both these observations denote an intimate connection 
between the air and combustion; it is well, therefore, 
to begin investigations by burning something and then 
try to find out whether the air has been altered in any 
way by the combustion of the substance in it. 

Phosphorus is commonly chosen for such investiga- 
tion because it burns very readily. It is a yellow, waxy 
substance, and is poisonous. The smell of burning 
matches is due to the phosphorus contained in them. 

Phosphorus should on no account be touched with 
the fingers, because the heat of the body is quite 
sufficient to cause it to ignite. Indeed it must always 
be kept under water, because it burns so readily and 
violently in air. 

If a piece about the size of a pin's head is burnt in 
a porcelain dish the bright yellow flame and the dense 
white fumes may be noticed. 

EXPERIMENT 17 

Aim. To find out any facts about the composition of 
air by burning phosphorus in it. 

Method. In order to test any change that takes 
place in the air due to the burning, it will be necessary 
to enclose a portion of air. 

Float a little porcelain dish in a trough of water. 
Remove a stick of phosphorus from the bottle with a 
pair of forceps ; put it between blotting-paper and cut 
off a piece about the size of a pea. Do not touch 
the phosphorus with the fingers; replace the stick 
immediately. 

Then put the piece of phosphorus into the dish and 
coyer with a stoppered bell-jar. While the bell-jar is 
being put into position the stopper must be taken out. 
You will readily see why this is necessary. Replace the 



THE FOOD OF THE PLANT 



35 




FIG. 15 



stopper. A certain amount of air has thus been 
enclosed in the bell-jar above the sur- 
face of the water (Fig. 15). 

Again remove the stopper and ignite 
the phosphorus by means of a hot 
glass-rod or wire. Quickly replace the 
stopper. 

Observations. The phosphorus burns 
brightly, producing dense fumes. The 
level ot the water inside the jar falls a 
little at first, then rises. When the 
burning is over, the water has risen 
about one-fifth of the height of the jar. 
The white fumes gradually dissolve in 
the water. 

As soon as the remaining gas is free 
from fumes, it is tested and the following observations 
made. (Several jars of gas will of course be necessary.) 

1. It is invisible, colourless, and odourless. 

2. If soluble in water, the solubility can only be 
slight, since the level of the water remains constant 
when the burning is over. 

3. It is neutral to litmus-paper. 

4. A lighted taper is extinguished and the gas itself 
does not burn. 

5. It seems slightly lighter than air. 

6. Lime-water is not turned milky by it. (Some- 
times a slight milkiness is observed ; this will be again 
referred to later.) 

Inferences. 1. Air is altered both in character and 
quantity by the burning of a substance in it. 

2. One-fifth of the air disappears ; since this cannot 
have escaped, it must have united with the burning 
substance. 

3. The gas that is left, which is four-fifths of the 
whole, is quite different in character. It can no longer 
support combustion. 

4. Air must be made up of at least two gases, an 
active gas that unites with phosphorus producing a white 



36 EXPERIMENTAL PLANT PHYSIOLOGY 

powder that dissolves in water, and an inactive gas in 
which substances will not burn. The proportion of the 
active to the inactive is one to four. 

It is not only by combustion that the active gas may 
be taken from the air. Some substances will absorb it 
without the application of any heat. 



EXPERIMENT 18 

Aim. To find out whether damp iron filings will 
absorb the active gas from the air. 

Method. Some iron filings are tied up in a piece of 
muslin and put into a glass measuring-cylinder. (If 

a measuring-cylinder is not 
available a test-tube can be 
used instead.) The cylinder 
is then filled with water and 
inverted over a basin of 
water. The muslin with 
the filings will remain in 
position if made to fit fairly 
tightly. 

Air is then very gently- 
blown into the cylinder until 
the level of the water inside 
is the same as that in the 

basin. This can be done by means of a piece of bent 
glass-tubing (Fig. 16). 

It may at first seem unnecessary to fill the cylinder 
with water and then blow the water out again, but 
there is no simpler way of getting the air inside at the 
same pressure as the air surrounding the cylinder. 
Try inverting an empty cylinder and note the result. 

Observations. Very soon the water inside the 
cylinder begins^to rise, and continues to rise until one- 
fifth of the cylinder is filled with water ; after this no 
further change takes place. 
On testing the remaining gas it is found to be exactly 




FIG. 16 



THE FOOD OF THE PLANT 37 

the same as the residual gas left after the burning of 
phosphorus in air. 

Inference. Damp iron filings take up the active gas 
from the air. 

The name of the active gas in the air is oxygen and 
that of the inactive is nitrogen. 

There are, in the air, other gases in small quantities, 
and these will be dealt with later. They were, of course, 
present in the residual gas left after the phosphorus 
had been burnt, since only the oxygen was taken from 
the air. It must therefore be remembered that the 
nitrogen tested was impure nitrogen, although the 
quantity of other gases contained in it was small. It is 
possible to prepare pure nitrogen, but it will not be 
very easy for us to do so at this stage. A pure specimen 
of the gas would give the same observations except in 
the case of the lime-water, which would not be turned 
milky at all, thus showing that the slight milkiness 
formed in the lime-water was due to some impurity 
and not to the nitrogen itself. 

We have already tested the properties of nitrogen 
and found it to be an inert gas in which a taper will 
not burn. The properties of oxygen must next be 
tested. In order to do this a specimen of the gas must 
be prepared. 

W hen phosphorus was burnt in air it united with the 
oxygen, forming a white powder called an oxide of 
phosphorus which rapidly dissolved in the water. It is 
extremely difficult to get the oxygen back again from 
the oxide of phosphorus, but many other substances 
containing oxygen give it up quite easily. Such a one 
is potassium chlorate. It is a white, crystalline sub- 
stance, and is used in the making of fireworks and 
matches. When potassium chlorate is heated, oxygen 
is given off. It is given off even more readily and 
with less heat if a black substance named manganese- 
dioxide is mixed with it. The manganese-dioxide 
remains unchanged at the end of the reaction, but in 
some way it helps the potassium chlorate to decompose. 



38 EXPERIMENTAL PLANT PHYSIOLOGY 



EXPERIMENT 19 

Aim. To prepare and collect oxygen and to test its 
properties. 

Method. Fit a hard round-bottomed flask with 
a cork and delivery-tube bent as in Fig. 17. Into 

the flask put about 
20 grammes of a 
mixture of three 
parts of pow.dered 
potassium chlorate 
and one part of 
manganese-dioxid e. 
Clamp the flask 
so that the end 
of the delivery-tube 
fits through a bee- 
pneumatic trough of water. 




FIG. 17 



hive shelf standing in 
Gently heat the flask. 

As soon as the heating has been carried on long 
enough to displace all the air in the flask, place an 
inverted gas-jar full of water over the bee-hive shelf 
and collect the gas that is coming off from the mixture 
in the flask. A round glass plate must be used for 
inverting the gas-jar. 

When the gas-jar is full of the gas, cover it under 
water with a glass plate smeared with vaseline and re- 
move it from the trough. Collect several jars in this 
way. 

Before removing the flame disconnect the cork. 
This is very important. If it is not done the water 
will be drawn up the tube, as the gas in the flask con- 
tracts, and the flask will then probably crack. 

Observations. 1. The gas is invisible. It has no 
colour or smell. 

2. It cannot be very soluble in water, because the 
bubbles of the gas rose through the water and did not 
seem to get any smaller. 

3. It is neutral to litmus. 

4. A lighted taper is put into a jar. It burns much 



THE FOOD OF THE PLANT 39 

more rapidly and brightly than it did in air. A glow- 
ing splint wnen put into a jar bursts into flame. 

5. It is slightly heavier than air. 

6. It does not turn lime-water milky. 
Inferences. Oxygen is an invisible, colourless and 

non-smelling gas. If soluble in water it must be only 
slightly so. It is neutral to litmus. Substances burn 
in it much more readily than they do in air. 

Other substances in the air. Thus it is seen that 
the air is made up almost entirely of oxygen and 
nitrogen. But there are also other gases present in 
small quantities. 

The leaves of a plant are continually transpiring ; 
it follows, therefore, that the air must always contain 
a certain proportion of water-vapour. It will now be 
shown that it also contains another gas which is most 
important in the life of a plant. 



EXPERIMENT 20 

Aim. To find out the effect produced by air on 
lime-water. 

Method. Place some lime-water in a shallow dish 
and leave it exposed to the air. 

Observation. The lime-water gradually becomes 
milky. 

Inference. Since lime-water in an open dish be- 
comes milky while that in a corked bottle remains 
clear, it seems probable that the change in the lime- 
water in the first case is brought about by contact with 
the surrounding air. 

Now neither oxygen nor nitrogen can produce milki- 
ness in lime-water. It is natural, therefore, to infer 
that there must be some other constituent in the air to 
which the milkiness is due. This constituent is the 
gas carbon-dioxide. The milkiness is caused by the 
combination of the carbon-dioxide with the lime of the 



40 EXPERIMENTAL PLANT PHYSIOLOGY 

lime-water to form a white powder named calcium- 
carbonate. 

EXPERIMENT 21 

Aim. To find out whether the amount of carbon- 
dioxide in the air is affected by the breathing of living 
creatures in it. 

Method. Take two glass dishes each containing a 
little lime-water. Leave one on the table. Breathe 
vigorously into the other through a glass-tube. 

Observations. The lime-water in the second case 
very quickly turns milky, while the other becomes so 
only gradually. 

Inference. The amount of carbon-dioxide in the 
air is considerably increased by the breathing of living 
things. 

EXPERIMENT 22 

Aim. To determine whether combustion affects the 
amount of carbon-dioxide in the air. 

Method. Two glass plates are taken. On one is 
placed a very short piece of lighted candle and on both 
a small dish of lirne-water. Bell-jars are then placed 
over the plates. 

Observation. The lime-water under the bell-jar 
containing the lighted candle becomes milky much 
more quickly than the other. 

Inference. The amount of carbon-dioxide in the air 
is increased by the burning of substances. 

(It must be noted here that it is only those sub- 
stances that contain carbon which produce carbon- 
dioxide on burning.) 

Carbon-dioxide is thus formed in the atmosphere by 
the breathing of all living things ; it is also produced 
when certain substances burn ; and it may be snown too 
that it is formed when animal and vegetable matter 
decay. 

Some of this gas must now be prepared in order that 
its properties may be tested more fully. 



THE FOOD OF THE PLANT 



41 




FIG. 18 



EXPERIMENT 23 

Aim. To prepare and collect carbon-dioxide and to 
test its properties. 

Method. Put about 20 grammes of limestone, or 
marble broken into small 
pieces, into a flat-bottomed 
flask, fitted with a thistle- 
funnel and deli very- tube. 

A two-necked bottle is con- 
venient, but an ordinary flask 
fitted with a two-holed rubber- 
stopper answers quite well. A 
rubber-stopper is better than 
a cork, as it is difficult to get 
the latter air-tight. 

The delivery-tube should be bent as shown in Fig. 18 
or Fig. 19. 

Cover the marble with water and add a little concen- 
trated hydrochloric acid through the funnel. The 
funnel need not necessarily have a tap, but, if there 
is no tap, care must be taken that the stem of the 
funnel reaches the liquid in the flask, otherwise the gas 
evolved will escape through the funnel. 

The gas is readily given off without the application 
of heat. It may be collected either over water as 
oxygen was collected (Fig. 18), or by down- ^7 
ward displacement of air (Fig. 19). 

