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PLANTS IN HEALTH AND DISEASE.

Published by the University of Manchester at
THE UNIVERSITY PRESS
(H. M. McKecunik, Secretary),

12, Lime Grove, Oxford Road, Manchester.

LONGMANS, GREEN & CO.

London: 39, Paternoster Row.
New York: 443—449, Fourth
Avenue and Thirtieth Street.

Bombay: 8, Elornby Road.
Calcutta : 303, Bowbazar Street.
Madras: 167, Mount Road.

PLANTS in HEALTH and DISEASE

being

An Abstract of a Course of Lectures
delivered in the University of Manchester during the session
1915-16

BY

F. E. WEISS, D.Sc.,
Harrison Professor of Botany,

A.-D.- IMMS, M.A., D.Sc.,
Reader in Agricultural Entomology,

AND

WILFRID ROBINSON, M.Sc.,
Lecturer in Economic Botany.

Manchester
AT THE UNIVERSITY PRESS
12, Lime Grove, Oxford Road.

LONGMANS, GREEN & CO.

London, New York, Bombay,
os etc.

1916.

PREFACE.

The course of lectures of which this little book is a summary
was undertaken with'a view to giving some assistance to those
who were endeavouring, to the best of their ability, during the
present crisis to increase the productiveness of their gardens or
allotments. Was it possible for the University to aid in any
way these practical men and women to accomplish their laudable
and patriotic endeavour? A knowledge of the structure and
life of the plants they cultivate could not fail to be helpful to
gardeners and allotment-holders in explaining the reasons for
many of the common horticultural practices. Familiarity also
with the common animal and fungal parasites of our garden
crops, and the methods of combating these pests would enable
them to save many doomed plants. For these reasons this course
of lectures on “ Plants in Health and Disease ” was instituted,
and, as the size of the audience indicated that the lectures met
a real need, it seemed desirable to issue to members of the
audience a short eight-page summary of each of the lectures.
As*we have received many enquiries from persons not attend-
ing the lectures both for single abstracts, and since the con-
clusion of the course, for complete sets, we have decided to
reprint them in book form. We trust, however, that the fact
will not be overlooked that this issue does not pretend to be
more than a somewhat brief summary of a course of lectures,
and that all the lecturers were tied down to very narrow limits
wherein to compress the subject matter of a much longer dis-
course. We would also point out that, as the lectures were
addressed to a Manchester audience, the lecturers often dealt
with the peculiar difficulties that are met with in this neighbour-
hood, and that the accounts given of the animal and fungal pests
do not profess to be exhaustive, but are descriptive of the more
common diseases occurring in the gardens and allotments in the
vicinity of our large industrial towns. The necessary condensa-
tion of many interesting points, which might with advantage
have been expanded, and the omission of the illustrations which
accompanied the lectures, will probably be less noticeable to
those who have attended the course of lectures than to new
teaders of this little volume; nevertheless, we hope that these
latter will also find in it some information, which may be of
value to them. Should this hope be realised we shall feel well

satisfied.
Tue UNIVERSITY, TF. E. WEIss.

MANCHESTER, A. D. IMs.
1sth March, 1916. W. ROBINSON.

TABLE OF CONTENTS.

PAGE

CHAPTER 1. F. E. Weiss
GENERAL FEATURES OF PLANT LIFE.

_ Nutrition and propagation, vegetative and seed reproduc-
tion. Annuals, biennials, and perennials. General account
of vegetative organs.

CHAPTER 2. fF. E. Weiss
Roots AND ROOT NUTRITION.

Absorption by roots and root pressure. Physical and
chemical nature of soil. Manures and their importance.
Bacteria in soil. Root tubercles of Leguminous Plants.
Rotation of crops. Trenching and ridging.

CHAPTER 3. F. E. Weiss .

STEMS AND LEAVES.

Mechanical requirements of stems and branches. Twin-
ing and climbing plants. The food conducting function of
stems. The work of leaves and their structure. The wilting
and recovery of leaves. Protection of leaves against exces-
sive drought. Direct effect of surroundings on leaf develop-
ment. Hardening off plants.

CHAPTER 4. F.E. Weiss
METHODS GF VEGETATIVE REPRODUCTION AND PROPAGATION.

_ Tubers. Bulbs and corms. Bulbils. Runners. Layer-
ings and cuttings. Budding and grafting.

CHAPTER 5. F. E. Weiss
FLOWERS AND THEIR FORMATION.

Conditions favouring the production of flowers. Structure
and functions of the various parts of a flower. Pollination
and Fertilisation. Self-fertile and self-sterile flowers.
Ripening of fruits and seeds.

CHAPTER 6. F.E. Weiss

SEEDS AND SEEDLINGS. ,
Vitality and longevity of seeds. Conditions favouring
germination. Seedlings. Variation. Natural and artificial

selection. Sports or mutations. Hybrids and the laws of
heredity.

I

9

33

41

vil

PAGE

CHAPTER 7. F. E. Weiss
MALFORMATIONS AND INJURIES.
Malformations arising as sports or by malnutrition.

Healing of wounds. Injuries due to lightning, frost, etc.
Harmful effect of smoke and fog.

CHaPrer §&. Wilfrid Robinson
FUNGI AS A CAUSE OF DISEASE IN PLANTS.

Common fungi, mushrooms, toadstools and moulds. How
they live. Saprophytes and Parasites. The “Damping-off”
disease of Seedlings caused by the fungus Pythium. Life
Story, means of spreading and living in the soil, pre-
cautionary measures.

CHAPTER 0. Wilfrid Robinson

DISEASES OF ROOTS AND TUBERS.
Fingers-and-Toes of Turnips, Cabbages, etc. Habits and
life history of the fungus. Remedial treatment. “ Liming ”

of the soil, etc. Similar’ diseases, ¢.g., Black Scab or Wart
Disease of Potatoes, “ Spongy Scab,” etc.

CHAPTER 10. Wilfrid Robinson

A DISEASE OF LEAVES, SHOOTS AND TUBERS.
The Late-Blight disease of potatoes. Symptoms and
means of spreading. Diseased tubers and the wintering of

the fungus. Treatment of the disease, spraying and the use
of resistant varieties.

CHAPTER 11. Wilfrid Robinson
DISEASES OF LEAVES AND SHOOTS (contd.).
Black-Leg or Wilt Disease of Asters. Tomato-leaf rust

or mould. lLeaf-blotch of Cucumber. Mildews' of Roses,
Gooseberries, etc.

CHAPTER 12. Wilfrid.Robinson
DIseEaSES OF LEAVES (condéd.).

Rusts. The Wheat Rust. Life Story of the fungus,
relation to the Barberry. _The Mint Rust and Hollyhock
Rust as examples of other Rust diseases.

CHAETER 13. A. D. Imms
INJURIOUS ANIMALS OTHER THAN INSECTS.

Injurious and beneficial Animals. Birds and the need
for scientific investigation with reference to their food at
different seasons of the year. Eelworms and more especially
the Stem Eelworm and the Knot Root Eelworm. Pul-
monate Molluscs (Snails and Slugs). The Black Currant
Gall Mite.

49

57

81

89

97

vill
PAGE
CHAPTER 14. A. D. Imms 105
InysuRiIous INSECTS.

General observations on Insects and their feeding habits.
Insecticides and their use. Earwigs. Destructive Cater-
pillars of Cabbage Butterflies, and the Currant, Winter, and
Codling Moths. The Gooseberry Saw-fly. The Pea Thrips.
Wireworms.

CHAPTER 15. A. D. Imms 113
InjuRious Insects (Contd.).

Crane Fhes; the Pear Midge; the Celery Fly: the Cab-
bage Root Fly; the Onion Fly; the Narcissus Fly; the
House Fly.

CHAPTER 16. A. D. Imms 121
INJURIOUS ANIMALS {(Conid.).

Aphides and their life-histories; preventive and remedial
measures. The Apple Sucker. Scale Insects. Greenhouse
Pests.and Fumigation Methods.

CHAPTER 17. A. D. Imms 129
BENEFICIAL ANIMALS, ETc.

Birds. The Testacella Slug. Earthworms. Centipedes.
Beneficial Insects. The Isle of \WWight Bee Disease. Litera-
ture bearing upon Economic Zoology.

INDEX ... ais as sis ete ave as aay we 187,

Plants in Health and Disease.

Chapter 1.

GENERAL FEATURES OF PLANT LIFE.

Nutrition and propagation, vegetative and seed reproduction.
Annuals, biennials, and perennials. General account of vegeta-
tive organs.

Gardeners being concerned in the cultivation of plants,
it is obviously important that they should be acquainted
with their structure and mode of life. They will find it
particularly useful to know in what ways plants respond
to their environment, for such knowledge will enable them
by varying their treatment to modify the development of
the plants they cultivate, to accelerate or retard their
growth and to ensure the production of a greater number of
flowers. The gardener’s aim is not always the same. The
flower lover is anxious to obtain a wealth of bloom. The
allotment holder on the other hand, concerned in raising
vegetables, may desire well-developed leaves, roots or
tubers. It is important therefore that we should study
both the vegetative and the reproductive organs.

The former are concerned mainly with nutrition, the
latter with the propagation of the plant. These two sets
of organs are often differently affected by external con-
ditions. A gardener knows that by over-stimulating the

I

2

development of foliage he may be endangering the pro-
duction of flowers. It is a common practice to reduce the
apply of water so as to encourage the formation of flower
buds. Some of our common British fruit trees when grown
in tropical climes will develop into luxuriant trees with a
wealth of foliage, but lack the flowers essential for the
production of fruit. The practice of pruning and root
pruning is based on the same phenomenon, and by reducing
the amount of vegetative organs the formation of flower
buds is encouraged. Yet though there seems to be this
opposition between vegetative and reproductive organs the
latter are really dependent upon the former. For it is
only at the expense of the food material absorbed and
worked up by the roots and leaves that the flowers are
produced. The activity of the vegetative organs must
therefore always precede the formation of flowers.

This is seen very clearly in the case of annuals. These
plants complete the whole of their life-cycle in one vegeta-
tive season which is usually much shorter than a year, in
some arid regions amounting only to a few weeks during
and immediately after the wet season. An annual com-
mences as a seedling at the beginning of the favourable
vegetative season, and after the production of a limited
number of leaves produces its flowers, which are kept sup-

lied with food material by the activities of roots and

eee until the seeds have matured, then the whole plant
dies down to be replaced next season by the offspring
developed from its seeds. Such is the life history of
the mustard and cress and also that of most of our com-
mon weeds like chickweed, groundsel, and some of our
grasses. The ubiquity of our weeds is due not only to
their effective means of dispersal, but also to their rapid
growth to maturity which enables them to produce two
and even three generations of plants in one season.

Slower in their development and exhibiting a more
marked contrast of vegetative and reproductive periods
are the so-called diennials. In these plants after the vege-
tative organs are produced they are employed throughout
the first summer season in manufacturing and storing a
large supply of food material, which is to be used in the
formation of flowers and seed during the second year,
after which the entire plant dies down having fulfilled
its existence and produced a vast number of offspring.
For the large store of food material which it has laid up

3

during the first year will enable it to produce a much
larger number of flowers than is commonly the case in
annual plants. In most biennials the food material is
stored in some underground part of the plant such as the
swollen ‘root’ of the turnip or beet, the leaves during the
first season forming a tuft or rosette close to the ground.
_ Lastly we have perennials which are much more varied
in character than the first two groups of plants. These
latter are always comparatively soft in texture, but
perennials include both herbaceous and woody forms
such as trees and shrubs. The herbaceous again are of
two types, firstly those which persist throughout the winter
like violets and primroses, and secondly those which die
down in the autumn leaving a persistent root or root-
stock underground, from which the plant renews its
growth in spring. Plants of this kind like the iris, peony,
larkspur, Michaelmas daisy and many other favourites
of our herbaceous borders have like biennials a large store
of food material in their underground organs. This
enables -them in most instances, not only to produce
annually a crop of flowers but to branch out underground
and develop into ever-spreading clumps, which in many
cases require repeated breaking up and thinning just as we
require to cut back our bushes and trees. In some cases
these underground portions of perennial plants do not
remain attached in one mass, but when ihe plant dies down,
a good deal of the underground part dies away too, leav-
ing isolated portions, so that in place of one individual
we find many fragments which would seem to be offspring
though they are really only remnants of the original
parent. Such offsets we have in the case of the tubers of
the potato, which represent the rounded swollen ends of
underground shoots that have become entirely separated
one from the other. Though this is really only a breaking
up of the original plant, it is often spoken of as vegetative
reproduction but must not be confused with seed reproduc-
tion, which is always the result of the fertilisation of
flowers. It is important to differentiate these two methods
of propagation, particularly as in the case of the potato
the tubers used for setting in spring are termed “seed
potatoes,” though they have really nothing to do with
seeds. Vegetative reproduction does not replace seed re-
production but is an additional means of propagation,
often of the greatest use both in nature and in cultivation.

4

The potato, for instance, though it produces flowers in
this country very rarely contrives to ripen its seeds in our
climate and can only be propagated in England by its
tubers, which are indeed the sole reason for its cultivation,
for these tubers richly stored with food material are of
the greatest importance as a staple food of mankind. The
Jerusalem artichoke has the same way of vegetative repro-
hie and a very similar process obtains in all bulbous
plants.

Most perennial plants produce a crop of flowers and

it every season, in some cases after a shorter or longer
period of immaturity as is usual for instance in shrubs
and trees. These flowers may make their appearance in
spring before the foliage. But in that case the flowers are
produced at the expense of the food material built up by
the leaves of the preceding summer, while the fruits are
generally matured by the activity of the leaves of the same
season.

Let us now consider some of the effects of external
factors upon the growth of plants. If we germinate a seed
under suitable conditions on the surface of the soil we
find that when the young root breaks through the seed-
coat it bends downward and penetrates into the earth.
That this is due to the effect exerted by gravity on the
young growing root can be demonstrated by slowly rotat-
ing a growing plant on a horizontal axis, when it will be
found’ that the root will grow out horizontally as gravity
acts first on one side of the root and then on the other, and
thus its effect is eliminated and the root is not affected.
The main stem of a plant is equally sensitive to the force
of gravity, but responds in a different manner growing
in the opposite direction to it and, if laid down horizon-
tally, bending upwards at right angles.