Observations. 1. The gas is colourless 
and invisible. It has a faint, pungent 
smell. 

2. The bubbles appear to get rather 
smaller as they rise through the water, 
thus suggesting that the gas is some- 
what soluble. 

The solubility may be further tested in the following 
way: A jar of the gas is opened under water; the 
plate is replaced so tnat a small quantity of water is 
enclosed; the jar is well shaken and again opened 
under water. The water then rises very gradually in 



FIG. 19 



42 EXPERIMENTAL PLANT PHYSIOLOGY 

the jar, therefore some of the gas must have been dis- 
solved in the water. 

3. If a little moist blue litmus-paper is put into the 
gas, or, better still, into the solution of the gas in water, 
the litmus-paper slowly turns red. 

4. If a lighted taper is put into the gas it is immedi- 
ately extinguished. 

To show well the extinguishing power of carbon- 
dioxide ignite some turpentine in a saucer, then invert 
a bell-jar filled with the gas over the flames. 

5. The gas can be collected by downward displace- 
ment of air. It is therefore heavier than air. 

6. If a little lime-water is poured into one of the jars, 
or if the gas be allowed to bubble from the end of the 
delivery-tube into some lime-water, the lime-water 
becomes milky. 

Inferences. Carbon-dioxide is an invisible, faintly 
smelling gas. Its solubility in water is greater than 
that of oxygen. It is slightly acid. It will neither 
burn itself nor allow other substances to burn in it. It 
is heavier than air and turns lime-water milky. 

Summary. The knowledge that has now been gained 
respecting the composition of the air may be summar- 
ised as follows : 

It is a mixture of gases. 

About four-fifths of the whole is the inert gas 
nitrogen. 

About one-fifth is oxygen, a gas in which substances 
very readily burn. 

Carbon-dioxide, a heavy gas in which, substances will 
not burn, is present in varying but small amount. 

(The quantity differs according to the locality. In 
country air there are from 3 to 4 parts in 10,000, but 
in towns the proportion is greater.) 

In addition water-vapour is present. 

The air also contains other rare gases and various 
impurities, but for the purposes of this book these need 
not be taken into account as they are present in such 
small quantities. 



THE FOOD OF THE PLANT 43 

The relation that exists between a plant and the 
surrounding air can now be dealt with. Answers to 
the following questions will be sought. 

Does a plant get any food from the air ? If it does, 
what is the nature of the food taken, and under what 
conditions is it obtained ? 



II. THE ABSORPTION OF FOOD FROM THE AIR 

Experiments dealing with the absorption of food 
from the air are more difficult than those connected 
with absorption from the soil, because invisible gases 
are here being dealt with. 

A few preliminary experiments will be made. 



EXPERIMENT 24 

Aim. To find out whether air can pass from the 
atmosphere into and through a leaf. 

Method. A piece of straight glass-tubing drawn out 
to a point at the lower end and a piece 
of glass-tubing bent at right angles are 
put through a two-holed cork. The 
cork is then fitted into a conical flask 
or a bottle partly filled with water. 

In the upper end of the straight tube 
a stout leaf is fixed ; a laurel leaf is a 
suitable one for the purpose. 

A piece of rubber-tubing with a clip 
is attached to the end of the right- 
angled tube. 

The junction between the leaf and 
the tube and all the joints in connec- 
tion with the cork are then made air- 
tight (Fig. 20). 

If a big succulent leaf such as a 
funkia can be obtained the apparatus may.be set up 
more simply. In this case the straight glass-tube can 




FIG. 20 



44 EXPERIMENTAL PLANT PHYSIOLOGY 

be dispensed with and the leaf stalk inserted through 
the cork (Fig. 21). 

The amount of air in the bottle is now reduced by 
drawing^ it out by suction through the right-angled 

tube, and then the end 
is closed by means of 
the clip. 

Observations. Bub- 
bles appear in the water, 
in one case at the end of 
the drawn-out tube, and 
at the end of the petiole 
in the other. The bub- 
bles continue to come off 
for a considerable time. 

Inferences. Since the 
evolution of the bubbles 
continues for some time the air thus given off cannot 
all have been inside the leaf at the start. The bubbles 
must therefore be air which has come into the flask 
from the atmosphere to take the place of that which 
has been drawn out. This air must have passed through 
the leaf, since no other route exists. 

Therefore it is possible for the air to pass into the 
tissues of a plant. 




FIG. 21 



EXPERIMENT 25 

Aim. To find out whether air passes into a leaf 
equally through both the upper and under surfaces. 

Method. Apparatus similar to that set up for the 
last experiment is required. Two leaves of the same 
kind are used. One of these is smeared with vaseline 
on the upper surface, and the other on the lower. 

Observations. Air is easily drawn through the leaf 
which is yaselined on the upper surface, but it is ex- 
tremely difficult, and, in some cases, impossible, to get 
any bubbles from the leaf which has been vaselined on 
the lower surface. 



THE FOOD OF THE PLANT 45 

Inferences. Air is taken into the leaf chiefly, and, 
in some cases, entirely, through the lower surface. 

The observations made in the above experiment 
recall the results obtained in the experiments on tran- 
spiration (page 28), when it was found that transpira- 
tion takes place most actively from the lower surface 
of the leaf. In this case, as in that of transpiration, the 
number of stomata is the determining factor. 

The use of the stomata is thus twofold : they provide 
a means whereby the water-vapour can pass out of the 
leaf, and, further, it is througn the stomata that the 
air enters. 

The two foregoing experiments have shown that it is 
possible to draw air through a leaf. The presence of 
air in the leaf under natural conditions will now be 
demonstrated. 

EXPERIMENT 26 

Aim. To show the presence of air in a leaf. 

Method. Some water is boiled until it is free from 
air. A leaf is then put into the water and gently 
heated. If air is present in the leaf it will expand on 
heating ; as a result there will not be room for it all in 
the leaf and some must escape into the water. 

Observations. Bubbles of air come out into the 
water. They are specially noticeable from the lower 
surface of the leaf. 

Inferences. Air is present in a leaf, and, when forced 
out, it escapes chiefly from the lower surface. 

By these preliminary experiments it has thus been 
shown that air is actually present in the tissues of a 
plant, and, further, that air can be drawn into the 
plant from the surrounding atmosphere. 

Starch Formation in the Plant. The main question 
can now be dealt with : it must be determined whether 
the plant really feeds on the air ; that is, whether it uses 
any part of the air to build up its own plant-tissues. 



46 EXPERIMENTAL PLANT PHYSIOLOGY 

This is not easy. The question must be attacked in 
an indirect manner, and the process of reasoning must 
be carefully followed, otherwise the value of the experi- 
mental evidence may be lost. 

A simple substance commonly found in plant-tissues 
is selected ; and, by experiment, the conditions under 
which this substance is formed in the plant are de- 
termined. 

Now, one of the simple substances most commonly 
found in the tissues of a plant is starch. Potatoes and 
wheat contain a great deal of starch ; and, as it will be 
shown presently, starch is not confined to those parts 
of a plant that we use as food. 

Starch is therefore chosen for the investigation, and 
the conditions under which it is formed are investi- 
gated. It is necessary to have some simple test by the 
application of which the presence of starch can always 



be recognized. 



EXPERIMENT 27 



Aim. To show the effect produced by iodine on 
starch. 

Method. Take a small quantity of starch, powder 
it and mix it into a paste with a little water. Then 
add to it a few drops of very weak solution of iodine in 
potassium iodide. 

Next take several substances which are known to 
contain starch, and others that are known to be with- 
out it. Suitable substances containing starch are, for 
instance, a bean seed, a slice of potato, and a starched 
collar; as examples of substances without starch, a 
piece of white chalk or of washing soda may be used. 
Each of these substances should be treated in turn with 
the iodine solution. 

Observations. The starch is turned a purple blue 
colour by the iodine solution, so also are all the sub- 
stances which contain starch. Those without it, on 
the other hand, just take on the brown colour of the 
iodine solution. 



THE FOOD OF THE PLANT 47 

Inference. Starch is turned blue by iodine solution. 

This is the method that is always employed to detect 
the presence of starch. This knowledge can now be 
applied to the case of a green leaf. 

EXPERIMENT 28 

Aim. To test for starch in a green leaf that has 
been picked in the afternoon of a sunny day. 

Method. The working out of this experiment is 
complicated by the fact that the leaf is green. This 
green colouring matter, or chlorophyll, must therefore 
first be got rid of, otherwise it will mask the blue 
iodine reaction. A liquid must be used which will 
dissolve out the chlorophyll from the tissues of the 
leaf, but which will leave the starch unaltered. 

It is evident from common observation that chloro- 
phyll is not soluble in water; vegetables that have 
actually been boiled in water still retain their green 
colour. Chlorophyll is, however, soluble in methylated 
spirit. 

Chlorophyll dissolves out slowly in cold methylated 
spirit, but it comes out much more quickly on heating. 
This must be done very carefully, as metnylated spirit 
ignites so readily. 

It is well to boil the leaf in water first. As it boils, 
bubbles of air are seen to escape from the leaf, especi- 
ally from the under surface. By this preliminary boil- 
ing the air in the leaf is expelled and its place taken 
up by water. When the escape of bubbles has almost 
ceased the leaf should be taken from the water and 
boiled in methylated spirit. The green colouring 
matter is then readily dissolved, as the passage of the 
spirit into the leaf is rendered easy. 

When the leaf is quite colourless it should be washed 
in water and a little dilute solution of iodine then 
poured over it. After a few minutes it should be again 
washed in water. 

Observations. The leaf becomes a dark blue colour. 



48 EXPERIMENTAL PLANT PHYSIOLOGY 

Inference. Starch is present in a green leaf that 
has been picked in the afternoon of a sunny day. 

Now it will be remembered that the aim stated on 
page 46 was to investigate the conditions under which 
starch can be formed in the plant, and to ascertain, if 
possible, the part taken by the air in its formation. 



EXPERIMENT 29 

Aim. To find out if light and darkness influence 
the formation of starch in the leaf. 

Method. Over both sides of a leaf of a grow- 
ing plant pin carefully pieces of silver paper. The 
paper should be turned over at the edges and fast- 
ened so that the pins 
do not injure the leaf. 
Cover another leaf 
with two pieces of 
paper, each of which 
has a hole cut in the 
centre. The two holes 
must be alike in size 
and shape, and the 
papers must be placed 
over the leaf-surfaces 
FlG 22 ^ so t^k the holes ac- 

curately coincide. 
In this way one leaf is completely in the dark ; the 
other is darkened except for tne central portion. The 
leaves should be left for two days, or longer if the 
weather is not sunny ; then they should be picked in 
the afternoon and tested for starch. 

Observations. No starch is present in the leaf that 
was wholly covered. 

Starch is present in the uncovered part of the second 
leaf (Fig. 22). 

Inference. Starch is formed only under the influ- 
ence of light ; it cannot be formed in darkness. 




THE FOOD OF THE PLANT 49 

It is thus proved that light is necessary for the 
formation of starch in leaves. A further experiment 
may be made to amplify and confirm the above, or 
may be substituted for it if preferred. 

EXPERIMENT 30 

Aim. To find out if light and darkness affect the 
formation of starch in the leaf. 

Method. Choose a potted plant that has a large 
number of leaves. Take off one leaf; boil it in water; 
then put it into a bottle of methylated spirit. Carefully 
label the bottle with the date. Put the plant into the 
dark. The next day take off another leaf and treat 
this in the same way as the first. Similarly, each day 
for a week remove one leaf. Then bring the plant into 
the sunlight again and, in the light, continue the col- 
lection of leaves for a second week. 

At the end of the fortnight examine all the leaves 
for the presence of starch. 

Observations. The first leaf gave a dark blue starch 
reaction. In the second the reaction was fainter. After 
that the amount of starch gradually decreased until 
there was none remaining. 

On the eighth day, that is, the first after the plant 
had been taken from the dark, a little starch was again 
found. Each day after this the amount of starch in- 
creased. 

Inferences. A green leaf loses its starch if kept in 
the dark and regains it ,when brought back again into 
the light. 

EXPERIMENT 31 

Aim. To find out which light-rays are most effective 
in starch formation. 

Method. The light-rays are made to pass through 
coloured screens (blue or red) before they reach the 
plant. The screens may be of red or blue glass, or 
coloured solutions may be used. 

If the screens are to be of glass, two wooden boxes, 

D 



50 EXPERIMENTAL PLANT PHYSIOLOGY 

large enough, when standing on end, to contain potted 
plants, are covered with dull black paper both inside 
and out : or, better still, the inside only may be papered, 
while the outside is painted black. Margarine boxes 
are a convenient size. 

The covers of the boxes are replaced, one by a sheet 
of blue glass, the other by a sheet of red. 

This is a useful piece of apparatus, and it is therefore 
worth while to cover the boxes carefully. To keep the 
sheets of glass in position a beading may be put round 
the edges of the boxes and the glass fitted into this with 





. FIG. 23 



FIG. 24 



small brass catches, or, if preferred, the glass may be 
put into frames and fitted to the boxes with hinges and 
a latch. 

A plant depleted of starch is then put into each box, 
and the boxes are placed so that a good light falls on 
the coloured screens (Fig. 23). . 

For carrying out the experiment by means of 
coloured solutions, special doiible bell-jars are prepared 
(Fig. 24). By means of these double bell-jars the 
working of the experiment, is rendered very simple. 
The coloured solution is put into the outer jacket of 
the jar, and underneath the jar is placed a plant whose 
leaves have been depleted of starch. The whole 
apparatus is then placed in a good light. 

The blue solution is made oy adding ammonia to a 
solution of copper sulphate. At first a precipitate is 



THE FOOD OF THE PLANT 



51 



formed, but, on the addition of more ammonia, the 
precipitate is dissolved and a blue solution is formed. 

The red solution is prepared by dissolving potassium 
bichromate in water. A saturated solution should be 
made. This substance is extremely poisonous. 

The double-jacketed bell-jars are very convenient for 
this experiment, but they are not essential and they 
are expensive. 

To carry out the experiment without the double bell- 
jar it is well to use a water plant. Elodea canadensis, 
the American pond-weed, serves the purpose well. 

Some pieces of Elodea are depleted of starch by being 
kept in the dark in the usual way. They are then 
placed in two bottles of water in which 
is small shot acting as ballast. The 
bottles are corked and the joints thor- 
oughly waxed. The coloured solutions 
are put into two large glass jam or 
pickle jars, and the small bottles con- 
taining the Elodea lowered into the solu- 
tions (Fig. 25). As before, the whole 
apparatus must be put into a good light, 

Observations. When the usual starch 
test is applied it will be found that in 
each case a great deal of starch has 
been formed by the plant in the red light, but very 
little by the plant which has been subjected to the 
blue rays. 

Inference. The red rays of light are more effective 
than the blue in starch formation. 




FIG. 25 



EXPERIMENT 32 

Aim. To find out whether the formation of starch 
is influenced by temperature. 

Method. Elodea, the American pond-weed, or some 
other water-weed, is used for this experiment, as the 
temperature of water can be kept more uniform than 
that of air. 



52 EXPERIMENTAL PLANT PHYSIOLOGY 

Put two vessels, each containing a piece of Elodea, 
into the dark, until the plants are depleted of starch. 

Then surround one of the vessels with ice, put a 
thermometer into each, and place both in a good light. 

Observations. No starch is formed in the plant 
contained in the vessel that is surrounded with ice, but 
it is formed in large quantities in the control experi- 
ment. 

Inference. Warmth is necessary for the formation of 
starch. 

EXPERIMENT 33 

Aim. To find out if the colour of the leaf affects the 
formation of starch in it. 

Method. Choose a plant whose leaves are green in 




ct 



a. The variegated leaf that was tested. The green part is shaded. 

6. The leaf after it had been boiled in methylated spirit. 

c. The leaf after treatment with iodine. The part which turned blue is shaded. 

FIG. 2G 

part only. Variegated maple gives very good results, 
but anv variegated leaf can be used. 

In all experiments that depend on the iodine test it 
is advisable to select thin leaves, as it is difficult to see 
the colour reaction through a very thick skin. 

From the results obtained in Experiments 29 and 30, 
it is clear that the afternoon is the best time for the 
working of this experiment, as then the leaf has had 
sufficient time to make a good supply of starch. 



THE FOOD OF THE PLANT 53 

Pick a leaf. Make a careful drawing of it, showing 
accurately the position of the green and the white 
portions. Then test for starch in the usual way. 

Observation. Only that part of the leaf which was 
originally green is turned blue by the iodine (Fig. 26). 

Inference. Starch is formed in the green part of 
the leaf only. 

EXPERIMENT 34 

Aim. To determine whether the absorption of air 
by a leaf is essential to the formation of starch in it. 

Method. The leaves of a calceolaria or other small 
potted plant are depleted of starch in the usual way. 

Three of the leaves are then smeared with vaseline ; 
one on the upper surface, one on the lower, and the 
third on both surfaces. 

The plant is put again into the light, and, when a 
sufficient time has elapsed, the leaves are tested for 
starch. 

It will be necessary in this case to get rid of the 
vaseline before applying the iodine test. To this end 
the leaves may be put into xylol or petrol until the 
vaseline is dissolved. 

Observations. Starch has been formed plentifully in 
the leaf whose upper surface is vaselined. Very little 
is found in the leaf that had vaseline on the lower 
surface. In the leaf smeared with vaseline on both 
surfaces no starch reaction is obtained. 

Inference. No starch is formed if the absorption of 
air is prevented. 

Three conclusions have now been drawn relating to 
the formation of starch in the leaf : 

1. It cannot be formed in the dark. 

2. It cannot be formed in the absence of chlorophyll. 

3. It is formed in green leaves exposed to light under 
ordinary atmospheric conditions. 

Now starch is made up of the three elements carbon, 



54 EXPERIMENTAL PLANT PHYSIOLOGY 

hydrogen, and oxygen. Hydrogen and oxygen, to- 
gether, form water, which is taken in from the soil by 
the roots of the plant. But it has already been shown, 
by the use of culture-solutions, that the plant is not 
dependent on the soil for its carbonaceous food (page 
13). It follows, therefore, that a plant must obtain 
carbon in some form from the air. 

The air, it will be remembered, is made up of oxygen, 
nitrogen, carbon-dioxide, and water-vapour, together 
with various rare gases and impurities in minute 
quantities. The only possible source of carbon is, 
therefore, the carbon-dioxide (a compound made up of 
carbon and oxygen) which is present in the air. 

The importance of the element carbon is very great. 
It is essential to all living bodies. 

It is not easy to determine practically by any direct 
method whether the plant absorbs the carbon-dioxide 
of the air. The reason for this will be understood 
when the section on the breathing of the plant has 
been studied, An indirect method will therefore be 
adopted, and an experiment will be carried out to 
ascertain if starch continues to be formed in an atmos- 
phere that is deprived of carbon-dioxide. 

There are three substances that have the power of 
absorbing carbon-dioxide. These are : 

1. Lime-water. 

2. Caustic potash solution. 

3. Soda-lime. 

In any particular case the substance that is most 
suitable must be selected. For instance, if the aim is 
to show that carbon-dioxide is being absorbed, then 
lime-water should be used because the absorption of 
carbon-dioxide is quickly demonstrated by the milky 
reaction given. Wnen, however, the aim is the com- 
plete absorption of all carbon-dioxide present, then a 
strong solution of caustic potash is more suitable. 
Again, soda-lime can be used when a solid substance is 
required. 



THE FOOD OF THE PLANT 59 

Inferences. Grape-sugar can be recognized by the 
red-brown precipitate formed when a solution of it is 
boiled with a few drops of copper sulphate solution and 
excess of potash ; in the case of cane-sugar the same 
precipitate is formed in the solution, but not until it 
nas been boiled with an acid. 

These reactions may cause a little difficulty at first, 
but the tests must be satisfactorily applied before pro- 
ceeding further. In order to give more practice several 
plant-storage organs should be tested for sugar. 



EXPERIMENT 38 

Aim. To test for sugar in the carrot, onion, beetroot, 
turnip. 

Method. Extracts of the vegetables are made and 
each is tested as in Experiment 37. 

Observations. Grape-sugar is present in the carrot, 
onion, and turnip. 

Cane-sugar is present in the beetroot. 

The chemical changes that underlie these reactions 
will easily be understood by those who have some 
knowledge of the subject. 

Copper sulphate and potash react, giving a flocculent 
precipitate of copper-hydroxide : 



CuS0 4 +2 KOH = Cu(OH) 2 +K 2 S0 4 

The copper-hydroxide precipitate is then dissolved 
again by the addition of excess of potash. 

Copper-hydroxide is made up of cupric-oxide and 
water. Thus : 

Cu(OH) 2 -CuO+H 2 

Now certain sugars have the power of taking oxygen 
from other substances and " reducing" them to less 
oxidised forms. The equation 

= 2Cu 2 0+0 2 



60 EXPERIMENTAL PLANT PHYSIOLOGY 

shows how cupric-oxide may be reduced to cuprous- 
oxide, which is a red-brown precipitate. 

The previous experiment has shown that grape-sugar 
is a reducing sugar. Cane-sugar is not a reducing sugar, 
but is converted into one when it is boiled with a trace 
of mineral acid. The change which takes place is 
termed hydrolysis and consists in the addition of a 
molecule of water to the molecule of the non-reducing 
sugar, by which it is changed or "inverted" into a re- 
ducing sugar. Thus : 

C 12 H 22 O n +H 2 = 2C 6 H 12 6 

Cane-sugar. Water. Grape-sugar. 

EXPERIMENT 39 

Aim. To determine whether the starch formed in 
the leaf is converted into sugar. 

Method. A few leaves are picked in the afternoon 
of a warm, sunny day. They are then put on damp 
blotting-paper in a well-corked bottle and placed in the 
dark for about three days. 

When the leaves are picked they contain a large 
quantity of starch. It has already been shown that 
starch does not remain in the leaves if the plant is kept 
in the dark. In this case, however, the removal of any 
substance from the leaf is prevented by its separation 
from the plant. 

At the end of three days the following tests are 
made : 

1. Some of the leaves are tested for starch. 

2. Some of the leaves are tested for sugar. 

3. Some leaves freshly gathered from the plant are 
tested for sugar. 

Observations. No starch is found in the leaves 
although they have been separated from the plant. 

Those, however, that were submitted to the second 
test were found to contain sugar. 