Detailed microscopic examination has shown that
plants have special regions of perception and it was found
by Darwin that as regards its sensitiveness to gravity the
seat of perception was the root-tip. and that if this was
cut off the root ceased to respond tg gravity. In the stem
it is known that the perceptive region is not so limited in
extent. But while the main root and the main stem tend
by their response to gravity to grow in a vertical direction,
lateral roots and lateral branches do not respond in a
similar manner but tend to place themselves more or less
horizontally or obliquely. A most curious feature of plant

5

life, and one very difficult of explanation, is the fact that
when the tap root or the main stem of a plant is destroyed,
a lateral branch will take its Page and assume the vertical
position. The importance of this position is self evident
when we consider the functions of the root and of the
stem.’ The former acts as the absorbing organ, extracting
from the soil the water and valuable salts necessary for
the growth of the plant; it is obviously essential therefore
that the root should grow downwards in search of moisture.
It is also important that the lateral roots should not grow
in the same direction as the main root, so that they can
search out other regions of the soil in their quest for food
material. The spreading habit has the further advantage
that it anchors the plant more firmly in the soil and pre-
vents it from being easily uprooted by the wind.

The stem on the other hand growing away from the
soil is in an advantageous position for exposing the leaves
it bears to the full rays of the sun and thus enabling them
to fulfil their main function in the life of the plant. This
function is to absorb as much light as possible and by con-
verting the light rays into energy to build up the organic
material upon which the formation of flowers and fruits
depends. ti ht being therefore of such vital importance
to plants we find that in the course of their evolution they
have acquired the power of responding in their growth to
the stimulus of light. It is a familiar fact that stems of
plants will bend towards the light if instead of being
illuminated from all sides they receive the light from one
side only. Growing in the open the main stem is erect
and the leaves are usually expanded horizontally, that is
at right angles to the direction of maximum illumination
which is from above. When shaded on one side the stem
inclines towards the light and the leaves are placed
obliquely, often indeed horizontally, when a plant is grown
in a window, in which case it is more or less shaded on
three sides. Roots as they are underground do not nor-
mally exhibit in their growth any response to light, but if
a young seedling plant is grown with its roots in a glass
jar it will be found that if illuminated from one side only
the roots will bend away from the light. In nature this
can be observed in the roots which grow out from the stem
of the ivy when clinging to a wall. They are: generally

roduced from the shaded side of the stem and always
ee away from the light and towards the wall. These.

6

climbing roots are not sensitive to gravity as they have
a very definite function to perform in fixing the plant to
the wall, and are not concerned in the absorption of food
material.

Wherever bending, such as described above, takes place
in growing organs, this is due to differences in the amount
of growth on different sides of the stem or root. The con-
cave side grows less than the convex side. Thus when a
stem, which has been laid horizontally, bends upwards,
this is due to the greater amount of growth of the side
nearer the ground. If a stem illuminated from one side
bends permanently in that direction this is due to the fact
that light retards the rate of growth and the side away
from the light growing more rapidly the stem becomes
convex on this side and bends towards the light. The
fact that light retards growth and therefore causes plants
to be short and “stocky” is of course a well-known
phenomenon, while the lack of illumination acting like
darkness causes more rapid growth and we get long
“leggy” plants, when they are insufficiently lighted as
when grown in the shade, in deep frames or pits, or not
close up to the lights in greenhouses.

There are one or two other factors influencing the
growth of plants which it may be useful to refer to at this
juncture. Besides being sensitive to gravity roots are
also sensitive to contact, and when a root tip comes in
touch with a solid body such as a stone in the soil it bends
away from it. This is brought about as in the case of
other bending movements, by the fact that contact causes
retardation of growth on this side of the root and this side
becomes therefore convex, the root-tip pointing away from
the obstacle met with in the soil. In this way it is possible
for the root to make its way even through a stony soil,
avoiding or rather growing round all obstacles with which
it may be met, bending to right or to left in its progress
downwards. This sensitiveness to contact is however not
only possessed by main roots which grow downwards; it
is equally important to lateral roots.

It has been found in the somewhat infrequent cases
occurring in nature that where the soil is drier on one side
of a plant than on the other the root system develops
more abundantly in the moister soil. Experimentally, too,
we can prove that a root will grow towards moisture just
as the stem of a plant will grow towards the light.

7

Lastly, we have plenty of evidence that in addition to
their need for water the roots of plants, just like the living
parts of all plants or animals require for their growth that
life-giving constituent of the air, which we also breathe
in, namely, oxygen. This need of the roots for air is one
of the fundamental facts which governs not only the dis-
tribution of plants in nature, but dominates our whole
agricultural and horticultural practice. It is this need for
air which causes the farmer to drain his fields and the
gardener to grow his plants in porous and well-drained
pots. If we turn a plant out of such a pot we shall see
by the dense felting of the roots in close contact with the
sides of the porous pot and among the crocks at the bottom
of the pot how eager the roots are for air. If on the other
hand we do not secure drainage or if we over-water
pot-plants we soon find that they show signs of ill-health.
As a matter of fact their roots are being suffocated and
may die away unless we alter our treatment. Probably
more plants are lost by over-watering than by insufficiency
of water. Normally, therefore, though water is a prime
necessity of plants its provision must not interfere with
the respiratory process of plants, and we must always
provide a porous soil for our crops, that is a soil with
sufficient air. This is one of the reasons for the practice
of hoeing. For apart from the clearing out of weeds,
hoeing prevents the ground from becoming caked, a con-
dition which would prevent the free access of air into the
soil. At certain stages in their growth this need seems
greater than at others. Speaking generally we may say
the more actively growth is procceding, the greater the
need for air. Germinating seeds for instance require a
large amount of air, and when the formation of new roots
is proceeding in the case of cuttings and layerings a porous
soil is essential. When a potato starts its new growth and
is rapidly developing its new shoots and roots we. find
that the skin, a hard and impervious layer, becomes inter-
rupted by numerous breathing pores which enable an active
respiratory process to take place. These pores can easily
be seen with the naked eye on the skin of a sprouting
potato. They are equally clearly seen on the twigs of
trees such as those of the horse-chestnut. On the leaves
the pores through which the plant takes in various con-
stituents from the air are not visible except with such
magnification as the microscope affords.

8

As the leaves of most plants are expanded in a drier
medium than that which surrounds the roots, these pores
naturally allow a large amount of moisture to escape par-
ticularly in dry weather. If we cover a plant with a bell-
jar we can see this moisture condensing on the sides of
the glass, and in carefully conducted experiments we can
actually measure the amount of water so lost. From such
calculations we estimate that a fair-sized tree standing in
the open may lose ten gallons of water in the course of
a summer’s day. This loss of water from the leaves might
at first be thought to be an accidental phenomenon due
to the possession by plants of delicate expanded leaf struc-
tures. As a matter of fact, however, this process of trans-
piration is of vital importance to the plant. In the
first place it can be shown by experiment that leaf trans-
piration exerts a considerable amount of suction, and it is
in part by this means that avater is raised in tall trees to
the upper branches. In shrubs and herbaceous plants this
suction alone would be sufficient to raise the water into the
leaves and flowers, though the roots too are equally con-
cerned in the ascent of sap. Nor must we consider the
water which is given off by the leaves as so much waste. It
is indeed important that some of the water absorbed by the
roots should be driven off. For the water contained in the
soil contains the mineral salts which are necessary for the
plant, in very dilute solution, and these salts require there-
fore to become concentrated in the plant. This can only
be done by driving off some of the superfluous water and
that is effected by the process of transpiration. The leaves
are therefore acting as a condensing apparatus and thus
performing a very needful function. .

I trust that what I have said regarding the sensitive-
ness of the Various parts of the plant to external forces
and surrounding conditions will have impressed the reader
with the power of response inherent in plants, and this
will indicate how by artificially selecting our medium and
method of treatment we can considerably modify the
course of development of the plants we cultivate.

Chapter 2.

ROOTS AND ROOT NUTRITION.

Absorption by roots and root pressure. Physical and chemical
nature of soil. Manures and their importance. Bacteria in
soil. Root tubercles of Leguminous Plants. Rotation of crops.
Trenching and ridging.

We have seen that the sap is raised in plants in part
by the transpiration of water vapour which takes place
through minute pores which are found scattered over the
surface, particularly the lower surface, of leaves. In
addition to this suction exerted by the leaves there exists
a definite upward pressure of the sap by the roots known
as root pressure. ‘Vhis upward force is due to the fact that
the roots of plants are covered in by a porous membrane,
and that the cells of the root contain a cell sap of greater
concentration than the water contained in the soil. When-
ever such a condition of things occurs a physical law
determines that water passes through such a permeable
membrane from the less dense to the. denser liquid.
Water is thus attracted into the cells of the root and,
causing them to swell, it forces the liquid up certain con-
ducting channels of the root and into the stem.

If, during a period of active root absorbtion, we cut
a plant above the surface of the soil we find that water
will be forced out of the exposed wound and, fixing a
glass tube tightly in the place of the stem which has been
removed, we find that the liquid which exudes can be
forced up to a considerable height.

' Particularly in spring when root absorbtion is very
active the pressure exerted by the roots may be very con-

9

IO

siderable and causes the “bleeding” of plants, as_ the
active exudation of sap is called. This occurs sometimes.
when trees and shrubs are pruned too late in the spring
after the sap has begun to rise. The active absorption of
water takes place in the young roots a little way behind
the root-tip, where the root can be seen to be covered bv
a mass of delicate hairs which, possessing very thin walls,
offer great facility for the penetration of the water. It is.
important, therefore, in transplanting young plants from
a seed bed or pan to injure as little as possible the delicate
young roots, as by pulling instead of carefully digging
up young plants the absorbtive part of the root anchored:
by its many hairs may be left behind in the seed bed.
Plants injured in this way will inevitably suffer a set-
back, as they will not be able to absorb nutriment
efficiently until they have developed new roots to take
the place of those which have been injured.

Let us now consider the nature of the soil in which
the roots are growing. We have already seen that to be
suitable for the growth of ordinary plants it must be well
drained, whether we are dealing with a plot of ground
or with pot plants. If the earth becomes water-logged
the roots are deprived of the necessary air and soon die
away. The physical condition of the soil is therefore as.
important as ifs chemical composition. In nature, soils
are not all equally porous. Some therefore require the
careful attention of the cultivator. Clay soils particularly
are liable to retain too much water and need to be
specially treated for successful cultivation. The retentive.
power of a soil depends largely on the size of the particles
of which it is built up. Gravel or coarse sand allows the
water to pass through it more rapidly than fine sand, and
even the finest sand is eomipeeen of larger particles than
those of clay. We can easily prove this by stirring up:
sand and clay in water. The particles of sand soon settle
down and allow the water to clear while the water in
which clay has been stirred up remains muddy for a long
time. If, however, we add a little lime to this cloudy
liquid we find that it clears rapidly. This is due to the
“coagulation” of the very fine particles of clay which
become collected into larger groups, and these being
heavier than the single particles of clay, fall to the bottom
of the water and allow it therefore to become clear. It is
upon this action of lime upon clay that the practice of

Il

“liming” heavy soils depends, as the admixture of lime
causing the ultimate particles of the clay to become col-
lected together, the ground becomes more coarsely grained
and therefore more porous. It is then easier to work, being
lighter and more open, and is also warmer. This physical
change which is effected by lime is not the only benefit
derived from its use. Near our large towns the soil is
usually rendered acid by the fumes produced by the com-
bustion of coal. In foggy weather this acidity of the
atmosphere becomes very noticeable. It has been shown
by experiment that such acid soil, and even ordinary soil,
when watered with acid rain-water collected in towns, is
very detrimental to plant growth. This acidity can be
removed by adding lime to the soil, a practice therefore of
the utmost importance to those who cultivate gardens or
plots near our industrial towns.

Heavy soils can also be lightened by the admixture
of sand to the soil. Good natural loams, which are easily
worked soils, consist of fine sand with some clay and a
little lime.

Lastly, farmyard manure, quite apart from its im-
portant chemical effect upon the soil in enriching it with
many valuable food substances, is of great value in
improving the texture of all soils. To sands it gives great
water-retaining power, while it renders clay soils more
porous and friable.

Let us now examine some of the chemical needs of
the roots of plants. By a chemical analysis of the ashes
left after burning plants it has been ascertained that they
are mainly built up of four chemical elements, namely,
hydrogen, oxygen, carbon, and nitrogen, to which must be
added smaller quantities of lime (calcium), magnesium,
potash, sulphur, phosphorus, and iron. Of the four
essential substances, the first two, hydrogen and oxygen,
combined in the form of water are present in all soils in
that form. Carbon, though present in the soil largely in
the form of lime or calcium carbonate, is mainly obtained
by the plant through its leaves from the atmosphere,
where it occurs as carbonic acid. Nitrogen, on the other
hand, of which there is a large supply in the air, cannot
be made use of by the plant in this free form, but is
mainly absorbed in the combined form as nitrates by the
roots. In comparison to the other substances dissolved
in the water, the nitrates are the most important salts in

12

the soil as far as the nutrition of plants is concerned. We
can realise therefore the usefulness of nitrate of soda as
a fertilizer. In nature the nitrogen for plant food are
largely obtained from the humus, or decaying vegetable
matter, which is present in all soils. In cultivation,
where plants are removed when they die down or where
they are taken up to be used as vegetable, it becomes
necessary to replace the humus, which would naturaily
form, by leaf mould or other decaying matter such as
manure. The nitrogen contained in rotting manure or in
humus is, however, not maihly in the form of nitrates. It
is contained in highly complex organic compounds,
while in manure it occurs iy in urea, a compound of
ammonia. These organic compounds require to be acted
upon. by bacteria, which are found in the soil and in dung
before they are available as plant food. On the other
hand we find.in the soil micro-organisms of another kind
which have the power of combining the free nitrogen of
the air with the oxygen and ultimately build up the
nitrates so important to plants. These nitrifying bacteria
are constantly at work, and when a field lies fallow the
soil will be found at the end of the fallowing to be richer
in nitrogen compounds than it was at the beginning. In
addition to these organisms referred to above, there’ are
special forms which are always associated with the roots
of plants belonging to the pea family. If we pull upa
pea or bean we find that the roots bear curious swellings
called root tubercles, which are found when examined to
contain numerous minute bacteria. The roots must not
be considered diseased, though they mav look like it.
They are in a normal condition, and the bacteria inhabit-
ing these nodules do not injure the plants but enrich
them with nitrogenous material which they obtain from
the air found in the interstices of the soil. It is therefore
particularly important to keep the ground around peas
and beans open by hoeing, so that the roots may not only
get the necessary oxygen for breathing purposes, but also
the nitrogen they require for their nutrition. It is bv the
activity of these nitrifying bacteria that leguminous
plants, as those belonging to the pea family are called, can
grow in very poor soil, that is to say, in soil in which
there are very few nitrogen compounds, and yet produce
seeds which are very rich in nitrogen, and therefore very
important as food for mankind. Of further interest in

13

connection with these plants is the fact that when they
have yielded their crop they have not exhausted the soil
‘of its nitrogen compounds, but will be found to have
actually enriched it. This renders leguminous plants a
valuable crop to alternate with other crops which deplete
the soil of nitrogenous material. In agriculture it is often
found expedient in the case of poor or exhausted fields to
dig in a leguminous crop such as lupins or clover, which
are often grown for the purpose of enriching the soil.

With a view to increasing the number of the nitrify-
ing organisms in the soil attempts have been made in this
and other countries to introduce more of these bacteria,
particularly into pots or beds in which sweet peas or other
members of that family are to be grown. In England
preparations of these bacteria were distributed some years
ago as “nitrobacterine,” but the use of this preparation
was not found profitable in all cases, probably as in many
soils there is already a sufficient supply of these bacteria.
Professor Bottomley has, however, now discovered a better
way of cultivating these bacteria in peat, with which they
can be easily distributed. Peat, which represents the
partially decayed vegetable remains, differs from leaf
mould or humus in undergoing decay under very wet con-
ditions. | As a consequence it remains permeated with
certain substances which render it acid and unsuited to the
growth of most plants, though heaths, azaleas, rhododen-
drons, and other members of the heather family grow
well in peaty soil. When rendered alcaline, however, peat
has been found to favour the dévelopment of roots, and
therefore the whole growth of plants, and containing as it
does a large amount of organic material, much of which
is rendered soluble when alcaline, it has a high manurial
value. This is said to be still further increased by inocu-
lating the peat with nitrifying bacteria, which grow very
vigorously in alcaline peat and thus increase the available
plant foods. “ Bacterised peat,” as it is called, is not yet
-on the market, and has therefore not been extensively
tried, but experiments which have been conducted at Kew
Gardens with pot plants, and on a farm near Norwich, are
of considerable promise.*

_* The Spirit of the Soil.” An account of the nitrogen fixation
in the soil by bacteria, and of the production of auximones as

promoted by bacterised peat. By G. D. Knox. Constable,
London, 1915.

14

It should not be forgotten that there are numerous
other micro-organisms in the soil, many of which are not
only useless to plant life, but may be actually injurious
particularly by preventing the full development of the
nitrifying bacteria. Dr. E. J. Russell, of the Rothamstead
Experimental Station, has discovered that if soil is par-
tially sterilised, either by steam or by volatile disinfect-
ants, some at any rate of the harmful organisms are
destroyed, and the subsequent activity of the useful
bacteria is greatly increased. The effect of such partial
sterilisation has been found particularly beneficial in the
case of richly cultivated soil, such as that in use in green-
houses and market gardens, and sterilisation has been suc-
cessfully adopted in many instances. The chemical
antiseptic used by Dr. Russell was toluene, one of the
coal tar products. Its action as well as that of steam is
generally believed to destroy the numerous protozoa, which
are microscopic animal organisms. Some of these are
known to feed on bacteria, and are therefore very likely
destructive of the nitrifying bacteria.*

Before leaving the subject of the nitrogen nutrition
the chief function of the roots of plants, it should be
mentioned that excessive use of nitrogenous manures has
been found to render many plants more liable to disease,
particularly such as are produced by various fungi. It
is well, therefore, to practice moderation.

As regards other substances than nitrogen which it
has been found useful to add to the soil as fertilizers,
the most important are potash and phosphoric acid, the
ei in the form of various phosphates, including bone
meal.

These plant foods must be used in differing propor-
tions according to the richness of the soil, and also
according to the requirements of particular crops. Speak-
ing generally, gardens and allotments which receive an
abundance of stable manure do not require much in the
way of other fertilizers; but in heavy soils basic slag
used occasionally will be found a useful way of adding the
necessary phosphorus, while nitrate of soda is often valu-
able to push on early crops, such as lettuces, peas, etc. In

* See “Reports on the Partial Sterilisation of Soil ” Journal
of the Board of Agriculture, January, 1912, January. 1913, and
May, 1914.

15

agriculture it is usual to practice a rotation of crops
whereby plants with different requirements as regards
muneral salts succeed each other, and with the periodic
introduction of a leguminous crop prevent the exhaustion
of the soil. Though this is not so necessary in horticul-
tural practice where the soil is generally richer, yet it is
equally useful here to vary the crops in different portions
of the plot, firstly because as shown above peas and beans
enrich the soil in nitrogen, and secondly because such
alternation often puts a stop to certain plant diseases
which can only propagate themselves when the same plant
is grown year after year on the samé plot. This is the
case with such diseases as “club root,’ of turnips and
cabbages, and the “wart disease” of potatoes.

Another feature of some importance must be touched
upon. Probably every gardener has noticed that the
deeper soil of his garden differs very considerably from
the surface soil. On heavy land the subsoil, as it is
called, consists partly of solid clay, and is not so porous
and crumbly as the surface soil. Itis also of less nutritrve
value for it contains little or no decaying organic matter.
Both physically and chemically, therefore, it is less suited
to the nutrition of plants than the surface soil. It is
important, therefore, when cultivating deep-rooted plants
to improve the subsoil. This is done by ¢renching, a
process which may be regarded as the cultivation of the
subsoil. The surface soil is lifted off by removing one
spit of soil, and then the subsoil may be improved both
chemically and physically by digging in farmyard
manure, or other forms of decaying organic matter. In
the case of clayey subsoil, the addition of lime should
not be forgotten. If the clay of the subsoil is too solid
it may be necessary to replace it by loam, or it may be
improved by burning it, whereby it 1s rendered more
friable. By thus improving the subsoil, a much better
nutrition of deep-rooted crops will be secured. In the case
of lighter soils, where there is less difference, at any rate
physically, between the surface soil and the subsoil, it is
a common and beneficial practice.when trenching to put
the surface soil at the bottom of the trench and cover it
with the subsoil, to which manure has been added.

Another useful process for the amelioration of soil is
that of ridging, this 1s particularly beneficial in the case
of heavy and tenacious soils. By digging up the soil and

16

heaping it in parallel ridges a larger surface is exposed
to the weathering, particularly to the action of frost
during the winter. The effect of frost is to disintegrate
the large masses of soil, and so to render the earth more.
permeable both to water and to air. By exposure to the
atmosphere moreover the decay of organic matter in the
soil is promoted, and thereby valuable nutritive matter
is rendered soluble and available for plant nutrition. It
is, therefore, sometimes recommended, before ridging, to
spread any stable manure which is to be incorporated with
the soil evenly over the plot. A trough is then dug two
spades wide, one spadeful being deposited to the right
and the other to the left of the trough. The soil of the
latter can also be lifted, and turned, or merely forked over.
This operation of ridging should be commenced in the
autumn, and can of course only be undertaken on ground
that is to remain empty during the winter.

Further information regarding the nature of soils and
manures can be found in the following excellent books :—

E. J. RUSSELL. “Lessons on Soil.” Cambridge Nature
Study Series, 1/-.

E. J. Russerz. ‘A Students Book on Soils and Manures.”
Cambridge Farm Institute Series, 1915, 3/6.

A. D. Harr. “ The Soil.” John Murray, London, 5/-.

A. D. Hay. “ Fertilizers and Manures.” John Murray,

London, 5/-.

Chapter 3.

STEMS AND LEAVES.

Mechanical requirements of stems and branches. Twining and
climbing plants. The food conducting function of stems. The
work of leaves and their structure. The wilting and recovery of
leaves. Protection of leaves against excessive drought. Direct
effect of surroundings on leaf development. Hardening off
plants. ;

The stem is not as important an organ of plants as are
the roots and leaves. The latter are essential for the
nutrition of plants, and their particular work can only be
carried on if the leaves are fully exposed to the sunlight.
There is consequently considerable competition among
plants for “a place in the sun,’ and in the course of
evolution there has been a development of plants of
increasing size, which by overlapping the smaller ones
have been able to reach the light. This has led gradually
to the production of trees and shrubs, but in herbaceous
plants too, the stem and branches have the function of
displaying the leaves to the best advantage. This and
certain mechanical principles which are involved account
for the peculiarities of branching typical of different
plants. In addition to the stem and branches the leaf
stalks are also concerned in the ultimate positions in which
the leaves are expanded. By varying growth in length
and direction the leaves -become so placed that there is
little or no overlapping and the dovetailing of the various
leaves causes the formation of a regular “leaf mosaic.”

17

18

This is very clearly seen in the arrangement of the leaves
on the horizontal branches of a tree, or in the case of ivy
growing up a tree trunk or on a wall. Even in plants
possessing no stem, and in which therefore the leaves are
found close to the ground forming a rosette, the leaves
are found to overlap very little and particularly when they
are stalked it can be clearly seen that young leaves which
are formed near the middle of the rosette do not overlap
the older ‘leaves, as the latter become pushed out more
soe pe from the centre by the elongation of the leaf-
stalks.

The function of the stem being to bear and display
the leaves it follows that, to carry the foliage and to resist
the strain which winds will exert upon leafy plants the
stem has to meet certain mechanical requirements. The
rigidity, and at the same time the elasticity of the stem
is attained by the development of certain thick-walled
cells which collected into groups are so arranged as to
give the greatest amount of strength with the smallest
expenditure of material. These strengthening cells are
displayed in plants on the principles which have been
adopted by engineers in the manufacture of rigid, and at
the same time elastic structures. We have, firstly, the
hollow cylinder as is seen in grass-haulms. The slender
grasses can maintain their erect position by virtue of the
mechanical properties of their straw. The bamboo cane
is perhaps the ‘most powerful of all in this form of con-
struction. In other cases we find the stems developing
internally a system of girders which give them the strength
they require.

There are some plants in which the stem has not
sufficient rigidity to grow erect as it does in most instances.
In such cases the stem may trail along the ground, often
covering a large area and the plant is then often modified
as a shade plant unless it grows among very short vege-
tion, as in the case of some mountain plants. Sometimes
by using the support of the rigid stems of other plants
twiners and climbers are able to reach the light though
they have to compete with much stouter vegetation.
Twiners, such as the scarlet runner and the hop, have no
special climbing structures, but their slender stems ending
in a heavy terminal bud are bent over to one side, and on
close examination can be seen to rotate very slowly, either
clockwise or counter clockwise. This circular movement

1Q

of the top of the stem causes it to grow round any upright
or nearly vertical object in its neighbourhood, and then
as the slender stem tries to straighten itself out it grips
firmly the support round which it has grown. As the
rotation is always in the same direction for a given species,
it is necessary before giving such a plant any artificial
aid to see in which way the rotation takes place, for if one
twines the plant in the wrong direction it will unwind
itself again when left alone. It is difficult to keep a
twining plant to a horizontal course as it of itself will
never twine round a horizontal support. Climbing plants
on the other hand can fix their special climbing organs or
tendrils to horizontal supports, partly owing to the sensi-
tiveness of these organs to contact. In some cases, as for
instance in the Virginia Creeper (Ampelopsis) the contact
stimulus causes the tips of the tendrils to swell up into
sticky suckers, which enable this creeper to fix itself to
a vertical wall. More frequently however when the ten-
drils come into touch with some object, they grow round
it by virtue of the fact that the side which is touched
grows less rapidly, so that it curves round the support.
After it has grasped the latter the tendril contracts spirally,
and thus tightens the climber to its support. Tendrils are
zenerally formed from parts of the leaf, the leaf-tips in
peas, the leaf-stalks in the Nasturtium and Clematis.*

Besides carrying the leaves the stem has the further
function of conducting to these leaves the food material
absorbed by the roots. As we have seen the forces at
work in connection with the supply of water to the leaves
are, firstly, the root pressure (lecture 2), and secondly, the
transpiration current in leaves (lecture 1), which latter
causes a considerable suction to be exerted on the water
in the stem. The special channels through which the sap
rises are the vessels, long continuous passages, running in
the wood of the stem. But this upward passage of water
with the inorganic salts it contains, is not the only con-
duction required. The complex substances formed, as
we shall see, in the leaves require to be conducted away
partly to nourish the flowers and fruits, partly to enable
further growth and development of the roots to take place.
This elaborated sap passes through other channels which

* Further information concerning this interesting group of
plants can be obtained by the perusal of Charles Darwin’s
*“ Movements and Habits of Climbing Plants.”

20

lie outside the wood and are spoken of as the bast-tubes.
These are very delicate, and to prevent them from being
crushed they are protected on the outside by strong resist-
ant fibres called the bast fibres or hard bast. :

With regard to the essential work of leaves their
primary function is without.doubt to build up organic
food material from the water taken up by the roots, This
they are able to combine with carbon which they take
from the atmosphere, where it occurs plentifully in the
form of carbonic acid, the gas we breathe out from our
lungs. In the presence of sunlight all parts of the plant
which contain the characteristic green colouring matter are
able to break up the carbonic acid, retaining the carbon
and giving back the oxygen which was previously com-
bined with the carbon. In doing this they are constantly
ere as it were the air and enriching it with the
ife-sustaining oxygen needed by man and all forms of
animal life. The carbon which the plants retain is com-
bined with the elements of water to form organic material
called carbohydrates, of which starch and sugar are the
most important to plants.

Within certain limits the brighter the light the more
active is this process of carbon nutrition, or assimilation
as it is called. Hence the great importance of giving
plants as bright and sunny a position as possible, and
hence also the reason why plants seek to secure the
most favourable positions for their leaves. The formation
of leaf mosaics, mentioned above, is a good instance of
means adopted to secure the most effective display of
leaves. Unless specially adapted for growing in shade,
as are for instance most ferns, the majority of plants will
not show their best development unless grown in an open
situation. Apart from the essential influence of light upon
the process of leaf nutrition, it has been found. by experi-
ment that this important nutritive process is affected by
temperature being within certain limits proportional to
the rise in temperature. This explains why in sheltered
places, where the air is heated up in winter many plants
are known to make better growth than actually in the
open. Shade can be partially counteracted by warmth.
In frames in which plants often get less light than in the
open, the warmer temperature enables them to grow more
rapidly than were they exposed to the colder air. It
must not be forgotten too, that not only is the process of

21
leaf nutrition stimulated by warmth but root-absorption
also increases with a rise in temperature, so that all nutri-
tive processes are favoured by warmer conditions of soil
or climate.

A secondary but important function of leaves was re-
ferred to in a preceding lecture, namely, the importance
of getting rid of a certain amount of water vapour into
the atmosphere. This process of transpiration aids the
upward passage of sap and also causes the concentration
of the necessary mineral salts, which are only contained
in dilute solution in the soil.