No sugar reaction is obtained in the case of the 
freshly gathered leaves, 



THE FOOD OF THE PLANT 61 

Inferences. The starch formed in the leaf is con- 
verted into sugar, a soluble substance; and in this 
form it travels about the plant. 

The change by which starch is converted into sugar 
is one of hydrolysis (page 60), similar to that by which 
cane-sugar is converted into grape-sugar. 

It may be represented as a chemical equation. Thus : 



Starch. Grape-sugar. 

The exact value of x is not known. 

It has already been shown that, when cane-sugar is 
boiled with a mineral acid, it is converted into grape- 
sugar. 

In the following experiment it will be shown that the 
process whereby starch is converted into sugar is similar 
to that by which grape-sugar is formed from cane- 
sugar. 

EXPERIMENT 40 

Aim. To find out what happens to starch when it is 
boiled with a mineral acid. 

Method. Small pieces of potato, or powdered starch, 
can be used in this experiment. To tnis, water and a 
few drops of hydrochloric acid are added. The whole is 
then boiled. At intervals of two or three minutes a 
little of the liquid is removed and treated with iodine 
in order to test for starch. 

When the iodine solution no longer gives any starch 
reaction the remaining liquid is tested for sugar. 

Observations. Each time the iodine test is applied 
the amount of starch is found to decrease. Finally no 
starch is left. The remaining liquid shows the presence 
of a reducing sugar. 

Inference. Starch is converted into sugar when 
boiled with a mineral acid. 



62 EXPERIMENTAL PLANT PHYSIOLOGY 

It has thus been proved: firstly, that the starch 
formed in the leaf is converted into sugar ; and, secondly, 
that it is possible to convert starch into sugar by 
boiling it with a mineral acid. 

Now it is quite evident that the starch present in the 
leaf is not naturally hydrolysed by a mineral acid in 
this way. The hydrolysing agent in the case of the 
leaf is a ferment termed " diastase." This diastase can 
be extracted from the leaf, but the operation is beyond 
the scope of this book. 

The action of diastase differs from that of a mineral 
acid in that it is able to effect the change at the 
ordinary temperature. 

The soluble sugar into which insoluble starch has 
now been converted travels down the stem, and some 
of it is afterwards reconverted into starch, in which form 
it is stored as a reserve-food in tubers and bulbs and 
other such structures. 

Eventually the starch and sugar formed in the plant, 
together with the substances taken up in solution from 
the soil, are used by the plant for the building up of its 
solid framework and for the formation of the living 
substance, the protoplasm, contained within it. 



EXPERIMENT 41 

Aim. To determine where the food material, manu- 
factured in the leaf, passes through the stem. 

Method. A ring of tissue about an inch wide and 
reaching as far as the wood, is removed from the stem 
of a branch of a tree or from the main stem of a young 
potted seedling- tree. A two-year-old sycamore answers 
well. 

The removal of this ring of tissue, consisting of 
bast, cortex, and bark (page 18), does not inter- 
fere with the passage of the watery solutions up 
the stem, since they ascend through the wood only 
(Experiment 8). 

Observations. The following observations were made 



THE FOOD OF THE PLANT 



63 



in an experiment in which two young sycamore trees 
were used. The figures 29 (a) and 29 (6) are photo- 





FIG. 29 (a) 



FiG. 29 (6) 



graphs of the two plants taken on May 14th before the 
ring of tissue was removed. 

The stems were then ringed a few inches above the 
earth. 

The ringing of the stem made no difference to the 
unfolding of the new leaves. A circular swelling 
formed round the stem immediately above the 
ringed portion. In the case of 6, dormant buds 
below the ring began to develop. The plants were 



64 EXPERIMENTAL PLANT PHYSIOLOGY 

again photographed on June 20th (Figs. 30 (a) 
and 30(6)). 

After this the petioles of the leaves began to droop. 
This was especially the case in plant a, where, by the 





FIG. 30 (a) 



FIG. 30 (b) 



end of the month, the leaf petioles were hanging 
vertically and the leaves were curled up. 

The plant b did not suffer so much. The new shoots, 
developed from the dormant buds below the ring, 



THE FOOD OF THE PLANT 



65 



continued to grow and thrive. The swelling- above the 
ring increased, and tissue-formation took place, almost 
covering the original wound. Figs. 31 (a) and 31 (6) are 





FIG. 31 ( ) 



FIG. 31 (b) 



photographs of the plants taken on July 1st. Fig. 32 
shows the lower part of plant b taken on the same day. 
From this date plant a withered away. There was 
no attempt at new bud formation in the axils of the 
leaves. The leaves did not fall off. 

E 



66 EXPERIMENTAL PLANT PHYSIOLOGY 

Plant b fared better. The lower shoots throve in a 
normal mariner. The leaves of the main shoot drooped 
more and more until they also hung vertically. They 
did not fall off, but good winter resting buds were 
formed in their axils and also a large terminal bud. 




FIG. 32 



Inference. Putting all these facts together the 
following conclusions are drawn : 

1. The ringing of the stem does not at first cause 
any apparent check to the growth of the plant. 

2. Later the growth is checked, and in the case of a 
the whole plant died. 

3. The difference in the behaviour of the two plants 
must be due to the development of the dormant buds 
below the ring in the case of b 



THE FOOD OF THE PLANT 67 

4. The only consistent explanation of these facts is 
that the food-material, elaborated in the leaves, passes 
through the plant in the tissue outside the wood. Assum- 
ing this, the facts recorded can be explained thus : 

In plant a the leaves continue to unfold after " ring- 
ing," as the water supply has not been checked ; the 
expanded leaves then manufacture starch ; the- starch 
is converted into sugar; it is then further combined 
with the substances from the watery solutions coming 
from the soil. The final product of this assimilation 
then passes down the stem until it is stopped by the 
cut ring. As a result no further manufactured food- 
stuff' ever reaches the root ; the root is unable to per- 
form its functions properly and to grow with the growth 
of the shoot above ; the plant ultimately dies as a 
result of the starvation of the root. 

In plant b the initial stages are similar to those of 
plant a, but later the root is Kept from starving because 
nourishment is sent to it by the new leaves that grow 
below the ring. The root-development, which thus 
takes place, is not sufficient to maintain proper develop- 
ment of the main shoot above the ring ; as a result of 
this the leaves of the main shoot cannot form their 
cork-layers and so they droop, but do not fall off; but 
the foot-development is sufficient to keep the plant 
alive for a considerable time and to enable the shoot 
to form its winter resting buds. 

Since this explanation tits all the observed facts, it 
may be concluded that the food-material manufactured 
in the leaves passes to different parts of the plant 
through the tissues outside the wood. 

Comparison of the Feeding-process in Animals and 
Plants. The feeding-process in plants and animals can 
now be compared. 

Animals are unable to utilise, as food, the constitu- 
ents of the air and the soil. They cannot build up 
starch from carbon-dioxide and water. They must, 
therefore, feed on the starch and sugars that the plants 
have already manufactured. 



68 EXPERIMENTAL PLANT PHYSIOLOGY 

Thus it is seen that plants only can live indepen- 
dently on this planet. Without them the simple con- 
stituents of air and water could not be used for food 
and animal life could not be sustained. 

The important conclusion is now reached that all 
animal life, our own included, depends for its very 
existence on the activity of the green plant. 

It is now possible to consider more fully the cause of 
this dependence of the animal world upon plant-life. 

Carbon-dioxide and water cannot of themselves form 
starch. Whenever it is required to build up any sub- 
stance from two simpler substances some form of energy 
must be supplied. Familiar instances of this occur in 
the experiments made in the chemical laboratory, where 
energy, in the form of heat, is constantly being applied 
to bring about chemical combination, 

So it is in the case of starch formation. 

To write the equation 

Carbon-dioxide + water = Starch -f- oxygen 

is not to state the case correctly. 
The right equation is : 

Carbon-dioxide + water + energy = Starch + oxygen. 

What then is this energy ? and why cannot animals 
make starch in a similar way ? 

The answer to these questions is that the ultimate 
source of all this world's energy is the sun, and only 
chlorophyll is able to absorb this energy for the building 
up of food. 

Health Value of keeping Plants Indoors. There is 

one interesting and very important point which may be 
referred to here. 

It will be remembered from the lime-water test (Ex- 
periment 21) that in breathing, or respiration, a large 
proportion of carbon-dioxide is given out. 

Further, we know from experience that a room gets 
"stuffy" when several people are in it. 



THE FOOD OF THE PLANT 69 

The conclusion is therefore drawn that the "stuffi- 
ness " is due to excess of carbon-dioxide in the air, and 
that the reduction in the amount of carbon-dioxide 
will render the atmosphere healthier. Here then is 
the health reason for having green plants in the house. 

It must not, however, be forgotten that this advan- 
tage holds only during dayligtit, since no absorption 
of carbon-dioxide can take place by the plant in the 
dark. 

(Note. It has recently been suggested that the un- 
healthy atmosphere produced in a room containing 
several people is due to increase in amount of moisture 
rather than to that of carbon-dioxide; and, further, 
that the "stuffiness" is lessened when the air is kept in 
motion.) 



SECTION II 
THE BREATHING OF THE PLANT 

IN the breathing process of animals a larger propor- 
tion of carbon-dioxide is given back to the air than is 
taken from it. This was proved in Experiment 21, 
where it was seen that the amount of atmospheric 
carbon-dioxide is being continually increased as a result 
of the breathing of animals. 

Now plants, like animals, are living things. They 
feed. Tney have the capacity for growth. Do they 
also breathe ? 

The first step towards finding an answer to this 
question is to determine whether or no plants give out 
carbon-dioxide. 

Just at first this may look like a contradiction of 
what has been already learnt. But it is not neces- 
sarily so. 

It has been seen that, given certain conditions, a 
plant takes in carbon-dioxide from the air in its feeding 
process (page 56). But this absorption of carbon- 
dioxide for food does not, in any way, hinder the plant 
from giving off the same kind of gas in the absolutely 
distinct process of breathing. 

In trying to find out, however, whether there is 
evolution of carbon-dioxide, great care must be used 
to prevent the plant from taking back, as food-material, 
the carbon-dioxide that it may Have given out by 
breathing. If the reabsorption of the carbon-dioxide 
is not prevented, the lime-water test will, of course, 
fail. 

Obviously, then, the plant must not be allowed to 
obtain food from the air while breartiing experiments 

70 



THE BREATHING OF THE PLANT 75 

cient to extend for at least ten inehes on each side 
of the V-shaped bend. The apparatus is now made 
air-tight. 

The level at which the liquid stands in the two arms 
of the tube is marked witn gummed paper and the 
thermometer read. The temperature may then be 
kept constant by means of a wet cloth. 

Now since no substance is put into the flask to 
absorb the carbon-dioxide, it follows that there will be 
no change in the level of the coloured liquid if the 
amount of oxygen inspired equals the amount of 
carbon-dioxide expired, provided always that there are 
no other factors which, nullify the conclusions arrived 
at. If, however, the volume of oxygen taken in ex- 
ceeds that of carbon-dioxide given out, the column of 
coloured liquid in the tube will move towards the flask ; 
conversely, the column of liquid will move in the 
direction away from the flask should the volume of 
carbon-dioxide expired exceed that of oxygen inspired. 

Observations. For a time the level of the liquid 
remains practically stationary. Soon, however, the 
liquid is slowly pushed down the arm of the tube 
which is adjacent to the flask. (It is not generally 
possible to get the column of liquid further than the 
bend of the tube, as the gas from the flask manages to 
escape when the end of the column of liquid stands at 
the bend.) 

Inference. It may be roughly estimated from this 
experiment that the amount of oxygen inspired is 
normally equal to that of carbon-dioxide expired, but 
that, in the apparatus set up, the normal result is soon 
falsified by the conditions of the experiment itself. 

(Note. When seeds are germinated in a confined 
space so that the supply of external oxygen is limited, 
the seedlings are able to utilise the oxygen contained 
withirf themselves for their respiration. This utilisation 
of the oxygen contained within the tissues of the plant, 
for respiratory purposes, is termed Intramolecular 
Respiration. iBihpn ipf^^^"^ respiration is taking 



76 EXPERIMENTAL PLANT PHYSIOLOGY 

place carbon-dioxide continues to be evolved, thus 
there ceases to be any relation between the amount 
of oxygen taken in and that of carbon-dioxide given 
out. 

The change in the level of the liquid which was 
observed in the above experiment, after the apparatus 
had been set up for some time, was largely due to 
the fact that intramolecular respiration had begun. 
Another factor which caused movement of the column 
of liquid was the evaporation of water-vapour from 
the surface of the peas and paper.) 

EXPERIMENT 46 

Aim. To determine the amount of carbon-dioxide 
that is given out in the process of respiration. 

Method. From Experiment 45 it is seen that, 
within very rough limits, the amount of carbon-dioxide 
expired is equal to the amount of oxygen inspired. 

Assuming, then, that these amounts are equal, the 
quantity of oxygen measured in Experiment 44 may 
be taken also as a measure of the amount of carbon- 
dioxide.expired. 

EXPERIMENT 47 

Aim. To find out whether a plant breathes in the 
dark as well as in the light. 

Method. The foregoing experiments can be worked 
again, the apparatus being kept in the dark. 

Observations. Similar observations will be obtained. 

Inference. Plants breathe always, night and day. 

EXPERIMENT 48 

Aim. To find out whether a plant can live if it is 
deprived of oxygen. 

Method. In this experiment some substance must 
be used that has the power of absorbing oxygen from 
the air. Pyrogallie acid answers this purpose. In 




FIG. 37 



THE BREATHING OF THE PLANT 77 

order to demonstrate its absorptive power a small 
quantity of the acid, dissolved in water, may be put into 
a well-corked bottle and left for a few hours ; then, by 
the extinction of a lighted taper which 
is thrust into the bottle, the absence of 
oxygen is proved. 

Two healthy bean-seedlings are sus- 
pended, by means of cotton, in two bottles ; 
at the bottom of one bottle is a solution 
of pyrogallic acid ; in the other an equal 
volume of water. The seedlings must not 
touch the liquids. The bottles are then 
well corked (Fig. 37). 

Observations. The seedling which is 
suspended over the water continues to 
grow, while the one placed in the bottle containing 
pyrogallic acid very soon dies. 

Inference. A plant cannot live if deprived of oxygen. 
In other words, a plant dies if it is unable to breathe. 

(Note. In the case of the seedling that was suspended 
over pyrogallic acid, a little further growth is observed 
before the seedling begins to wither. This is due to 
the fact that the oxygen contained within the se*edling 
itself is being used up in its respiration 
(page 75). 

EXPERIMENT 49 

Aim. To find out whether the breath- 
ing of plants affects the temperature of 
the surrounding air. 

Method. A glass funnel is filled with 
peas that have been soaked for two days 
and are just beginning to germinate. The 
funnel is then supported in a tumbler at 
the bottom of which is a little water. A 
bell-jar is placed over the whole. The 
bell-jar is fitted with a one-holed cork, 
and a thermometer is passed through the hole so that 
its bulb dips down into the peas (Fig. 38). 




FIG. 38 



78 EXPERIMENTAL PLANT PHYSIOLOGY 

A control experiment is set up in which the peas are 
replaced by cotton-wool or sawdust. 

Observations. On reading the thermometer it is 
found that the temperature of the germinating peas is 
higher than that of the control experiment. 

Inference. The breathing of plants causes the tem- 
perature of the surrounding air to rise. 

Health Value of Plants Indoors. It has now been 
proved that plants, like ourselves, continually take in 
oxygen and give out carbon-dioxide. This may at first 
sight seem to contradict what was said on page 68 as to 
the health value of plants in the house. 

It is evident that the breathing of the plant helps to 
make the air "stuffy," just as our own breathing does. 
But during the day the green plant takes in carbon- 
dioxide as food and gives out oxygen and thus helps to 
purify the air; the amount of carbon-dioxide absorbed 
as food far exceeds that expired in the breathing process. 
Thus a purifying of the air is effected by the presence of 
the plant. But this advantage lasts only during the 
day. As the evening approaches the feeding process 
slackens and finally stops, but the breathing never 
ceases, and so, during the night, plants, like ourselves, 
only vitiate the air, and should therefore be removed 
from the rooms in which we sleep. 

The Significance of the Breathing Process. The true 
meaning that underlies the need for continuous breath- 
ing in all living things can now be explained. 

Something of the process has been learnt. It has 
been found that all living things, plants as well as 
animals, breathe constantly as long as they continue to 
live (Experiment 47) ; and, further, that aeath follows 
when breathing ceases (Experiment 48) . So far as the 
interchange of gases is concerned, breathing consists in 
the taking in of oxygen and the giving out of carbon- 
dioxide; out this is no explanation of the fact that 
breathing is essential to life. G 

As long as a plant or an animal lives, it is constantly 



THE BREATHING OF THE PLANT 79 

expending, in its growth and in its varied activities, the 
energy which it possesses within itself; and, unless this 
energy can be restored, it must die. 

As a result of the absorption of food, new plant or 
animal tissues are constantly being built up, and these 
tissues form a storage of energy in a potential form. 

The meaning of the word " potential" in this connec- 
tion will be readily understood from an illustration. 
Water, stored in a reservoir, has "potential" energy. 
It can be used to drive an engine or to turn a mill- 
wheel. But, before it can be of any use whatever, it 
must be released from the reservoir and allowed to 
expend its energy in movement to a lower level, i.e. the 
" potential" energy must be converted into "kinetic" 
energy, or, in other words, the energy of power must 
be changed into the energy of movement. 

Similarly, plant substances which are formed during 
assimilation must be broken down in order that the 
energy which is stored up in them may be liberated. 
The oxygen that is taken in in respiration acts as the 
destroying agent. It enters into combination with 
chemical compounds, formed in the plant, and breaks 
them down; as a result of this, energy is liberated. 
Some of this energy is dissipated in the form of heat, 
as was shown by the rise in the temperature of the air 
surrounding germinating peas (Experiment 49), but 
the greater part of the liberated energy is available for 
use in the further growth and activities of the plant. 

It is thus seen that breathing is a destructive process 
by means of which energy is liberated. For this reason 
it is continuous, taking place in darkness as well as in 
the light. 

Exactly opposite is the process of assimilation. Here, 
from the carbon-dioxide taken in from the air and the 
water absorbed from the soil, a plant manufactures 
substances such as starch. This is a building-up process, 
and, for this reason, energy must be supplied. It 
follows, therefore, that it can take place only in the day- 
time, in 4he light of the sun, the earth's one source of 
energy. 



80 EXPERIMENTAL PLANT PHYSIOLOGY 

The equation that represents the building up of starch 
was given on page 68 : 

Water + carbon-dioxide + energy = Starch+ oxygen, 

The following equation may now be compared with 
the above ; 

Starch + oxygen= Water + carbon-dioxide -f energy. 

This equation represents the destructive action of 
oxvffcn on starch. 

The first equation shows the assimilation or building- 
up process for which energy must be supplied. The 
second equation is that of respiration or breaking-down 
in which energy is liberated. 

Summary. The main facts that have been learnt 
concerning the relation between the plant and the sur- 
rounding air may now be tabulated. 

Air is used by the plant in connection with the 
processes of feeding (carbon-assimilation) and breathing 
(respiration). 

Carbon-assimilation. Respiration. 

Carbon-dioxide is taken in. Oxygen is taken in. 
Oxygen is given out. Carbon-dioxide is given 

out. 

The air is purified. The air is rendered im- 

pure. 

Only green parts of plants Every part of the plant 
can use carbon-dioxide for breathes, 
food. 

Energy is required, there- Energy is set free, there- 
fore carbon -assimilation fore respiration takes 
takes place only in sun- place always, in dark- 
light, ness as well as in the 

light. 



SECTION III 
THE GROWTH OF THE Pl^ANT 

Introductory. When a seed is put into the ground 
and given suitable conditions it begins to germinate. 
First the root emerges and penetrates into the soil, 
after this the shoot grows up into the air. In time, a 
large branching system of roots is developed in the 
soil, and, above the ground, a stem with many branches 
bearing leaves, flowers, and finally fruits. 

I^ilnjotj^asy_M.say exactly what is meant by growth. 
It is not simply increase in size and bulk. A sponge, 
wlieu__t)lace(l in wiiJeF," swells and increases in weight^ 
but it-has not grown, and, when taken out ojf_the water, 
it shrinks to its original size. Growth necessarily im- 
plies a permanent change in form and can only take 
place in living things. 

It has already been seen that two processes are con- 
tinually going on in the plant, one building up the 
tissues and the other breaking them down. jGrrowth 
takes place .,.when the process of building up is greater 
than^that of breaking. cJQwn. 

Tn" order to study the elementary conditions and 
phenomena of growth a large number of seedlings at 
various stages are required. 

Useful seedlings for the purpose are those of the 
broad-bean, French-bean, giant-sunflower, and Indian 
corn. These are all large seedlings and can be grown 
satisfactorily indoors. 

The Germination of the Seed and the Growth of 
the Seedling. The morphology of the growing seedling 
is well and fully described in most elementary text- 
books of botany. This section will therefore be eon- 
si F 



82 EXPERIMENTAL PLANT PHYSIOLOGY 

fined to simple experiments dealing with causes and 
methods of growth, and the experiments should be 
worked in conjunction with the morphological study of 
the germination of the seed. 

For this reason it is sufficient here to trace briefly 
the course of events in some individual case, say in the 
germination of a broad-bean. 

When a broad-bean seed is planted in a pot of earth, 
and kept damp and in a warm place, it very soon begins 
to grow. First it absorbs some of the moisture and 
consequently swells. This swelling produces so much 
pressure on the coat, or testa, that a rupture is caused. 
The testa bursts at its weakest point, that is, at the 
point where there was already a small hole, the micro- 
pyle. The first root, or radicle, whose tip was just 
beneath the hole, protrudes through the ruptured coat 
and grows down into the soil. Soon after this, the first 
shoot, the plumule, bent like a hook and with its yellow 
leaves all crowded together at the tip, pushes its way 
through the soil up to the light and the air. 

The seed germinates because it has been supplied 
with all the conditions necessary for its development. 

These conditions, which together make up its environ- 
ment, may be examined separately in order to find out 
which are essential to the growth of the seedling. 



A. Water as a Necessary Factor in Germination 

EXPERIMENT 50 

Aim. To find out whether a seed will germinate 
without water. 

Method. Some bean seeds are planted in a pot of 
well-dried soil, and others in a pot of damp soil. The 
seeds in the damp soil are watered regularly in the 
usual way. 