Let us now consider how the structure of leaves is
adapted to perform its important functions. For both of
them it is essential that the air should be in close contact
with the cells of the leaf. It is esscntial to prevent too
great a loss of water from the leaves and also for pro-
tective purposes it is necessary that the surface of the
leaf should be covered with a more or less resistant layer.
Consequently the passage of air to the interior of the leaf
is effected through microscopic pores called stomata, which
are usually found in greatest number on the under sur-
face of leaves. These pores have the power of opening
and closing so that the aperture can be regulated to the
needs of the plant. They open with the approach of day-
light and thus enable the carbonic acid to enter the leaves
when the conditions are favourable for leaf nutrition. At
the same time, of course, water vapour escapes outwards
through the pores. The inner parts of the leaf are so
arranged that the cells containing most of the green colour-
ing matter are near the upper surface of the leaf where
they will receive most light | Throughout the interior of
the leaf there are wide spaces through which the air can
circulate readily between the cells. The veins which bring
up the supply of water and which also have to collect
and conduct away through their bast portion the organic
material formed by the leaf, become very finely branched
so as to be in contact with all parts of the leaf.

If we consider that the larger the leaf surface the
larger the amount of light absorbed and hence the greater
the nutrition, we might wonder why all plants have not
larger leaves. The limitation in the size of leaves is
largely due to mechanical considerations. A slender stem
cannot bear leaves beyond a certain size. Moreover,
though a large leaf would increase the process of carbon

a?

assimilation, the loss of water from a large leaf -surface
might be more rapid than the absorption of water by the
roots. Whenever this happens it causes the welteng or
drooping of ieaves, and should that occur too frequently
the plant will not survive. As it is, wilting of leaves in
the open occurs in hot summer days and affects mainly
plants with large delicate leaves, large at all events in
proportion to their roots, such as Doronicums, Calceolarias,
and young Cauliflowers. In the case of pot plants, which
are not only losing water through their leaves but also
through the sides of the porous pots, it 1s a common
phenomenon to find plants wilting during the heat of the
day, and to prevent this it is necessary to shade the houses
during the summer months, at least on very bright days.
Wilting is particularly noticeable in recently transplanted
plants, in which there is always sure to have been some
injury to the roots so that absorption cannot keep pace with
transpiration. Transplanting is therefore best undertaken
in wet or dull weather, or at the end of the day, when
during the succeeding night the stomata of the leaves will
be closed, and consequently at the start, at all events, there
will be no undue loss ef water. Cuttings which have not
yet produced an adequate supply of roots require in
many cases to be kept in a moisture-laden atmosphere in
a frame. It is interesting to note that when plants in the
open flag at noon on a hot summer’s day, they vecover
again at night even without further water supply. When
the leaves croop the stomata usually tend to close, and
at night they are certainly closed so that the roots have
then the chance of replenishing the drooping leaves with
water. Watering the plants while the sun is still on them
should be avoidéd. Drops of water on the leaves are apt
to act as lenses and focussing the rays of light upon the
leaf often cause burning of the tissues and consequent
spotting of the leaves. Watering the soil has also some
dangers unless properly done. Merely damping the sur-
face of the soil, apart from the tendency to cause it to
cake, stimulates the growth of surface roots, which will
constantly require watering in dry weather. It is better
to give plants a good soaking from time to time The
water penetrating to some depth causes the development
of. deep roots, which will keep the plant supplied with
water even in dry weather.

It is often thought that leaves, or at any rate the leaves

23

of some plants, have .the power of absorbing water. It
1s not an uncommon practice to spray the leaves of plants
in the greenhouse when they are shaded, and also in the
open when the sun is no longer upon them and their sub-
sequent recovery, if they were drooping before, is attri-
buted to their having absorbed water. This is however
not the case. The spraying of the leaves, besides cooling
the foliage, which is often considerably heated by the sun,
has the effect by the evaporation of the water to render
the atmosphere around the plant moisture-laden, where-
fore the transpiration of the leaves is at once decreased.
It is this decrease of transpiration and not absorption by
the leaves which causes the revival of the plants. No
leaves with an impervious covering can take in water and
there are very few leaves which are not so protected.
Mosses and Filmy Ferns alone among leafy plants can
take in water through their leaves, which are adapted to
growth in moist if not dripping conditions.

It is interesting to note that when the stomata are
closed and transpiration is impossible, some plants are still
able to get rid of their superfluous water. At night the
leaves of the Nasturtium and the Fuchsia for instance are
able to force out little drops of water through water-pores,
which are found at the termination of their veins. Many
grasses, too, have water-pores at the tips of their leaves,
and the drops of water exuded by them often look like the
formation of dew.

When growing in very dry soil or in hot arid climes,
plants may experience a great difficulty in supplying their
leaves with water, often during a very prolonged period.
In such cases many remarkable deviations from the normal
struciure are produced, with the object of protecting the
leaves against drought. In most cases the leaves are small,
often reduced to mere scales or needle-shaped structures.
Such are the leaves of many of our moorland plants grow-
ing in sandy, well-drained soil and exposed to strong and
drying winds. In still more arid regions the leaves may
disappear altogether, and we get such curious plants pro-
duced as the fleshy Cacti of the New World and the
succulent Euphorbias (Spurges) of the Old World. In
both of these groups of plants the green stems undertake
the process of assimilation, and they also store up sufficient
water during the wet season to enable the plant to last
out during a prolonged drought:

24

It is sometimes stated that these strange modifications
are the result of the direct action of the environment on
vegetation. This is impossible of proof, and we may
assume that it is largely by natural selection of forms or
varieties most suited to these extremes of climate that
desert plants have in the course of ages sprung from
ordinary types of vegetation. Nevertheless we must not
forget that to some extent climate has a direct modifying
action. If, for example, we grow a seedling gorse in a
moist greenhouse, we find that it will persist for a long
time in producing small ordinary leaves, which precede
the spines in the young plant, and this effect is certainly
a direct action of the surrounding conditions. It is also
well known that the texture of leaves produced by plants
grown in a greenhouse differs greatly from that of leaves
of the same species growing out of doors. It is necessary
therefore before transplanting into the open plants raised
under glass, to get them gradually acclimatised to their
new surroundings. By transferring them from greenhouse
to frame and keeping the latter well ventilated, or by
placing plants in a sheltered place in the open, the leaves
have an opportunity of becoming 4ardened to more
rigorous conditions of existence. The outer layer of the
young leaves can become strengthened, and they as well
as the new leaves wili all be modified to suit the new
environment. In transferring a plant from a dry to a very
moist house it will often be noted that ithe older leaves
are shed. Being fully developed they are unable to adapt
themselves to the new conditions. Not able to transpire
freely owing to the moisture-laden atmosphere they become
overcharged with water and this probably causes them to
fall. The new leaves will be suitably modified to suit
the more humid conditions.

Chapter 4.

METHODS OF VEGETATIVE REPRODUCTION AND
PROPAGATION.

Tubers. Bulbs and corms. Bulbils. Runners. Layerings ‘and
cuitings. Pudding and grafting.

As explained in my first lecture vegetative repro-
duction is the multiplication of the plant by means of
structures, which partake of the nature of vegetative organs
and are not the result of the fertilisation of flowers.
This means therefore that the offspring, however numerous
they become in the course of years, are not so much
descendants as actual portions of the original individual
just as the most recently formed buds of a tree are really
part of the.same tree, which ten or a hundred or even a
thousand years before bore similar buds. The only
difference is that the buds which the tree produces year
after year all remain attached to the same parental stem,
while in the case of tubers or bulbs the parent plant dies
down each year, so that these structures become so many
separate individuals. The very large number of distinct
plants which thus arise will perhaps best come home to us
if we consider that in the case of a potato plant pro-
ducing yearly only six new tubers, we should have at the
end of ten seasons obtained over ten millions of tubers.
If an average of ten tubers were produced by each plant,
the number of tubers at the end of ten generations would
be 10,000,000,000 potatoes. As the millions of new plants
which are thus developed are, properly speaking, all parts

25

26

of the same plant they should, apart from slight differences
of nutrition, not only be all similar but actually of the
same constitution. “It is true that bud variation does
occasionally arise in plants, but it is a comparatively rare
phenomenon, and consequently we find that as a rule
plants, raised by vegetative methods, maintain the char-
acter of the parents and do not show the sporting with
which one is familiar when raising plants from seed. THe
purity of the strain is thus easily maintained by vegetative
propagation.

Let us now examine some of nature’s methods of
vegetative reproduction. In the first place, we have such
well-known examples as tubers which form the chief
method of propagation of the potato and of the Jerusalem
artichoke. A tuber is the swollen-up end of an under-
ground branch. This can be clearly seen by digging up
a young potato plant and following these subterranean
shoots from the parental stem to their tuberous tip. Both
the branch and also the tuber show the rudiments of
leaves, reduced to small scales and spirally arranged
round the potato. On an old potato the sickle-shaned
mark below each eye or lateral bud represents the insertion
of the leaf or leaf scar, and at one end of the potato where
these become more crowded together, we have the terminal
bud of the tuber. This is called the “rose” end. At the
opposite end we can generally find the scar or heel where
the branch of which the tuber is the dilated tip, was joined
to it. In all cases tubers are filled with stored food
material, mainly starch in the case of the potato. When
potatoes are set in the spring, the terminal bud grows
up to form the main stem of the new plant, while the
other eyes remain generally dormant. The new tubers are
formed from specialised underground branches produced
from the lower region of thc main stem. The latter is
therefore often earthed up so as to promote the develop-
ment of these side shoots. It is a very common practice
before setting potatoes, to place them side by side with
the “rose” end uppermost and allow the terminal shoot
to commence its development in daylight. By this means
the shoot does not elongate so rapidly as when grown >
underground, that is, in the dark, and thus the lateral
buds, which will develop into tuber producing shoots, are
morc closely crowded together and more numerous. This
method therefore results in a greater vield of new tubers.

aS

27

As there is generally a superabundance of food
material, it is possible to divide a tuber into several pieces,
and provided each piece has enough food material and a
sound “eye,” it will produce a new plant. It has been
found by practice that when cut for “sets” the pieces near
the “rose” end give the best results. When a potato is
divided up in this way it is necessary to leave the separate
pieces spread out to dry before they are planted, so that
they may form a protective layer before coming in con-
tact with the soil, the bacteria of which might cause them
to decay.

Recent] y a method of raising new potatoes in the dark
has received prominence in the press, and in many quarters
erroneous views have been formed concerning this mode
of cultivation. It has been thought by some that potato
plants could grow like mushrooms and were not dependent
upon light for their full development. This is very far
from being the truth. Of course it is well known that
potato tubers will sprout in the dark, as they do indeed
in nature underground, but they are only able to grow
without light while there is still a supply of food material
in the old tuber upon which they can draw for their
development. When that is exhausted the plants must
inevitably die, as without lhght they are like all green
plants unable to manufacture new organic material. It
has however been found by experiment that if tubers are
then half buried in fine dry soil spread out on a table
in the cellar, they will soon be surrounded by a crop of
small new potatoes close up to the old tubers. Large
tubers should be used for this method of cultivation, and
they should be placed three or four inches apart. It must
of course be remembered that these small new potatoes
are produced entirely at the expense of the food material
stored in the old potato. We are therefore by this method
of cultivation not increasing the food supply of the
country but merely replacing old potatoes by a crop of
more palatable new ones.

Bulbs and corms, which are the vegetative methods of
reproduction of many members of the Lily and Iris
Families, must be looked upon as specialised underground
buds, which by virtue of their store of food material are
able to lead an existence independent of the plants on
which they have been produced. Like the ordinary winter
buds of a tree, such as the horse-chestnut, they are pro-

28

tected on the outside by a few dry scaly leaves, while at
their centre will be found the foliage leaves and flowers
of the next season. But between these we find another
set of leaves which do not occur in the buds of a tree,
namely, thick storage leaves of a fleshy nature. It is the
possession of this internal supply. of food material which
enables these specialised bulbs to produce their leaves and
flowers in the next season, though separated from the
parent plant which formed these bulbs, while in the case
of the tree the winter buds expand in spring by making
of the food material stored in the branch to which they
ec.ong.

What is the nature of the parent plant on which the
bulbs are borne as lateral buds? Take up a tulip plant
after it has fiowered and you will find at the base of the
upright stem bearing the leaves the storage scales which
have made the growth of the stem and leaves possible,
and at the base of one or more of these scales will be seen
small buds, which are beginning to swell owing to the
organic material manufactured by the leaves, passing
down to them. This observation will teach us that it is
important, if we wish to save our own bulbs for planting,
to leave the old bulbs in the ground for some time after
flowering and not to cut off the foliage leaves but to pre-
serve them as long as they are fresh and green and capable
of manufacturing food material. This is the time, too, to
give the plant further food in the form of top dressing
or occasional supplies of manure water. It is also im-
portant to cut away the dead flowers, so that the food
material is not expended in the ripening of the seed vessel.

Another point of importance in bulb growing is to
secure the proper ripening of the bulb. In nature bulbs
after losing their leaves pass through a resting stage,
during which they are kept dry by the vegetation cover-
ing the soil in which they grow, whether they occur in
woodlands or meadows. In our gardens where the ground
above them is usually uncovered, water percolates down
to them, and if they are not very deeply buried slugs, too,
may attack them. It is therefore often advisable in the
case of damp soils, to take up the bulbs when the leaves
are dying away, and to allow the bulbs to dry in the sun,
storing them afterwards in a dry place until the time for
planting arrives.

Corms are very like bulbs and may often be mistaken

29

for the latter, but on cutting them across we find that the
dry scales which surround them on the outside are not
followed by fleshy storage scales, all the food material
being stored in the thick solid stem which makes up the
bulk of the corm. In their growth and development,
however, they follow very closely the life history of the
bulb. Crocuses and Gladioli possess corms, while Lilies,
Tulips, Hyacinths and Daffodils have true bulbs. The
bud-like nature of bulbs can’be clearly recognised by
examining the small swollen buds or duléils which arise
on the aerial stems of certain species of Lily, and which,
though unable to give rise immediately to a new flower-
ing shoot, can gradually be grown on to produce in time
amature bulb. ~

Runners, such as are produced in the Strawberry and
Violet, are delicate inferat shidets creeping over the surface
of the soil and becoming readily rooted at their nodes
when. in contact with the moist soil. In nature they cause
the very rapid spreading of these plants, and by the decay
of the portion of the stem which joins them to the parent
plant they may cause the increase of individuals. They
are conveniently used for propagating purposes and
should always be removed from the parental stock as they
draw nutriment from it and therefore impoverish the latter,
with the result that they reduce the number of flowers.