Observations. The seeds in the damp soil germinate. 
Those in the dry soil do not. 

Inference. A seed will not germinate without water. 



THE GROWTH OF THE PLANT 83 

That a seedling is able to absorb water has already 
been proved in Experiment 1. 

Now soak a broad-bean in water for a day. Remove 
it and squeeze gently. Water oozes through the 
micropyle. Is it then through the micropyle that the 
water was absorbed ? 

EXPERIMENT 51 

Aim. To determine whether water is absorbed by 
the broad-bean seed through the micropyle only, or 
whether it can be taken in through the 
testa as well. 

Method. Suspend a broad-bean in a 
beaker of water by means of cotton so that 
the micropyle is well out of the water 
(Fig. 39). 

Observations. The immersed portion of 
the bean becomes swollen first. Later the 
swelling extends to the part of the bean 
that is out of the water. FIG. 39 

Inferences. The broad-bean absorbs 
water through the testa as well as through the micro- 
pyle. The water that has been absorbed can pass from 
one part of the seed to another. 

The absorption of water by a seed is termed Imbibition. 

B. Food as a Necessary Factor in Germination 

The question of the food of the adult plant has 
already been dealt with in a long series of experiments. 
From these experiments it has been learnt that the 
plant gets food both from the soil and from the air. 
It remains now to find out whether these sources of 
food-supply are available and necessary for the early 
stages in the life of the plant. 

It wM be remembered that the food from the air is 
taken in only by the green parts of plants. It follows, 
therefore, that the seedling cannot take in food from 




84 EXPERIMENTAL PLANT PHYSIOLOGY 

the air during the early stages of its growth, since this 
source of supply is not available until some green 
leaves have unfolded. 

On the other hand, the substances dissolved in the 
water, which constitute the food-supply from the soil, 
are available to the plantT from the beginning of its 
growth. Whether or no these dissolved substances 
are necessary for germination can be shown by the 
following experiment. 

EXPERIMENT 52 

Aim. To find out whether a seed requires food from 
the soil in order to begin to grow. 

Method. Three broad-beans are planted. One is 
put into earth, one into cocoanut fibre, and the third is 
suspended over water in a corked bottle so that it is 
in a damp atmosphere. The bean that was planted in 
cocoanut fibre is watered with distilled water. 

Thus one only of the three seeds is provided with 
mineral food. 

Observations. All three seeds begin to grow. 

Inference. Food from the soil is not necessary in 
the initial stages of growth. 

It has thus been proved that a seed can germinate 
without nourishment either from air or soil. This does 
not imply that growth is possible without food,' alt hough 
it may appear at first sight as if it were a case of a 
"building-up" without a " breaking-down." But such 
a case is impossible. The growth of the seedling neces- 
sarily involves an expenditure of energy; and this 
energy must be constantly renewed or the seedling will 
die. Somewhere "potential " energy must be converted 
into " kinetic " energy. Or, in. other words, assimilated 
food substances must be broken down so that energy 
necessary for the growth of the seedling can be liberated. 

Where then does the seedling obtain this assftnilated 
food, since it cannot manufacture any for itself ? 

In answer to this question it may be said that the 



THE GROWTH OF THE PLANT 85 

food need not necessarily come from without. An ex- 
amination of any seed always reveals a supply of food- 
stuff either in the embryo itself, or packed around it. 
In the seeds suggested as types, the broad-bean, French- 
bean, and sunflower store food-material in the cotyle- 
dons of the embryo ; in the case of the maize the 
food-material is stored in the endosperm which sur- 
rounds the embryo. 

An experiment can now be made to find out whether 
the growing seedling uses the food that has been stored 
up in the seed. 

EXPERIMENT 53 

Aim. To find out whether a growing seedling uses 
the food that is stored up in the seed. 

Method. Three French-beans are planted. One (a) 
is allowed to grow naturally. In the case of the second 
seedling (b) the cotyledons are cut through at the base 
as soon as they are sufficiently above the surface of the 
earth to admit of the cut being made without injury to 
any other part of the seedling. When the cuts have 
been made and the cotyledons thus disconnected from 
the rest of the plant, it is better not to attempt to 
remove them, as such removal would probably cause 
injury to the shoot or root of the young plant. The 
cotyledons are cut off from the third seedling (c) as 
soon as the first foliage leaves have expanded. 

Observations. The results noted are, that 

(a) grows and develops ; 

(b) quickly withers ; 

(c) is not affected by the removal of the cotyledons. 
Fig. 40 is a photograph of two French-bean seedlings 

growing under similar conditions in one pot. From one 
of the seedlings the cotyledons were removed as soon as 
that operation was possible without injury to the rest 
of the seedlings. Up to that time the seedlings were 
equally healthy. 

Inference. The seedling uses the food stored in the 
seed until the unfolding of the green leaves enables it 
to build up food substances for itself. 



86 EXPERIMENTAL PLANT PHYSIOLOGY 

EXPERIMENT 54 

Aim. To determine the nature of the food that is 
stored in the cotyledons of the French bean. 




FIG. 40 

Method. Break a cotyledon across and place the cut 
end in iodine solution. 

Observation. The broken end of the cotyledon turns 
blue. 

Inference. Starch is stored in the cotyledons of the 
French-bean. 

The food-store in all seeds is not in the form of 
starch. Oils and fats are sometimes stored in sseeds ; 
and, in other cases, the store consists of proteins, sub- 
stances which resemble the white of egg. 



THE GROWTH OF THE PLANT 87 



C. Air as a Necessary Factor in Germination 

In the course of the experiments described in this 
book, it has been seen that a plant takes in two con- 
stituents of the air, namely, oxygen in the process of 
breathing, and carbon-dioxide as a food. 

Of these two gases, the carbon-dioxide is of no use 
to the germinating seed, since it contains no chlorophyll 
by means of which the carbon-dioxide can be assimi- 
lated ; this, however, is compensated for by the store of 
food within the seed itself. 

On the other hand, it has been proved that germinat- 
ing seeds do take in oxygen (Experiment] 43) ; and, 
further, that a seedling dies if the air which [surrounds 
it is deprived of oxygen (Experiment 48). 

It is thus seen that a germinating seed does not 
require carbon-dioxide from the air, hut it dies if de- 
prived of oxygen. 



D. Light as a Necessary Factor in Germination 

EXPERIMENT 55 

Aim. To find out whether light is necessary to the 
growth of a seedling. 

Method. Seeds of various kinds are planted in pots. 
Some of the pots are then kept in a dark room or large 
airy cupboard, the remainder are allowed to grow under 
the normal condition of alternating light and darkness. 
Temperature and other factors must be, as nearly as 
possible, the same for both sets of pots. 

Observations. The seedlings kept in the dark grow, 
but the growth is abnormal. The stems become long, 
thin, and are without strength, while the leaves remain 
small and yellow. Fig. 41 is a photograph of two broad- 
bean Seedlings, one of which has been grown in the light 
anil the other in darkness, all other conditions having 
been the same for both seedlings. 



88 EXPERIMENTAL PLANT PHYSIOLOGY 

Plants grown in the dark never reach maturity, but 
wither away after a time. 

Inferences. A seed germinates quite well in the dark, 
and the seedling continues to grow as long as the food 
that the parent plant has stored up for it in the seed 
lasts; but eventually it must have light in order that 




FIG. 41 



the leaves may expand, become green, and perform 
their work as feeding organs. The plant which is kept 
in the dark grows excessively long in its vain attempt, 
seemingly, to reach the light, and dies at last for lack 
of food. 

E. Life as a Necessary Factor in Germination 

i> 

If a seed is boiled for some time in water it is no 
longer able to germinate. Thus there is a vital differ- 



THE GROWTH OF THE PLANT 89 

ence between the boiled and the unboiled seed: one 
can germinate, the other cannot ; or, differently ex- 
pressed, one is living, the other is dead. 

F. Heat as a Necessary Factor in Germination 

Heat and cold are relative and not absolute terms. 
The seeds in our gardens do not germinate in the spring 
until a certain degree of temperature is reached; 
further, seeds, of any kind kept in a warm greenhouse 
germinate earlier than those of the same kind sown in 
the open. 

Accurate experimental work on this point is beyond 
the scope of this book. It can only be stated that 
some degree of warmth is necessary for germination 
and that the degree differs for different seeds. 

Plants that require a large amount of heat must grow 
in tropical countries ; in colder countries are to be found 
only tnose plants that can thrive at lower temperatures. 

In this country the limits of temperature between 
which plants grow arc a few degrees above freezing- 
point and about 50 C., but the plant's greatest activity 
occurs between the temperature 25 C. and 30 C. 

The conditions under which a seed can germinate 
have now been investigated. If these conditions are 
fulfilled seedlings will grow healthily and the method of 
their growth can then be studied. 

Direction of Growth. The first observations to be 
made are on the direction of growth of the various 
parts of the seedling. It is noticed that the root makes 
its way down into the soil, while the shoot grows up 
into the air. Further investigations will determine the 
causes which bring about this directive growth. 

EXPERIMENT 56 

Ain To show the direction of growth taken by the 
root and by the shoot of seedlings when the seeds are 
planted in various positions. 



90 EXPERIMENTAL PLANT PHYSIOLOGY 

Method. (a) A gas-jar is fitted with a cork. A 
small groove is cut in the lower end of the cork, and 
into this is fixed one end of a strip of sheet cork 
measuring about one inch by six. 

Three soaked broad-beans are then pinned on to the 
piece of cork. The beans are arranged so that the 
radicle points, in one case, downwards ; in the second, 
upwards ; and, in the third case, horizontally. The pins 
can be stuck through the cotyledons, but care must be 
taken to prevent their penetrating plumule or radicle. 

A little water is put into the bottom of the jar, but 







FIG. 42 



FIG. 43 



FIG. 44 



the strip of cork should not dip into it. The jar is 
then covered with black paper. This is a very simple 
way of showing the direction of growth taken by the 
root, but the following method is preferable for demon- 
strating the direction of growth of the shoot, as it does 
not necessitate the use of a closed jar. 

(fc) A gas-jar or lamp-chimney is lined with a roll 
of blotting-paper. The roll is filled up with moist saw- 
dust or moss so that the blotting-paper may be kept 
damp. Seeds are then carefully placed between the 
glass and the paper in the three positions given above. 



THE GROWTH OF THE PLANT 91 

Observations. 1. When the seed is planted so that 
the radicle points downwards, the radicle grows verti- 
cally downwards and the plumule vertically upwards 
(Figs. 42 and 43). 

2. If the radicle of the seed is made to point up- 
wards when planted, the radicle grows upwards for a 
little way, it then bends right over and grows down- 
wards ; the plumule curves and grows upwards (Figs. 
42 and 44). 

3. When the radicle of the seed points horizontally 
it grows horizontally for a little way ; it then bends at 
rignt angles and grows downwards ; the plumule grows 
upwards (Fig. 42). 

Inferences. The plumule always ultimately grows 
straight upwards and the radicle straight downwards 
whatever the position in which the seed is planted. 

(Note. Method (/>) of the above experiment is one 
that will constantly be found useful when early stages 
of growth are under investigation.) 



EXPERIMENT 57 

Aim. To find out whether light affects the direction 
of growth taken by the shoot. 

Method. A wooden box a margarine box is suit- 
able for the purpose is painted black on the outside 
and lined with black paper. A small hole is made in 
the cover of the box near one end. The box then 
stands on one of its shorter sides and a small pot con- 
taining a sunflower seedling is put inside. 

It will be necessary to remove the cover at intervals 
in order to water the developing seedling, but care must 
be taken not to alter the position of the pot. 

If the cover does not nt exactly, black paper should 
be pasted round the edges of it. 

The^box must then bs placed in a good light. 

The seedling is now illuminated from one point only. 

(See also the method used in Experiment 59.) 



92 EXPERIMENTAL PLANT PHYSIOLOGY 

Observations. After a while the tip of the shoot 
grows through the hole. 

If the method given for Experiment 59 be used, it 
will be seen clearly that the stem of the shoot places 
itself in the same direction as that in which the light 
is falling, while the leaves arrange themselves at right 
angles to the source of light. 

Inference. A shoot grows towards the light, the 
positions taken by the stem and leaves respectively 
being such as to secure for the leaves the maximum 
amount of light. 

The way in which the shoot reacts in response to 
the stimulus exerted by the direction of the rays of 
light is termed Heliotropism. The shoot grows to- 
wards the source of light, placing itself in the same 
line as the rays of light. It is described therefore as 
being positively hdiotropic. 



EXPERIMENT 58 

Aim. To find out whether light affects the direction 
of growth of the root. 

Method. Get a wooden box a margarine box answers 
the purpose well, or a chalk box may be used. Take 
off one of the long sides and fix a piece of glass in place 
of the cover. Let the box stand on the remaining long 
side and fill it with earth. 

Place a row of broad-beans close to the glass. (See 
also the method used in Experiment 59.) 

Observations. The roots do not grow - vertically 
downwards but disappear from the side of the glass into 
the darkness of the soil. 

Inference. .4 root grows away from the light. 

The direction of growth taken by the root is therefore 
said to be negatively hdiotropic. 



THE GROWTH OF THE PLANT 93 



EXPERIMENT 59 

Aim. To find out which rays of light have the 
greatest influence in determining the direction taken 
by the different parts of the plant. 

Method. The three boxes prepared for Experiments 
31 and 57 can be used again in this investigation. The 
two, which are respectively fitted with sheets of red 




FIG. 45 

and blue glass, are ready for use. In the third box the 
cover must be replaced by a sheet of plain glass. 

Three tumblers are almost filled with water. A piece 
of coarse net is put over the top of each and kept in 
place by a rubber band. Damp sawdust or fibre is 
placed over the net, and on this mustard seeds are 
sprinkled. 

One of these tumblers is then put into each box and 
the bftxes are placed in a good light. 

Observations. In every case the shoots of the 
seedlings grow towards the light and the roots away 



94 EXPERIMENTAL PLANT PHYSIOLOGY 

from the light, the angle of inclination being greater 
for the shoot than for the root. The seedlings growing 
in whitq light are most affected (Fig. 45, A). For those 
growing in blue light the directive influence is rather 
less (Fig. 45, B). The seedlings upon which the red 
light falls are only slightly inclined from the vertical 
(Fig. 45, c). 

Inferences. The blue rays have the greatest influence 
in determining the direction taken by the different 
parts of the plants ; very little influence is exerted by 
the red rays. 

EXPERIMENT 60 

Aim. To find out whether moisture affects the 
direction of growth of the root. 

Method. Two small sieves or gravy strainers are 
suitable for this experiment. These must be filled with 




FIG. 46 

damp sawdust and planted with mustard seeds. The 
meshes must be large enough for the roots to pass 
through. The sieves are placed over two tumblers or 
beakers, one of which contains water, while the other is 
left empty. The sawdust must be kept damp. 

Observations. The roots of the growing seedlings 
come through the holes of the sieves and begin to grow 
downwards. 

In the case of the seedlings growing over water the 
downward growth of the roots is maintained. Iji the 
other case, however, the root-tips soon turn upwards 
and creep along the damp surface of the sieve (Fig. 46). 



THE GROWTH OF THE PLANT 95 

Inference. Roots grow towards moisture. The 
attraction that water possesses for the root is sufficient 
to overcome its natural tendency to downward growth. 

The response produced by the presence of moisture 
on the direction of growth of the root is termed Hydro- 
tropism. Roots are said to be positively kydrotropic. 

The glass-fronted box made for Experiment 58 can 
be used to show very prettily the direction of growth 
taken by each part of a seedling. 

EXPERIMENT 61 

Aim. To show clearly the direction of growth taken 
by all the parts of a seedling bean. 

Method. Three or four broad-bean seeds are planted 
in the glass-fronted box used in Experiment 58. The 
beans are arranged so that the radicle in each case is 
close to the glass and points downwards. 

Three directive forces, as has already been seen, act 
on the root of the developing seedling. One of these 
forces pulls the root ver- 
tically downwards ; the 
other two draw it away 
from the glass towards 
darkness and the mois- 
ture of the soil. As a 

T. ,1 . ' "".- ,, \ nesuiiani 

result the root grows vertically * direction 

obliquely in a direction downwards O f roof 

which is the resultant 

of the three forces that act upon it. This is shown 
graphically in the figure. 

It is thus seen that the roots can be made to grow 
close to the glass by tilting the box forward so that 
the plane in which the glass lies is that of the resultant 
direction taken by the root. 

It is advisable to cover the glass with a piece of 
black paper. Thus, by reducing^ the amount of light 



Pull 




Pull into box 



Resultant 



96 EXPERIMENTAL PLANT PHYSIOLOGY 

that would otherwise reach the roots, the box need not 
be tilted at so great an angle. 

Observations. The primary root grows straight 
down to the base of the box. The secondary roots 




FIG. 47 



grow out almost at right angles to the primary root, 
while the shoot grows vertically upwards (Fig. 47). 

By the foregoing experiments it has been proved 
that the main root of a plant always turns downwards 
when allowed to grow naturally in damp soil. The 
cause of this downward growth must now be investi- 
gated. 

When a body is dropped it falls at once to the 
ground. This is a direct result of a force by r . which 
everything is attracted to the centre of the earth. 
This force is termed the Force of Gravity, and the 



THE GROWTH OF THE PLANT 97 

earth's centre, to which everything is attracted, is 
termed the Earth's Centre of Gravity. 

Experiments must now be carried out to determine 
whether this force of gravity is responsible also for the 
downward growth of roots. To this end the effect 
produced on the direction of growth of the root, when 
the force of gravity is rendered inoperative, must be 
studied. 

A careful examination of Fig. 48 will show clearly 
how the force of gravity may be rendered of no effect. 
The same bean-seedling is shown in eight positions 
which are arranged symmetrically round a central 
point. The positions 
lorm a series of pairs 
in which the action of 
gravity on the root-tip 
is equal and opposite for 
the two members of any 
pair; for instance, the 
effect of the action of 
gravity on the seedling 
when in position (6) 
would be neutralised by 
the effect produced when 
in position (/), and simi- 
larly for the other pairs. 
Generally, the root in 
all the positions shown 

on the right-hand side of the figure, when acted on by 
gravity, must be pulled in such a way that the outer 
side of the root-tip (that drawn with the thickened line) 
is in every case pulled down. On the other hand, the 
root, when on the left side, is affected in such a way 
that the inner side of the tip (that drawn with a thin 
line) is pulled down. If then the seedling can be 
arranged so that it takes up each position for the same 
length of time, the net result, due to any attraction 
that gravity may have for it, will be nil. 

Th< simplest way to achieve this is to make the seed- 
ling rovolve slowly and evenly by means of some clock- 

G 




98 EXPERIMENTAL PLANT PHYSIOLOGY 

work mechanism. An apparatus of this kind is called 
a Klinostat. 

To make a simple Klinostat. A cheap clock can 
easily be converted into a klinostat. 

The minute-hand must be removed, as it is not re- 
quired, and will be found to be in the way if it is not 
cut off. A thin rod, about four inches long, is then 
attached to the axis of the hour-hand so that it pro- 
jects horizontally from the axis. 

As the hour-hand revolves the attached horizontal 
rod must necessarily revolve with it, and to this rod 
can be attached the seedling that is to be experimented 
with. 

EXPERIMENT 62 

Aim. To find out whether the downward growth of 
the root is caused by the attraction of gravity. 

Method. A pad of damp moss is wrapped round the 
end of the klinostat rod and kept in position by a 
rubber band. Care must be taken that the moss is 




FIG. 49 

made secure, otherwise it will slip and not revolve with 
the revolution of the rod. 

Some germinating peas are fastened to the pad by 
means of pins. 

The clock is wound up and set under a bell-jar. 

Observations. The root does not grow dowrwards, 
but continues to grow in whatever direction it was 
placed on the damp pad. 



THE GROWTH OF THE PLANT 



99 



The shoot also continues to grow in the direction in 
which it is placed and does not turn upwards (Fig. 49). 

Inferences. The direction of growth of both the root 
and the shoot is influenced by gravity. In the case of 
the root the influence is a positive one, the root being 
attracted towards the centre of the earth ; the influence 
is negative in the case of the shoot. 

Geotropism is the term used to denote the way in 
which the plant reacts in response to the stimulus 
exerted by gravity. The main root is said to be posi- 
tively geotropic, since it grows towards the earth's 
centre. Shoots, on the other hand, are negatively 
geotropic. 

EXPERIMENT 63 

Aim. To show the curves taken by the root and 
shoot in response to gravitational stimulus. 

Method. A broad-bean is suspended over a gas-jar 
of water and left to ger- 
minate. When the main 
root has reached almost to 
the bottom of the jar and 
the lateral roots are well 
developed the rnouth of 
the jar is closed with a 
cork. This can be done 
by boring a hole in the 
cork large enough to fit 
the top of the root and 
then cutting the cork into 
two pieces. The cork is 
made secure with paraffin 
or plasticine. The appara- 
tus is then inverted (Fig. 
50). FIG. 60 

Observations. The 

shoot curves and continues its growth vertically 
upwards. 




100 EXPERIMENTAL PLANT PHYSIOLOGY 

The primary root bends over and grows vertically 
downwards. 

The lateral roots turn obliquely downwards. 



EXPERIMENT 64 

Aim. To find out what part of the root is sensitive 
to gravity. 

Method. Two broad-bean seedlings with radicles of 
about one to two inches long are fixed to a strip of 





FIG. 52 



sheet cork and suspended in a damp atmosphere with 
their radicles in a horizontal position. 

The strip of cork may be suspended by means of 
cotton in an inverted bell-jar standing in a dish of 
water. A greased glass-plate can be put over the base 
of the jar so that the air within may be kept damp 
(Fig. 51). 

The seedlings are drawn carefully before being 
attached to the cork (Fig. 51, a and 6). 

They are again drawn after twenty-four hours (a' 
and V). 



THE GROWTH OF -THE PLANT 101 

Observations. The tips of both seedlings have turned 
downwards. 

By measuring oft on the seedlings of and b' the 
length of the radicles of the seedlings a and 6, it is seen 
that the downward curve has begun in both cases just 
behind the point which marks the position which the 
root-tips occupied when placed on the cork. 

Inferences. The root-tip and region immediately 
behind it is sensitive to gravity. The remainder of the 
root is unaffected by gravitational stimulus. 



EXPERIMENT 65 

Aim. To find out what provision the plant makes 
against injury of the primary root. 