Many plants which do not possess natural means of
vegetative reproduction can be caused to give rise to new
individuals by separating certain portions, generally
lateral shoots, and inducing the same to develop new roots.
In some instances the formation of these roots is promoted
before the branch is separated from the plant; this process
is known as layering. The lower branches of such shrubs
as Gooseberries and Red Currants may be bent down and
partially embedded in a shallow trench dug round the
bush, and filled with light and porous soil. When stimu-
lated by moisture the buried portions of these branches,
the tips of which must be allowed to project beyond the
trench, will produce what are termed adventitious roots,
and when these are sufficiently well-established, the branch
may be severed from the parent plant, and the new
individual will lead an independent existence. It is
generally found advisable to cut back the projecting por-
tion of the branch to two or three buds. This method
can also be employed for Rhododendrons and for more

20

delicate shrubs, as well as for climbers like Clematis. A
similar process has been found advantageous for many
herbaceous plants such as Violas, Carnation and Pinks.
In these cases, however, it is usual to make a slight
incision in the buried portion of the shoot, which should
always extend to one of the knots or joints. Such an
incision stimulates the production of roots, particularly at
the end furthest removed from the parent plant. This is
no doubt due to the accumulation at this point of food
material which has been produced by the leaves at the tip
of the layered shoot, the food material being unable to
be carried across the incision. Even in dealing with
shrubs it is sometimes found advisable to make an in-
cision partially across the branch which is to be layered,
and to peg it down so as _to keep it in position until
the new roots are formed. It is of great importance that
the soil in which the new roots are to form should be well
aerated, more particularly in the case of herbaceous plants
which are more liable to injury by rotting. It is advis-
able, therefore, to add a considerable amount of sand to
soil in which layerings are to be embedded.

In the case of upright stems or branches, which are far
above the soil, the production of adventitious roots can be
promoted by removing a ring of outer tissues to the depth
of the wood, and then tying a handful of wet moss round
the shoot. Such ringing causes the food material formed
in the upper part of the plant to accumulate above the
wound, and this promotes the rapid development of roots.
When these have become sufficiently established the
rooted portion of the stem or branch may be severed from
the parent.

In many plants the formation of adventitious roots is
so rapid that shoots completely detached from a plant
can establish themselves as cuttings, producing their own
roots when placed in suitable conditions. In all cases in
which cuttings are made the first need is to cause the cover-
ing in, that is, the healing of the cut end of the shoot.
This is done by the development from the actively grow-
ing cells of a peculiar wound tissue, called callus, which
by growing over the wound and developing a layer of
cork, prevents the destruction of the exposed celis and
the entrance of harmful bacteria. The growth of callus
is promoted by the aeration of the tissues, and it is there-
fore important that the soil in which cuttings are placed

31

should be even better drained than that supplied to normal
plants. A soil made porous by the plentifial admixture
of sand should always be selected, and in the case of
cuttings which are being struck in pots, it is a common
practice to insert them close against the inside of the
porous pot, through which the air has a free access to the
cuttings. The danger of over watering cuttings is even
greater than in the case of well-established plants. Cut-
tings taken from woody plants produce their wound
tissues as well as the adventitious roots, which are formed
later, at the expense of the food material which is stored
in the shoot. They are not therefore so dependent on
warmth and light in the early stages as are herbaceous
cuttings. They. can indeed be struck in the autumn or
winter after the leaves have fallen, or in the early spring
before the foliage has been developed. If taken in the
autumn they often do not produce their roots until the
following spring, and they are always jater in the develop-
ment of their leaves than are well-established plants. In
the case of herbaceous cuttings which have no store of
food material, it is necessary that they should be able to
continue to form new organic food material in their leaves
so as to promote the growth of callus and the development
of roots. It is esential therefore that they should have
pleniy of light; but in the first few days before they have
adapted themselves to their new conditions, they are liable
to lose too much water by evaporation, and it is important
during this period to keep them slightly shaded, or to
grow them in a moisture-laden atmosphere in a closed
frame or greenhouse. It is better to prevent the loss of
too much water by protecting the leaves in this way than
by excessive supply of water to the soil, as herbaceous
plants are very liable to decay by the action of bacteria
on the cut end of the shoot. As herbaceous cuttings have
to continue to manufacture food material, they also require
a greater amount of heat than do woody cuttings, the
process of leaf nutrition being stimulated by an increase
of temperature. Herbaceous cuttings must, therefore, not
be taken too late in the autumn unless they are to be grown
in artificial heat. :

A few plants can be raised from root cuttings. This
is possible where plants are endowed by nattire with the
power of forming adventitious buds on their roots. Rasp-
berries, Pears and Apples are all examples of plants which

32

often produce suckers from their roots in the neighbour-
hood of the parent plant. Such growths can be separated
and developed into new individuals. In the case of Rasp-
berries indeed, this is a common method of propagating
the canes.

Lastly, it has been found possible in the case of some
plants with somewhat fleshy leaves to cause these, or even
portions of a leaf, to produce adventitious buds. This is
the case with many Begonias, particularly those belonging
to large-leaved varieties. If the leaf is placed on damp
soil, the midrib having been cut in several places, new
plants may arise from each portion as with the stimulus
of warm temperature and moisture the leaf produces a
considerable growth of callus, from which adventitious
buds soon arise. The fleshy scales forming the buibs of
most lilies are capable when separated from the parent
bulb of producing small adventitious buds from which
new plants can be grown, and this is a common method of
propagating the plants.

Space prevents a detailed discussion of the processes of
wit bag and grafting, but from the botanical point of
view the processes may be regarded as a special case of
making cuttings in which the latter, instead of being
planted in soil, are inserted in the tissues of a nearly
related plant with which they become united by the
development of wound tissue or callus. No adventitious
roots are formed by the graft as the scion relies for its
supply of water entirely upon the roots of the stock.

There has been much discussion as to whether, as a
result of grafting, there is any influence of the stock upon
the scion, or vice versa. A considerable amount of infor-
mation on this question has accumulated, but it is largely
of a negative character, and what positive evidence exists
is of a doubtful nature.

Chapter 5.

FLOWERS AND THEIR FORMATION.

Conditions favouring the production of flowers. Structure and
functions of the various parts of a flower. Pollination and
. Fertilisation. Self-fertile and self-sterile flowers. Ripening of
fruits and seeds.

In speaking of the difference between vegetative and
reproductive organs, in the first chapter I have already
mentioned that certain external conditions which favour
the development of the former affect adversely the forma-
tion of flowers. Long horticultural practice has also
proved that the development of floral organs can be
stimulated in various ways which are well known to
gardeners. Thus reduction in the supply of water and
consequent stoppage of the luxuriant development of
leaves is one of the chief methods employed. It has been
found by scientific experiments that these various horti-
cultural practices are based upon definite physiological
requirements. In recent years much has been done to
confirm and extend our knowledge of this subject. It
was proved that bright illumination of plants is essential
for the production of flowers. At first it was thought
that certain rays of light influenced the development. in
the plant of special flower-forming substances, and early
experiments seemed to indicate that these were produced
by definite rays of light from beyond the blue end of the
spectrum—rays of light which are known to have great
chemical activity. More recently, however, it seems to

22

34

have been established that it is probably the intensity
rather than the quality of light which is required for
flower production. Another factor which is important in
this respect is the concentration of organic materzal in the
stem and leaves of the plant, while an increase of water
and inorganic salts tends to the development of foliage
rather than flowers. It is for this reason that a reduction
in the supply of water is so helpful in producing abundant
flower buds. It is sometimes thought that a wealth of
blossom on our fruit trees predicts a fine summer; from
what has been said above, it is obvious, however, that it is
the fine summer or dry autumn of the previous year which
is responsible for the prolific bloom. Flower buds on our
fruit trees are, of course, already developed in the late
autumn. Dealing with herbaceous plants, it has been
found possible to arrest the development of flowers even
if the flowering shoot has commenced to make its appear-
ance. Thus in specimens of the common House Leek a
rosette of vegetative leaves may be caused to appear on
the fiower shoot if the plant 1s copiously watered and
uluminated with light passing through a red or blue
screen. Conversely, the runners of some plants may be
made to bend upwards and develop flowers if water is
withheld and the plant placed in a very bright light.

The horticultural practice of pruning and root pruning
is also intended to further floral development. Each tree
or shrub must be treated differently according as to
whether flowers normally make their appearance on long
shoots or on spurs, 2.2. on new or old wood. The removal
of non-flowering shoots is therefore what is aimed at. In
summer pruning the stoppage of the growth of purely
vegetative shoots will actually stimulate the older
branches to form flowers, as they will have an increased
amount of food material at their disposal. The increased
production of flowers effected by roct pruning is often
remarkable. It will be sometimes observed that apple
and other fruit trees produce long and vigorous new
shoots, particularly in an upward direction, which are
caused by the development of deep roots able to obtain
an abundant supply of water even in fairly dry soil. The
removal of these deep roots stimulates the growth of
fibrous roots in shallow soil, where there is a less abundant
supply of water in the summer and early autumn; the
result of root pruning is therefore the production of

35

numerous spur shoots, on which alone flowers are produced
in the case of the apple and pear. In some of our orna-
mental shrubs, however, such as Guelder Rose, Weigelia
and Forsythia, flowers are borne on the shoots of the pre-
vious year, and it is consequently a mistake to cut these
back in the autumn, as is so often done thereby reducing
the beauty of these shrubs. Old wood, on the other hand,
and branches which have already flowered, should be
pruned away at the end of the summer. Some plants
which are nearly related, differ in the manner in which
they bear their flowers and fruit, and it is therefore very
important before pruning to know exactly on which kind
of branches the flowers will be borne. For instance, the
Morello Cherry bears its flowers all along the shoots pro-
duced during the previous summer, while others flower
at the base of the shoots on short spurs. These kinds
must therefore be pruned differently from the former. A
similar difference is found in the case of currants. Black
Currants flower all along the shoots formed in the pre-
vious summer, while Red Currants produce their fruits on
small spur shoots found on older wood.

We must distinguish between the methods causing the
production of flower buds as described above, and prac-
tice—such as forcing, which has for its object to cause an
early unfolding of the same. Unless bulbs or aerial buds
already contain the rudiments of flowers no amount of
forcing will cause these to be formed. The application
of moist heat, often in faintly lighted pits, 1s indeed
inimical to the production of flower-forming substances
and promotes vegetative growth rather than flower pro-
duction. On the other hand, if flower buds are present
their expansion can’ be materially accelerated. By these
means flowers can be obtained in mid-winter when they
will be particularly appreciated. Most storage organs,
whether tubers, bulbs, or winter buds, require to pass
through a resting period before they enter upon a new
period of growth. Various methods can be adopted there-
fore to accelerate either the process of ripening the buds
or of reawakening the dormant structures. While dryness
promotes the former, heat and moisture effect the latter.
Tt has been found however that the resting period can be
shortened by various means. For example particular
varieties of potato, which will not sprout in the autumn
and cannot therefore be used for the culture of early

36

potatoes, if placed for a fortnight in a cold chamber
slightly above freezing point will readily develop when
planted. A similar treatment is advantageous for
rhubarb plants which are to be forced. Lift them out of
the ground and let them dry and be exposed to light
autumn frosts and they will then respond more readily
to methods of forcing. This practice of thoroughly cool-
ing plants which are to be forced is now regularly used
in the case of bulbs, retarded Lily of the Valley crowns,
Lilacs, Spirzeas, etc. Abroad, other methods have come
into vogue with the same object in view. It has been
found that the immersion of the branches of plants for ten
to twelve hours in a hot-water bath of from 85 deg. to
100 deg. Farenheit, has a remarkable effect in accelerating
the unfolding of winter buds. The roots should not be
so treated. It is therefore best to invert the plants and
allow the stem and branches to hang down into the hot
water. The effect of the latter is quite local; so that
when partially immersed only those buds which have been
under water are affected. Six weeks after this treatment
lilacs will be in full bloom, if subsequently grown in the
usual way. Professor Johannsen, of Copenhagen, has dis-
covered that exposure of plants to ether vapour for twenty-
four to forty-eight hours has a similar accelerating effect.
Both methods are largely used on the continent.

Let us now consider the structure of flowers and the
function of thei various parts. So manifold is the
appearance of flowers that it might seem at first difficult
in a short space to make any general statement on their
structure; yet as regards the essential organs of reproduc-
tion we find considerable agreement. It is more par-
ticularly in the structure of the brightly coloured petals
that we find great variety, and largely owing to the
adaptation of the flowers to the visits of insects. The
colour and scent is developed to direct them to the honey
which the flowers provide, and special honey guides, spots
in the Rhododendrons and lines in Pansies and Violas,
as well as differences in colour, guide them to the nectaries.
In making their way to these the insects come in contact
with the delicate stamens of the flower and the pollen
contained in the pollen sacs at the tip of the stamen
becomes dusted on to the insect. When it visits the next
flower, some of this pollen will be dusted off on the stigma
at the top of the immature seed vessel, and thus the insect

37

effects the pollination, which is the necessary preliminary
to the fer/zlisation of the flower. The pollen grains left
on the sticky surface of the stigma germinate there, and
a delicate tube grows down from them to the immature
seeds, which become fertilised by the fusion of the vital
element of the pollen grain with that of the ovule. Even
though a flower may produce both of these essential
organs, the poilen producing stamens and the ovule con-
taining seed vessel, self-pollination does not usuallv take
place as the stamens and seed vessel mature at different
times. This can easily be seen in such a large flower as
that of the Nasturtium, in which the stamens may be
observed ripening one after the other and liberating their
pollen, and only after the last of the pollen sacs has shed
its pollen does the three-pronged stigma open and is then
ready to receive pollen brought from another flower in
which the stamens are opening. All the various and
wonderful mechanisms, by means of which flowers ensure
their pollination, have the purpose of securing the ferti-
lisation of the ovules with pollen from another flower and
if possible from another plant, for as Darwin has shown *
cross fertilisation usually causes the production of more
numerous and stronger offspring than self-fertilisation.

Gardeners who grow exotic plants, the flowers of which
are adapted to the visits of insects not found in this
country, have often to perform this service themselves and
to pollinate artificially the flowers using usually a fine
camel-hair brush. In plants like the Tomato, tapping the
stems with a stick, wrapped round with a cloth to prevent
injury to the plant, will generally cause the pollen to drop
out of the opened anthers on to the stigma of these pendant
lowers. If tomatoes are grown under glass, we must take
care that during the period of flowering the house is kept
dry, as the pollen sacs will not open in a moist atmosphere
and the plants cannot be pollinated. Of course by the
method described self-pollination only can be effected,
crossing can be obtained by using a fine paint brush and
pushing the hairs up between the stamens first of the
flower and then of another.