Method. A broad-bean seedling with a radicle of 
about an inch long is selected. Ihe growing part of 
the radicle is cut off. The seedling is then suspended 
over water in a gas-jar with part of the radicle clipping 
down into the water. The gas-jar is covered witn 
black paper. 

Observations. The seedling is not killed by the re- 
moval of the growing part of the radicle. Very soon 
secondary roots are given off from the radicle. Gener- 
ally, but not invariably, one of the secondary roots 
grows vertically downwards and in this way functions 
as the primary root (Fig. 52). 

Inference. One of the secondary roots which would 
normally grow horizontally usually takes a vertical 
position if the primary root becomes injured. 



EXPERIMENT 66 

Aim. To find out what provision is made, in some 
plants at any rate, against injury to the primary shoot. 

Method. Two broad-beans are planted in a pot. 
From one of the seedlings the shoot is cut off as soon 
as it comes through the soil. 



102 EXPERIMENTAL PLANT PHYSIOLOGY 

Observations. In a few days the damaged seedling 
produces two new shoots which emerge from the ground 
by the side of the cut stem (Fig. 53). On taking this 
seedling out of the earth and examining it, the two 




FIG. 53 

shoots are seen to be borne in the axils of the coty- 
ledons. 

Inference. In the broad-bean, buds are borne in the 
axils of the two cotyledons. These buds usually re- 
main dormant, but they have power to develop should 
the primary shoot be injured. 



THE GROWTH OF THE PLANT 103 



EXPERIMENT 67 

Aim. To find out in what part of the root growth 
takes place. 

Method. A broad-bean seed germinates in damp 
sawdust or fibre. When the root measures about one 
and a half inches the seedling is removed from the 
sawdust, carefully washed and dried with blotting- 
paper. The root is then marked by horizontal lines 




FIG. 54 

into one millimetre divisions (1 nun. = ^ r inch approxi- 
mately). The marks are made with Indian ink by 
means of a small camel's-hair paint-brush. The ink 
will not run if the root is properly dried. It will be 
sufficient if a length of fifteen millimetres from the tip 
is measured off. 

Tt^e seedling is then pinned to a strip of sheet cork 
which is fitted into the cork of a gas-jar as described in 
Experiment 56. 



104 EXPERIMENTAL PLANT PHYSIOLOGY 

At intervals of twenty-four hours the seedling is 
carefully sketched. 

As the Indian ink marks tend to get blurred it is 
well to re-mark them frequently. 

Observations. In Fig. 54 are given drawings of a 
bean-seedling made at intervals of twenty-four hours. 

On the first day fifteen equal lengths of one milli- 
metre were measured off from the tip. 

After twenty-four hours it was seen that no growth 
in length had taken place in the division nearest the 
tip nor in the five divisions furthest from it, whereas 
the greatest growth had taken place in the third and 
fourth divisions from the tip. 

The next reading showed that the second and third 
divisions only were still growing in length, the fourth 
division and those above it had ceased elongating. 

In the final reading taken growth was confined to 
the second division. 

It was also observed that the root continued to grow 
in thickness after it had ceased to grow in length. 

Inferences. A root does not grow in length at the 
tip but in the region immediately behind the tip. 

The elongating region is very short, extending only, 
in the case of the bean, for a distance of about ten 
millimetres. 

The region of greatest elongation is about three 
millimetres from the tip. 

Each part of the root in turn very soon reaches its 
maximum length. 

Any part of the root continues to grow in thickness 
after ceasing to grow in length. 



EXPERIMENT 68 

Aim. To. find the connection between the growing 
part of the root and the region that is sensitive to 
gravity. 

Method. A broad-bean seedling having a radicle 
whose length is about one and a half to two inches 



THE GROWTH OF THE PLANT 



105 



long is selected. The tip of the root for a distance of 
fifteen millimetres is then divided off into millimetre 
lengths by means of Indian ink as described in Ex- 
periment 67. 

A strip of sheet cork is cut having a length equal to 
the diameter of a crystallizing dish or other glass 
vessel. The seedling is pinned to the strip of cork. 
Water is put into the glass vessel and the strip of cork 
then fixed above the water so that the root of the seed- 
ling lies horizontally (Fig. 55). 

Observations. Within twenty-four hours the tip of 
the root is found to have turned, vertically downwards. 




FIG. 55 

The part at which the curve has taken place is the 
region of the greatest growth. That part of the root 
which has ceased growing remains horizontal. 

Inferences. Gravitational stimulus is operative only 
on the growing portion of the root ; and when placed 
in a horizontal position the radicle curves downwards, 
the point of curvature being the region of greatest 
growth. 

EXPERIMENT 69 

Aim. To find out in what part of the shoot growth 
takes place. 
Method. Any rapidly growing plant can be used for 




(FIG. 56 a) 



; 



106 EXPERIMENTAL PLANT PHYSIOLOGY 

this experiment. A sunflower or 

broad - bean seedling answers well. 

The upper part of the stem, for a 

distance of about ten centimetres from 

the apex, is marked off by transverse 

lines into lengths of five millimetres 

each. The marks can be made with 

Indian ink by means of a small 

camel's-hair brush. 

Fig. 56 (a) is a drawing on a re- 
duced scale of a broad- 
bean seedling, the actual 
length of whose stem 
at the beginning of the 
experiment was about 
twenty-five centimetres. 
A length of ten centi- 
metres from the tip was 
marked off into five 
millimetre divisions. 
Fig. 56 (6) shows the 
lengths of the measured 
portion of the stem on 
the first, second, sixth, 
and tenth days, the scale 
to which the figure is 
drawn being three-quar- 
ters of the actual lengths 
measured. 
Observations. In the case of the 

broad-bean seedling observed it was 

found : 

1. That growth in length took place 
through a distance of about 4*5 centi- 
metres from the apex. 

2. That all the growing part does 
not elongate at the same rate; the 
region of maximum growth changes 
its position from day to day, keeping 
at a distance of about 1*5 centimetres 
from the apex. 



. 56 (ft) 



THE GROWTH OF THE PLANT 107 

Inference. The stem of a plant continues to grow 
i length throughout a much longer distance than is 
bserved in the case of the root (Experiment 67). 

The increase in length is most rapid at a point at 
Dme little distance behind the apex. 



INDEX 



BSORPTION of carbon - dioxide, 
55 

- of oxygen, 72 

- of water, 3 

Sid, hydrochloric, 11, 58 

- presence in root, 11 

r, absorption of food from, 43 

- a factor in germination, 87 

- composition of, 33, 42 

- presence in plant, 43, 44, 45 
nmonia, 50 

aimals, feeding process com- 
pared with plants, 67 



kEK, 20 
ust, 20 

eathing process, 70 
significance of, 78 



1LCIUM chloride, 24 
t,ne sugar, 58, 59 
irbon assimilation, 56, 80 
irbon-dioxide, 39 

- absorption of, 55 

- evolution of, 71 

preparation and properties of, 

41 

irmine, 10 
lustic potash, 54 
jntre of gravity, 97 
pbalt chloride, 27 
ppper sulphate, 50, 58 
Drtex, 20 
lilture solutions, 12 



1ASTASE, 62 

Erection of growth, 89, '95 



Direction of growth, action of 

light on, 91, 92, 93 
action of moisture, 94 



ELEMENTS, 2 

Energy, kinetic, 79, 84 

potential, 79, 84 
Eosin, 10 

FEEDING process, animals and 

plants compared, 67 
Food, a factor in germination, 

83-86 

storage in seeds, 86 

GASKS, tests for, 32 
Geotropism, 99 
Germination, 81 

factors necessary, 82-89 

necessity for water, 82 

necessity for food, 83, 84, 85 

necessity for air, 87 

necessity for light, 87 

necessity for life, 88 

necessity for heat, 89 

GRAPE sugar, 58, 59 
Gravity, force of, 96 

centre of, 97 

elimination of, 97 

action on growth, 98, 99 
Ground-tissue, 20 
Growth, 81 

direction of, 89 

region of in root, 103 

region of in shoot, 105 
Guard-cells, 29 



109 



110 EXPERIMENTAL PLANT PHYSIOLOGY 

HEALTH value of plants, 68, 78 
Heat, a factor in germination, 89 
Hcliotropism, 92 
Hydrochloric acid, 11, 58 
Hydrolysis, 61 
Hydrotropism, 95 



IMBIBITION, 83 

Injury, provision against, 101 

Intramolecular respiration, 75 

Iodine, 46 

Iron filings, 36 



KINETIC energy, 79, 84 
Klinostat, 98 



LEAF fall, 31 

Leaves, influence on ascent of 
water, 21 

starch formation in, 48-56 
Life, a factor in germination, 88 
Light, action on direction of 

growth, 91, 92, 93 

a factor in germination, 87 
Lime- water, 54 

action of air on, 39 

action of breathing on, 40 

action of combustion on, 40 
Litmus, 11 



MANGANESE dioxide, 37 

Marble, 10 

Medullary ray, 20 

Micropyle, 82, 83 

Moisture, action on direction of 

growth, 94 
a factor in germination, 82 



NEGATIVE heliotropism, 92 
Negative hydrotropism, 95 
Nitrogen, 37 
properties of, 35 



OSMOSIS, 15, 16 
Oxygen, 37 

absorption of, 72 

evolution of, 57 



Oxygen, necessity of, to plant life, 
76 

preparation and properties of, 

38 

Phosphorus, 34 
Pig's bladder, 14 
Pith, 20 
Plumule, 82 

Positive heliotropism, 92 
Positive hydrotropism, 95 
Potash, 54, 58 
Potassium bichromate, 51 

chlorate, 37 
Potato, 15 

Potential energy, 79, 84 
Pyrogallic acid, 77 



RADICLE, 82 
Reducing sugar, 59 
Respiration, 70 

absorption of oxygen, 72 
- action on temperature, 77 

amount of oxygen absorbed, 

73 

amount of carbon-dioxide 

evolved, 74, 76 

evolution of carbon-dioxide, 

71 

intramolecular, 75 

significance of, 78 
Root, absorption by, 3 

absorption of soluble sub- 

stances, 10 

growing part, 103 

hairs, 17 

pressure, 22 

rate of absorption by, 4 

sensitiveness to gravity, 100 



SALTS, soluble, 2 

Shoot, growing part, 105 

Soda-lime, 54 

Soil, absorption of food from, 2 

composition of, 1 

Starch, action of mineral acid on, 
61 

composition of, 53 

conversion to sugar, 57, 60 

formation, action of a'lr, 53 

action of carbon-dioxide, 

55 



INDEX 



111 



Jtarch formation, influence of 
light, 48, 49, 50 

influence of temperature, 

. 51 

'influence of colour, 52 

presence in plant, 46 

presence in cotyledons, 86 

tests for, 46, 47 

Item, passage of manufactured 
food through, 62 

passage of water through, 18 

structure of, 19 
Itomata, 29 
lugar, cane, 58, 59 

formation from starch, 60 

grape, 58, 59 

inversion of, 60 

reducing, 59 

tests for, 58 



TEMPERATURE, action on absorp- 
tion, 4 

action on starch formation, 

51 

increase due to respiration, 

77 

Testa, 82 
Transpiration, 23 



VASELINE, 28 



WATER, absorption of, 3, 4 

ascent of, 17 

a factor in germination, 82 

hardness of, 2 

Wood, passage of water through, 
18 



THE END 



Printed by BALLANTYNE, HANSON &> Co. 
Edinburgh <5r London