In some plants, such as the Cucumber and Marrow, the
flowers are of two kinds, some producing the seed vessel,

Darwin, C. “ The Effects of Cross and Self-Fertilization in the
Vegetable Kingdom.” (Murray, 9/-.)

38

which can be seen as a thick structure below the petals,
and others producing only pollen. In such cases, self-
pollination is impossible and cross pollination must be
effected either by insects or by the gardener. It is curious
that in spite of the many adaptations to cross fertilisation
there are many plants in which self-fertdzsation 1s the
normal process of seed production. This is the case with
most of our cultivated grasses. In the wheat and other
cereals, though the flowers open and the light powdery
pollen is. carried from plant to plant, the stigma of the
seed vessel is usually pollinated before the flowers open,
and as there is only one seed in each seed vessel it is
usually self-fertilised before the foreign pollen arrives.
Even in flowers, apparently specially adapted to the visits
of insects such as the Sweet Pea, which possesses both
scent and colour, the immature pod is already pollinated
in the bud stage of the flower, and if we wish to effect
any crossings between different varieties of this plant, we
must cut away the stamens before the flowers open and
introduce pollen from another plant. While such normal
self-pollination has for horticulturists the advantage that
it preserves the purity of the varieties we cultivate, it is
probable that it may gradually cause a weakening of the
race, and in nature even occasional crossing probably re-
invigorates the strain.

Quite a number of members of the Pea Family are
self-fertile, including the commonly cultivated forms of
peas and beans, while many other leguminous plants, such
as the clovers are self-fertile and cannot be fertilised with
their own pollen even when it is placed on the stigma of
the flower. An interesting analysis has been made of the
behaviour in this respect of the various members of this
Family, and it has been found that all the annuals\in
this group of plants are self-fertile, while the perennial
forms are self-sterile. It would seem as if it were of more
importance to ensure regular and uninterrupted produc-
tion of seed in the former, while in perennials, even if in
one season owing to the absence of insect visitors, seeds
are not produced, the persistence of the plant to the next
season when conditions may be more favourable will
enable it to reproduce its kind. :

The flowers of many varieties of Apple and Pear are
self-sterile, and disappointment has often been caused by
the failure of a fruit tree to bear when grown singly with

39

no other variety near to it from which pollen could be
carried by seed. Even in orchards, if a large number of
self-sterile varieties are grown close together, the yield of
fruit may not be found to be satisfactory. In both cases
it has proved very efficacious to plant near such trees a
Siberian or other Crab Apple, which produces a’ large
amount of effective pollen which can be carried by insects
from tree to tree.

After fertilisation, when the young plant is beginning
to be formed in the seed, the seed vessel too begins to
swell, and it is from observing the latter that we can
gather that pollination has been successful. Of course
the continued growth of the seed-vessel, as well as the
development of the embryonic plant and the storage in the
seed of enough food material for its subsequent germina-
tion, necessitates considerable activity on the part of the
vegetative organs particularly of the leaves. It is for this
reason advisable in the case of Peas, Scarlet Runners and
French Beans, when they begin to set their pods, to
encourage their further growth by the use of suitable
stimulants such as liquid manure.

It is obvious that as the formation of seeds and fruits
require increased supplies of food material, herbaceous
plants that are heavily fruiting will have less food where-
with to develop new flower buds, and graduallv the plant
will cease to produce flowers. If, therefore, we are culti-
vating plants for the sake of their flowers it cannot be
too strongly recommended to pluck off all flowers as soon
as they are dead, so that they should not begin to sct
their seeds. In the case of Sweet Peas, where the seeds
take up a large amount of food material, this is par-
ticularly important, but it 1s a good rule to follow in all
cases. The fact that a large supply of food material to
fruits may exhaust that available for the production of
flower buds will explain reduction in the number of
flowers on fruit trees, if an abnormally heavy
crop of fruit was produced in the previous season.
It is not, however, only the formation of flowers
which will be interfered with. Under special cir-
cumstances the vegetative organs, too, may suffer.
For this reason it is recommended that young
fruit trees, which have been recently transplanted, should
not be allowed to ripen many of their fruits during the
first season after transplantation, as this may interfere

40

with the proper development of the new root system, which
is always damaged to some extent when trees are moved.
Similarly, it is advisable when Raspberry canes are newly
planted to cut them down to within eight or ten inches of
the ground, so that they should not exhaust themselves
during the first year, as that would prevent the formation
of strong new shoots for the next season. =

As ripening fruits require a large amount of nutrition,
it is important to see that the stimulating treatment we
give to plants which are bearing, should not _be used for
the production of vegetative shoots. In Tomatoes the
growth of such shoots should be checked when the fruits
are maturing. It is also advisable to reduce the foliage
a little so that the fruits are not shaded. Excessive
defoliation, however, will prevent the fruits from attain-
ing their full size as the leaves are the seat of formation
for the organic material which the fruits require.

Wien fully mature the seeds inside the fruits will be
found to be surrounded with a hard seed coat, within
which we have the minute seedling either containing its
store of food material in its fleshy seed leaves as in the
Pea or Bean, or surrounded with a supply of nutriment as
in the case of the Wheat grain. The food material is
usually largely starchy or oily, but in addition there
is a smaller amount of the essential nitrogenous material
so important to the plant and so essential to man. Of all
cereals Wheat is the richest in nitrogenous material, while
all leguminous seeds, thanks to the help which their
parent plants have received from the nitrogen fixing
bacteria in their root tubercles, have a great abundance
of organic nitrogen compounds. This is the cause of
their great nutritive value to mankind.

Chapter 6.

SEEDS AND SEEDLINGS.

Vitality and longevity of seeds. Conditions favouring germi-
nation. Seedlings. Variation. Natural and artificial selection.
Sports or mutations. Hybrids and the laws of heredity.

Ripe seeds are surrounded by a hard seed coat which is
more or less impervious and prevents complete drying up
of the embryonic plant within the seed. The more re-
sistant the coat the longer the seed can preserve its vzzality.
The seeds of some plants germinate almost immediately
they are mature, but most of them are adapted for and
require a period of rest. During that time it is necessary
to keep them dry and fairly cool. Under such conditions
they can preserve their power of germination for some
years, though a certain number even of resistant seeds will
die. It is also more difficult to germinate old seeds, owing
probably to the drying of the seed coat, as well as to
changes which have taken place in the living cells of the
seed. When kept dry little alteration takes place
internally, and the seed can remain dormant, practically
no loss of matter taking place by respiration. For what
length of time this suspension of animation can last is not
definitely ascertained, but we may say with certainty that
we have no proof that seeds which have lain dormant for
a thousand years or more, like those which have been taken
from Egyptian mummy cases, can be germinated. That
so-called mummy wheat and mummy peas have been
originally obtained from Egyptian mummies must be re-
garded as purely legendary. Accurate investigations

41

42

which have been made regarding the longevity of seeds
have proved that so far a hundred years may be taken as
the longest period for which seeds have been known to
retain their power of germination. The seeds which enjoy
such prolonged vitality belong to a restricted number of
Natural Orders of which the Pea Family is one, remark-
able like the others for the very resistant coat or testa with
which its seeds are covered. :

After the resting period, seeds when placed in favour-
able conditions commence to germinate. Moisture and a
warn. temperature favour germination. Water is absorbed
by the seed coat as a whole, or may be taken in at certain
points. In some small smooth seeds the outer layers of the
seed coat are mucilaginous, and when wetted swell up
considerably and become slimy. This is the case with
the seeds of the flax plant (linseed) and with those of the
cress. The probable reason for this special provision is to
cause the seeds to become fixed in the soil so that the
seed leaves can be more readily withdrawn from the seeds.
There are generally present on the inside of the seed coat
certain layers of cells which swell up rapidly when the
seeds absorb water, and probably aid in the splitting of
the seed coat, thus enabling the embryo to be gradually
withdrawn from the seed. Warmth, the other factor
essential to germination, is required for certain important
chemical changes which need to take place before germina-
tion can be effected. The food material stored in the seed
is largely of a solid nature and requires to undergo trans-
formation so that it can be dissolved in the cell-sap and
can be conducted to the growing root tip and to the
developing leaves. By certain fermentative changes starch
is converted into sugar, oil and organic nitrogencus com-
pounds are broken up and pass from cell to cell. In
some cases as in peas and beans the food material is stored
in the two fleshy seed-leaves or cotyledons, while in other
seeds like those of the melon, the onion, and in all our
cereals it is found in cells outside the young seedling and
requires to be absorbed by the latter before it can be made
use of at the growing points. The re-awakening of the
vital processes indicated by these internal changes is
marked by the commencement of respiration, that indis-
pensable accompaniment of all life, whether animal or
* Ewart, A. J. “On the Longevity of Seeds.” Proceedings of

the Royal Society of Victoria, 1908.

*

43

vegetable. We can easily demonstrate the occurrence of
this breathing process in germinating seeds if we allow
moistened seeds to germinate in a closed jar. The latter
‘will soon. lose all the oxygen it previously contained, and
carbonic acid will be found to have taken its place, as can
easily be seen by the fact that if a lighted taper is intro-
duced into the jar it will be immediately extinguished.
‘Respiration which is inseparable from active growth is a
process of slow combustion, and is always accompanied
by a rise in temperature. This can best be seen when a
large mass of seeds are germinating together as, for in-
stance, during the process of malting, when barley grains
in large heaps are passing through the early phases of ger-
mination, the starch they contain in the resting stage being
transformed into the sugar known as maltose. By plung-
ing the hand into a heap of barley which is undergoing
this change one can readily detect the heat which is being
evolved. The active need for oxygen by germinating
seeds will make us careful to keep our seed beds porous
and to prevent the soil above seeds from becoming caked
and therefore impervious to air.

Details of the various methods of germination will be
found in most botanical textbooks and need not be dealt
with in this lecture. J might, however, mention that whilst
most sceds germinate best in the absence of light, there are
some small seeds which should be sown superficially, as
light seems to be beneficial to their development. This is
particularly the case with the seeds of some grasses. An
important point in sowing seeds, apart from taking care to
give them the proper depth, is to ensure that the soil with
which they are covered is fine and friable, so that the
seedlings have no difficulty in forcing their way up to the
light. In the case of mustard, cress and other members of
the same family (Cruciferae), care should be taken not to
over-water the seedlings as they are liable to “ damp-off ”
owing to the attacks of a parasitic fungus, which will be
dealt with in a subsequent chapter. When dealing with
some seeds which germinate irregularly, possibly owing to
their age or on account of their having been stored during
the winter when they should have been sown immediately
after maturing, it may be advisable to keep the seedpan
going for a considerable time as a succession of seedlings
may be produced, all of which may be quite healthy and
normal.

44

Let us now pass on to consider the nature of the seed-
lings themselves. It was pointed out previously that
Darwin had shown the beneficial effect of cross-pollination
in the more vigorous development of the offspring. Another
important feature of seed reproduction as compared with
vegetative reproducton, particularly if the seeds are
the result of cross-fertilisation is the occurrence of a con-
siderable amount of variation. The more dissimilar the
parents the more varied are the offspring, and the greater
therefore the scope for the play of natural or artificial
selection.

The term variation has been used for two very different
phenomena noticeable when examining a large number of
plants or animals of the same species or kind. It is well
lnown that the offspring of any two parents all have some
different individual characteristics, and a close observation:
of a number of seedling plants will show us that though
they all have a oar resemblance, we find that - they
differ slightly one from another in size,in the shape and
texture of their leaves, and when they grow up in colour and
conformation of their flowers. Indeed, if we had carefully
examined the seeds from which they have grown we should
have found that the latter showed already a considerable
range of variation in shape and size, and possibly also in
colour. Such slight individual variations are always
found to fluctuate around a mean or average which we
may look upon as the general character of the species or
race. It is these slight individual variations which
Darwin regarded as so important in the evolution of new
forms, those least suited to the particular conditions of
life gradually dying out and leaving those which were fit
to survive. Natural selection operating in this way was
thought to have produced the innumerable forms which we
know as natural species. In the same way man, by making
a choice of the plants most suitable to his purposes, has
by artificial selection produced the strains and varieties
now cultivated.

Recently experiments have been carried out in which
the seeds of certain plants have been carefully graded and
the largest sown separately with a view to ascertaining
whether by such constant selection the seeds could be in-
definitely increased in size. This was, however, not found
to be possible, for though the average size of the seeds was
at first considerably raised, a limit was reached beyond

45

which it was impossible to increase the size of the grains.
By this process of selection the investigator had probably
succeeded in isolating a pure strain of the particular
variety with which he was experimenting, characterised by
a larger seed than the sample with which he started, which
was, no doubt, a mixture of races, some producing smaller
and some larger seeds. But once a pure line had been
obtained, though it showed slight fluctuations, yet it could
not be improved by selection of the extremes of these
fiuctuating variations.

We know, however, of other variations which arise from
time to time in all species of plants, and which are very
dilferent' from the fiuctuations as we may term those
described above. Sometimes we come across new forms
which differ considerably from the normal type in one or
more characters, as, for example, the cut-leaved varieties of
so many plants. In this case we do not find a series of
forms intermediate between the cut-leaved individual and
those with normal foliage. This second type of variation
which arises suddenly Darwin calied a sport, and he con-
sidered it to be of comparatively little importance in the
evolution of plants, as its very infrequent occurrence would
cause it to disapeede in nature by constant inter-crossing
with the more numerous normal forms. By artificial selec-
tion, however, man can perpetuate and establish these
sports, as has obviously been the case with many of those
forms which took the fancy of the horticulturist. Itisjtothis
kind of variation that we owe so many of our interesting
and peculiar forms of cultivated plants. Recently a Dutch
botanist, De Vries, has endeavoured to show that this
second kind of variation, which he calls muéatiun, to dis-
tinguish it from the former or fluctuating variation, is of
more frequent occurrence than had been supposed by
Darwin. He observed that a certain large group of pianis
of Evening Primrose which had established itself in a wild
condition in Holland showed a very considerable amount
of mutation, and his experiments and other observations
led him to the conclusion that at certain periods, possibly
owing to changed environment, plants passed into a phase
of mutation, during which numerous new sub-species or
races might arise. Interesting and important as De Vries’
experiments are, his case cannot be considered proved until
we know more about the previous history of the plants
which show such considerable mutation. At present we do

46

not know whether the Evening Primrose which he investi-
gated was a pure race or of hybrid origin. =

To horticulturists a knowledge of Aybrids is of the
ee importance, a vast number of new and interesting
forms being continually produced through hybridisation,
At one time it was thought that hybrids were invariably
intermediate between the two parental forms, and that they
were generally sterile, and could therefore not be repro-
duced by seed. Though this is sometimes the case,
particularly where the parents are of different species, it 1s
by no means the rule, and certainly not in the case of
hybrids between two different varieties. Our exact know-
ledge of hybrids dates from the careful experiments made
in the latter half of the nineteenth century by Gregor,
Mendel, of Brinn. Unfortunately, his important investi-
gations did not become generally known until the begin-
ning of the present century. Mendel’s first observations
were made on various varieties of the garden pea, and he
obtained the striking result that in crossing two different
strains the offspring were not of immediate type, but
generally inherited the characters of one of the parents in
their entirety. Thus in crossing varieties, of which one
had round and smooth and the other wrinkled seeds, he
obtained seeds all of which were round. He therefore
considered this to be u dominant character, while he called
wrinkledness recessive. But when the flowers of the hybrid
plant were subsequently fertilised with their own pollen,
the seeds they produced were not all round, some of them
were wrinkled like those of one of the grand parents. On
carefully counting the number of these recessive types, he
found that one in four had reverted to the wrinkled tvoe.
Moreover, he found that of the round seeds some were of
pure type, and when further cultivated always produced
round seeds, while others were of hybrid nature and these
always produced reversions to the parents which had been
originally crossed.

He was able, finally, to demonstrate that of the off-
spring of every hybrid when self-fertilised one quarter
reverted completely to the female parent, one quarter to
the male parent, while half of the offspring remained of
hybrid nature. These resembled the parent, which pos-
sessed the dominant character but preserved the recessive
character in a dormant or latent form as was shown by its
reappearance in a subsequent generation. The accurate

47

numerical results, which Mendel obtained, can be readily
explained on mathematical grounds by what we know of
the various combinations which are possible between egg-
cells and pollen-grains when differing in two characters.
Of course, if the parental forms differ in several characters
the possible combinations are much more numerous and the
numerical chances of complete reversion to the parent forms
is much smaller. On the other hand, while we obtain a
large number of hybrid forms which will show reversions
to the original parents, we shall find some new combina-
tions which are pure forms and therefore breed true. These
we can isolate from the rest by the rejection of all strains
which show reversion, and thus we can obtain new and
permanent varieties. This no doubt has been the means
adopted by plant breeders in the past from practical
experience, but thanks to Mendel and those who have fol-
lowed up the path shown us by his investigations, a scien-
tific basis has been laid io the practice of hybridisation.

In the course of these scientific enquiries some remark-
able facts have come to light. It has, for instance, been
discovered in crossing a white and a yellow variety of the
Marvel of Peru (Mirabilis) that the hybrid produced was
not intermediate in character, that is, of pale yellow
colour, neither was it like either of the parents, but of a
pink colour and marked with red stripes. The white form
must evidently have possessed some chemical factor which
changed the yellow colour into red, while the character
producing striping, which could not show itself in the
white form, became visible in the hybrid by reason of the
coloured sap. The offspring produced by self-fertilisation
of this hybrid were of twelve different kinds: five with
different shades or striping in yellow, five corresponding
forms in red, and two white forms, which, though re-
sembling each other externally, differed in constitution as
could be seen from their progeny.

A somewhat similar and equally remarkable result was
obtained by crossing two white Sweet Peas belonging to
the variety, Emily Henderson, the offspring having a
partly-coloured flower, red with blue keel, probably very
like the ancestral form. Of the two white forms evidently
each one contained a special factor, which combined with
that of the other white form caused the formation of a
coloured sap. The offspring of the coloured hybrid were
mostly coloured but in different degrees, while white forms

48

similar to the two original parents were also produced, the
two colour-producing factors separating out in these par-
ticular descendants. :

Though in a comparatively short space of time great
advances have been made in cur knowledge of the laws
of hybridisation and heredity, much remains still to be
learnt in this important branch of scientific knowledge
and the co-operation of the plant breeder with the scientific
investigator is much to be desired. The success already
attained in this country is of good augury for the future.

Further information on some of the subjects discussed
oe = lecture can be obtained from the followifig

Si—

Darwin, C. “ Origin of Species,” and “Animals and Plants
under domestication.”

De Vries, H. “ Species and Varieties, their Origin by Mutation.”
Chicago, 1905. Also “ The Mutation Theory,” London, 1909.

Bateson, W. “Mendel’s Principles of Heredity,’ Cambridge
University Press, 1900.

Punnett, R. C. “ Mendelism,” Macmillan & Bowes, Cambridge,
1905, I/-.

Chapter 7.

MALFORMATIONS AND INJURIES.

Malformations arising as sports or by malnutrition. Healing
of wounds. Injuries due to lightning, frost, etc. Harmful
effect of smoke and fog.

Abnormal development of various parts of plants
whether of vegetative or of reproductive organs may be
regarded as malformations. Many of these arise as sports
or mutations, and may be transmitted to the progeny of
the plant. In the case of the vegetative organs such occur-
rences as pitcher-shaped leaves have been noted in many
imstances. Leaves of this type are of no special advantage
to the plant, they may indeed be a disadvantage, causing
water to collect on the leaf surface and thus increasing the
lability to fungal attacks. Among ferns an excessive
development of the margin of the frond leads to the
development of so-called crested varieties, which being
pleasing to some tastes, have been perpetuated by artificial
selection. We have no knowledge of the causes of such
excessive development of the leaves, nor are we certain
of the origin of similar unnatural developments in stems,
such as appear in the case of “ fasciation” (from the Latin
fascis, a bundle). ‘This term is applied to the abnormal
development of the stem into a broad flattened structure,
often caused by the fusion of several stems, or by the union
of lateral shcots with the main stem. Superabundance of

49

50

food supply may in some cases be responsible for this
abnormal growth. Sometimes these forms occur spon-
taneously without apparent cause, and have been cultivated
by horticulturists, as in certain species of Echznocactus
and in the flowering shoots of the Coxcomb (Celosia
cristata). ‘

Almost all cultivated plants are known to produce
occasionally variegated leaves, and in some cases leaves
which are entirely devoid of green colour. In some in-
stances these have been traced to lack of nutrition. We
know of course that if seedlings are raised in the dark or
potato tubers are allowed to sprout in the absence of light,
the leaves will be of a sickly yellow nature, as light is
essential for the production of the green colouring matter
characteristic of foliage. Similarly, absence of iron salts
in the soil will prevent the formation of chlorophyll. It
has also been noticed that young shoots arising below a
graft in the case of hollies are often white in colour. This
must be regarded as due to an interference in the down-
ward conduction of material caused by the artificial union’
of the tissues at the graft. Some cases are also known
of wild plants with variegated foliage, which under culti-
vation in richer soil have developed normal green leaves.
It has also been possible by growing plants in the green-
house at higher temperatures to change the variegated
into green leaves, and to prevent the formation of further
variegated leaves in the case of some plants.

Some botanists have considered that a special substance
or “virus” is developed in certain parts of a leaf which
has prevented the formation of the green colouring matter ;
but as we do not know anything of the nature of such a
substance it is perhaps simpler to consider that the chloro-
phyll granules in certain portions of the leaf have re-
mained in a more youthful condition, in which they pro-
duce the pale yellow colour which always precedes the
green colouring matter. Apparently this stoppage in the
development occurs more commonly in plants under culti-
vation than in their natural condition. Variegated leaves
.are obviously less efficient as nutritive organs, and varie-
gated plants are therefore often less resistant than normal
forms. Their foliage is more easily affected by heat or
frost and the leaves are less long lived.

There are soine cases in which variegation is considered
to be a disease produced by bacterial action. The so-

51

called “mosaic disease” of Tobacco plants and of
Tomatoes is supposed to be due to this cause, and certain
experiments indicate that it is highly infectious.

Most gardeners will have come across examples of
abnormal formations in flowers. Often the sepals may
become leafy as in Jack-in-the-Green Primroses, sometimes
the petals have a leaf-like appearance as in the Green Rose.
To the flower-lover these transformations are more curious
than beautiful, but to the botanist they are of considerable
interest as cases of reversion. For we must assume that
the various Horal organs are all modifications of leaves
which have become adapted to the special function of
reproduction, and in such foliaceous developments we may
see a retrogression to a more primitive type of leaf. More
rarely, but still occasionally, we may find stamens or pistil
becoming transformed into vegetative leaves; the latter is
often the case in the flowers of the Double Cherry. In both
cases sterility of the flower is caused. In other sports the
outermost leaves of the flower, the calyx, may become
coloured and delicate in texture like the corolla. This is
the condition in the hose-in-hose variety of the Polyanthus.
More frequently it is the inner leaves of the flower, the
stamens, that become petaloid. The “doubling” of
flowers is sometimes found to take place in wild plants
when transplanted into garden soil. In all probability
the tendency to doubling is independent of the cultivation
of the plant, but the rich nutrition which the plant receives
accentuates the effect. Certainly double varieties to be
kept in perfect condition require an abundant supply of
food, and are liable to degenerate when grown in poor soil.
Degenerate is perhaps not a good term to use, as the plant
is in a more perfect condition from the reproductive point
of view when the stamens and pistil are normally
develqped and fertile than when they are transformed into
showy petals. In that case the flower is generally com-
riteeely sterile, though it may sometimes retain a few
serviceable stamens or a receptive seed vessel.

From the doubling of flowers such as the Rose, we must,
distinguish the doubling of the so-called “flowers” of
Daisies, Chrysanthemums, and other members of the Com-
posites. In this Family the apparent flower is really a
head of small flowerlets or florets closely crowded to-
gether. These are often of two kinds, showy ray florets
and small central disk-florets usually of a different colour.

52

When these “flowers” are said to double, it is by trans-
formation of the tubular flowers of the centre into con-
spicuous strap-shaped flowers, like those of the margin of
the inflorescence. In that case they need not necessarily
become sterile, for in many Composites like the Dandelion,
all the florets are normally of the ray-floret type. Another
curious freak met with in some of the Composite when
they receive abundant nutrition is the development of some
of the ray-florets into small heads of flowers on stalks of
their own, so that the large central head is surrounded by
a number of lesser ones. This is the condition in the
Hen-and-Chicken Daisies, and similar modifications are
found in other plants.

A not uncommon abnormality in gardens is the
development of a terminal flower of large dimensions and
regular shaped at the top of the flowering spike of the
Foxglove; in the Snap Dragon, too, flowers showing radial
symmetry instead of the usual two-lipped condition may
be formed. A plant may indeed bear nothing but
“peloric” flowers as these are called.

Monstrosities may also occur in the development of
fruits. Double-fruited oranges with two whorls of carpels
(pegs) one inside the other are occasionally found, while
in one variety known as Buddha’s fingers, the various
carpels are only united below and taper off above into
finely divided pod-like segments.

“While most of these malformations are due to little
understood intcrnal causes, many cases of excessive devel-
opment of plant structures are produced as the response
to irritation. Thus, numerous kinds of galls and tumours
may be developed by animal and vegetable parasites.
insects and fungi, which injure the tissues and cause them
to swell or grow more vigorously. These will be dealt
with in later chapters.

Plants like animals have the power of protecting them-
selves against external injury and of healing any wounds
that are caused thereby. The most usual method is by the
formation of layers of cork, which will protect any exposed
part from the atmosphere, thereby preventing the excessive
loss of moisture through the wounded surface and also
reducing the chances of invasion of the tissues by disease
or decay producing organisms. This development of a
resistant layer of cork is due to the active growth of the
living cells of the plant in the exposed region. This active

53

growth requiring energy, which plants gain by their
respiratory process, we find that the healing of wounds
is marked in plants as in animals by a local rise in tem-
perature. The wound iisstie which is developed at first
is soft and termed callus; it is that excrescence of cells
which is produced as we have seen around the base of a
shoot in making cuttings. Within this callus layer imper-
vious cork is then formed and this is usually sufficient for
the healing of herbaceous plants or soft tissues. In trees
and shrubs, however, if the stem or branches are deeply
cut, the wound will subsequently be covered up by woody
layers as well. The formation of these liquified tissues
commences at the margin of the wound and they gradu-
ally cover over the entire wounded area. It is thus that
branches broken at their base become covered up'in nature,
and are found as knots buried in the wood. In cultivation,
of course, much larger branches are often amputated than
usually break off in nature, and though their stumps will
ultimately be covered up, it takes a considerable time in
the case of a thick branch, and before the wound is closed
up there is plenty of time for the wood exposed by the
cut to commence to decay. To prevent this from taking
place, it is advisable to coat the surface of the stump
immediately after the removal of the branch, with tar or
some antiseptic substance, which will prevent the entrance
of bacteria or of other harmful fungi.

Injury by lightning, if it is deep and considerable, may
be beyond the power of the plant to repair; but. where
only the outer layers have been Sermeete in which case
a single longitudinal fissure is generally found running
vertically down the side of the stem, callus and cork
usually heal up the gap completely.

* Frost is in all probability one of the most frequent of
external factors causing injury to plants. Though we
cannot as a rule prevent the havoc wrought by frost, it is
not without interest to note what is the effect of freezing
upon plant structures. It is particularly the young grow-
ing parts of plants which are nipped by frost, while the
mature leaves are often undamaged. On the other hand,
somewhat fleshy plants like the Nasturtium, do not resist
frost very weli. The cell sap containing a good many
organic acids and often sugar in solution, does not freeze
readily. The denser the sap is, the less danger there is
of its being frozen. As a protection against the effect

54

of frost, we find that with a lowering of temperature some
of the water from the cell-sap passes into the spaces
which exist between the cells in mature tissues. Thus the
sap becomes more concentrated and less liable to injury,
while the water which may become frozen in the inter-
cellular spaces does no damage if the latter are of fair
size. If they are small, then the expansion of the water
in freezing may tear the tissues asunder and thus injure
them. In the growing parts of the plant there are tiny
intercellular spaces, so that little water can be passed
out of the cells, and consequently the sap itself may be
frozen, in which case the protoplasm, the living substance
of the cells, is killed and the tissues become blackened.
A very sudden frost is always more harmful than a long
spell of frost preceded by a gradual lowering of the
atmospheric temperature, as plants are able to prepare
themselves by a condensation of their cell sap. It is the
same with recovery from frost. Plants in a frozen con-
dition may recover readily if they are slowly thawed.
When exposed to bright sunshine on a frosty morning they
may be permanently injured as often happens with Wall-
flowers. It is therefore well to shade frozen plants from
the direct rays of the sun, so that they are thawed less
rapidly. We must remember that in bright sunshine the
pores of the leaves open and the plants are rapidly trans-
piring, and if this takes place while the ground is still
frozen and the roots are unable to absorb water, the con-
sequences may be serious.

Those of us who have laboured in town gardens are
only too familiar with disappointment due to general sick-
liness of some of our plants and to the harmful effect
of a smoky atmosphere. Worse in winter than in summer
it is nevertheless pronounced even in the latter, and records
taken show that there are quite a number of gloomy days
in July and August, and that air pollution is quite appre-
ciable during these months. From actual measurements
made during the month of July, it has been calculated
that if we draw a circle of a mile radius around the Town
Hall of Manchester, 195 tons of impurities would be col-
lected during the month from this area, and of this
100 tons would consist of soot or other insoluble matter ;*
* See First Annual Report of the Sanitary Committee of the City

of Manchester on “ The Work of the Air Pollution Advisory

Board.” March, 1915.

55

and Manchester is by no means the worst town in- this
respect. It is gratifying to know that efforts are being
inade in many of our industrial centres to understand, and
let us hope, also to cope with this problem of air pollution,
and all such movements deserve the hearty support and
co-operation of gardeners and flower-lovers.

From what has been said in earlier chapters, it will be
clear that bright sunshine is the most important factor
in the nutrition of plants, as it is only in the presence
of light that the green chlorophyll of the leaves is able
to form starch, the food so essential for the further growth
of the plant. A murky atmosphere therefore lowers con-
siderably this prime nutritive activity of the leaves. If
in addition to ihat the leaf, as is always the case with
town evergreens, has become coated with a more or less
opaque layer of sooty matter, the normal power of leaf
nutrition is still further reduced. What wonder then that
our town and suburban gardens suffer many serious losses.
It is only hardy plants which can survive these adverse
conditions. Conifers are usually regarded as fairly
resistant plants, and so they are to drought or cold, but
smoke is most harmful to them as their breathing pores or
stomata are situated at the base of a depression on the
surface of the leaf, and this pit becomes partiallv filled
with soot and the pore is thus blocked. In ordinary leaves
these pores are usually on the protected under surface and
are therefore not endangered by the smoke, but in the case
of the needle-shaped leaves of the conifers they are freely
exposed to smoke and fog. As a consequence we find
that conifers do not thrive in the neighbourhood of our
industrial towns. A blackening of leaves is, however, not
the only noticeable effect of air pollution on vegetation.
We observe, particularly on evergreens, that many of the
leaves have brown spots or dead margins, and in both cases
this is traceable to the acidity of the atmosphere, which
kills the tissues, particularly those near the stomata or
breathing pores. It is particularly zz winter fogs that
the air becomes very acid, owing to the sulphur contained
in the coal, and our own sense organs enable us easily to
detect the presence of the sulphuric acid. Even in summer
the acidity of the atmosphere causes the discolouration
and the early fall of the leaves of trees in our parks and
town gardens.

50

It is obvious that the rain washes this acid into the
soil, so that not only the leaves but the roots also suffer
from its effect. Rain water collected in or near our towns
is always slightly acid, and a great difference can be
observed in the growth of seedlings, some of which are
watered with ordinary rain water and others with rain
water in which the acid has been neutralised. A consider-
able number of striking experiments on this point have
been made by Mr. Ruston, of the Leeds University. The
lesson they teach us is that the soil of gardens or allot-
ments near our large towns should be well treated with
lime, so as to neutralise as far as possible the acidity which
is caused by the rain washing the smoky atmosphere.
A consideration of the fact that this smokiness and acidity
of the air is greater in winter than summer suggests that
a dressing with lime in the spring before growth recom-
mences would be especially beneficial. Experience has
also taught us that many biennials are better wintered
under glass than when left in the open, even when they
readily withstand the frost. A glance at the condition
of the lights of a cold frame will show us what a coating
of soot we have prevented from collecting on the leaves.
A really penetrating fog, however, will get into frames
and greenhouses, and may often do considerable damage
to the delicate blooms of Orchids and to many other
flowers, besides causing in some plants the leaves to become
discoloured or to fall.*

* See F. W. Oliver on “ The Effects of Urban Fog on Culti-
vated Plants.” Journal of the Royal Horticultural Society,
vol. xili, 1891.

Chapter 8.

FUNGI AS A CAUSE OF DISEASE IN PLANTS.

Common fungi, mushrooms, toadstools and moulds. How they
live. Saprophytes and Parasites. The ‘‘Damping off’ disease
of Seedlings caused by the fungus Pythium. Life Story, means
of spreading and living in the soil, precautionary measures.

In preceding chapters you have become acquainted with
the normal uses of the parts of healthy green plants. We
are now to consider plants in disease and especially the
disturbances of the structure and functions of plants pro-
duced by parasitic fungi. It is obviously impossible
within the limits of a few chapters to deal with the whole
subject of diseases of plants caused by fungi, but by a
consideration of some of the diseases most common in
fields, gardens and greenhouses, the main features of the
subject may be illustrated. By this treatment you will
not only become more familiar with the causes of these
particular diseases, but also with the sort of* precautions
and remedial measures it is worth while to adopt in rela-
tion to the various types of plant disease.

Practically all parts of plants are subject to disease.
In some cases we have immediate and complete destruc-
tion of the plant or part attacked, but in others the death
may be delayed or the plant may support the parasite in-
definitely. We have diseases of seedlings, diseases of
roots, of stems and of leaves, and a very large class of

57

58

plant diseases comprises those which attack fruits. In
this lecture we shall consider the most common disease of
seedlings which is known as the “ Damping off” disease.

Before dealing in detail with this, however, let me
recall some of the more essential features in the structure,
nutrition and life history of the more common fungi, facts
with which you are no doubt, to some extent, already
familiar. In any of the fields and lanes around our large
towns, especially in autumn, it is possible to find examples
of the larger fungi such as mushrooms and _ toadstools.
Whilst the colour and form of the part which we see above
ground are often striking, in reality this is only the repro-
ductive part of the toadstool or mushroom plant, just as
much as the flower and fruit are the reproductive parts of
the higher plant. It is true that the mushroom as we
generally see it, appears above ground, sheds its spores
and decays all within a few weeks; but the vegetative part
of the plant lives in the ground for a very considerable
time before it enters on the reproductive phase.

The bricks of so-called mushroom spawn contain
quantities of fine interlacing threads of fungus, in manure
which are really the vegetative part of the mushroom.
Under the microscope these filaments are seen to consist
of long branched tubes. These tubes are divided up into
chambers or cells by cross partitions, and each cell is
lined with the jelly-like semi-transparent living substance,
protoplasm. Within this are drops of water and oil as
well as certain denser granules, but filaments of fungi
never contain the green colouring matter found in higher
plants. The protoplasm is the living part of the cell,
and food material is taken in from the soil or manure
through the protecting cell membrane. Such food
material often appears stored in the tubes as drops of oil.

Although the filaments of the vegetating mushroom.
plant are not bound together into a complex plant body
like the cells of flowering plants, yet the loosely inter-
woven threads behave as a whole, and after weeks or
months of vegetative growth give rise to the definite fruit
bodies consisting of numberless aggregated filaments.
‘These reproductive bodies usually possess a stalk bearing
an expanded cap from the under side of which project the
radiating gills. Upon the surface of the gills are borne
large numbers of minute round or oval spores, Each
spore is really a single tiny cell so small that it can only

59

be seen if very highly magnified. If, however, the stalk
is removed from a not over-ripe mushroom, and the latter
placed on a sheet of white paper, in the course of a few
hours so many spores are shed from the gills that a print
of the gills is produced on the paper. This print consists
of myriads of spores which have fallen like dust from the
gills. The spores are the reproductive cells of the fungus
and, falling to the ground, they grow out to produce tiny
filaments. They soon begin to absorb water and food
substances from decaying vegetable matter in the soil and
manure, and so the vegetable life of the plant is carried
on.
Many fungi are much more minute than the mush-
room, indeed are so small that they can only be pro-
perly studied under the microscope. As an example of
such a fungus we have Mucor, the common white mould
which often appears on damp bread or dung. If a piece
of bread is kept moist for a few days under a bell-jar
in a warm place the mould soon appears as a dense
growth of fungus covering the surface of the bread. Erect
silky threads stand up from the surface like a miniature
forest. Microscopic examination shows that the fungus
consists of two sorts of filaments: fine ones which branch
and ramify in all directions forming a felt on and in the
substratum, and the coarser erect ones which stand free.
The finer network on the substratum is the vegetative part
of this fungus, while the erect coarser aerial threads are the
reproductive organs. The individual branching filaments
are very similar to those described for the mushroom, ex-
cept that in this case there are no cross walls dividing up
the tubes. When the aerial filaments have grown for a few
days there appears at the extreme tip of each a minute
round swelling like an inflated ball about the size of a pin
head. The protoplasmic contents of this globular body or
‘spore-case soon become divided up to form a large number
of spores, and then it has the appearance of a miniature
ball full of shot. As the wall of the spore-case becomes
dry it breaks, scattering its spores into the air as a fine
dust in all directions; the spores of Mucor and, indeed,
of many fungi, are very light and easily carried by the
slightest current of air. When placed in water the spores
germinate in a few hours, the protoplasm within absorbs
water, the spore wall bulges in one place and grows out
forming a fine filament which, given suitable food

60

material, rapidly grows and branches to form a dense felt
such as we saw on the moist bread. : :

We have seen that suitable food material for this
fungus is provided by bread, dung, etc.; that is to say by
dead or decaying plant or animal matter, just as the
mushroom or toadstool lives on similar dead organic
remains in the ground. Fungi, which in this way draw
all their nutriment from the rotting remains of plants or
animals, are known as saprophytes, and although they
are unable to injure living plants they readily feed on
their remains after death.

Fungi, unlike green plants,.possess no chlorophyll
and are therefore unable to construct their own carbon
compounds such as starch and sugar from the carbonic
acid of the air. They, however, take in such complex
organic substances ready-made from the remains of plants
which have previously manufactured them, and saprophy-
tic fungi play an important part in Nature in living upon
and decomposing the dead organic remains of plants and
animals. The saprophytes as a rule cannot attack living
plants, and therefore do not give rise to plant diseases.

A large number of ‘fungi, however, are unable to live
even upon decaying plant remains, and derive nutriment
from the cells of living plants. Such fungi are sarasites,
and not only do they require to take in the carbon com-
pounds of their food material ready-made, but they can
only take their food substances from living cells. Now
it is clear that in obtaining substances forcibly from plants
while still alive, such parasitic fungi rob the plants
attacked of materials which otherwise would have been
used for their own life and growth, and may harm them
more directly in doing so. A plant which harbours a
parasitic fungus is spoken of as the host plant, and in
most cases the host suffers injury not only because it is
robbed of substance by the fungus, but also because the
work of the particular parts of the plant infected are
seriously interfered with. We shall see for example how
the stores of food material in roots like turnips, or tubers
like potatoes, are raided by fungi, and on the other hand
how mildews and rusts prevent leaves from manufacturing
food supplies.

Most of the larger and more prominent fungi are sapro-
phytes and live, as we have seen, on decaying organic
matter. Some of them, however, are parasites. The

61

bracket fungi and the honey agaric, for instance, live as
parasites on trees. The vegetative filaments of these
ungi ramify in the tissues of the tree, gaining entrance
often through wounds, and only the reproductive bodies
are produced outside. But very many diseases of plants
are caused by fungi which, like the saprophytic mould
Mucor, are so small that they can only be studied with
the aid of the microscope. Such a minute parasitic fungus
called Pythium de Baryanum often causes the “ Damping-
off ” of seedlings.

The “Damping-off” disease very frequently appears
when the seeds of many plants are sown too thickly and
grown under conditions which are too warm and moist.
Young seedlings begin to die off in patches and soon
ptesent a very characteristic rotten appearance. The disease
1s very commonly met with in gardens and greenhouses,
occurring in seed-beds of all kinds a few days after the
germination of the seeds. It is most abundant in very
wet weather and when the beds are kept too shaded and
-badly ventilated; crowding of seedlings also favours the

rogress of the disease. At first a few individual seed-
ings are attacked at or near the surface of the ground,
the tissues in this region having a water-soaked appear-
ance. Soon the cells collapse, and being unable to
support the weight of the cotyledons the seedlings
fall prostrate. Those immediately around are similarly
affected, the disease spreading through the seed-bed in
ever widening circles until practically the whole may be
destroyed. When the plants fall over they become pale
and rotten, and soon the whole bed is reduced to a moist
mass of decaying seedlings. This mass is seen, on closer
examination, to be covered with the very delicate threads
of a fungus somewhat like the mould on bread. Often
the filaments can be seen to have spread from the first
diseased seedlings to the outer parts of the circle. These
filaments most often belong to the fungus Pythium, which
is so called because of its ability to produce rotting. The
fungus continues to grow in the dying seedlings, and the
filaments may form a dense felt over the whole seed-bed.

If a seedling is examined under the microscope shortly
after it is attacked, the collapse of the tissues just above
the ground is seen to be due’ to the destruction of the
cells at that spot. This destruction is caused by the fila-
ments of the fungus which at this stage are seldom visible

62

to the naked eye. They are, however, almost exactly like
those found on the mouldy bread, and they run both
between and within the dead and dying cells. They can
also be traced into and between adjoining living cells,
and when the seedling falls prostrate the remainder of
the plant is soon invaded, for it is kept moist by contact
with the-damp soil. As the tissues further decay the
filaments of the fungus spread out over the soil reaching
across to other seedlings which then succumb in the same
sort of way. The fungus attacks the cells of the seed-
lings by first extracting water, then boring through the
cell walls and finally killing the living protoplasm and
feeding upon the cell contents. Since the fungus grows
‘very rapidly a seedling may be reduced to a putrid mass
in a few hours. :

The life story of Pythium is typical of the most
thorough and destructive of plant parasites. The fila-
ments. at first grow in the air spaces between the cells, but
later they enter and kill the living. protoplasm. The
spread of the fungus at first is due to the growth over the
soil from its earlier victims to healthy seedlings which
are attacked in turn; but soon a more rapid means of
spreading comes into play. When a seedling has become
thoroughly infected by the filaments of the fungus the
ends of many of the branches of the latter begin to swell
out into globular bodies very like the spore-cases of Mucor,
only much smaller. These swollen bodies are full of
protoplasm and serve a somewhat similar purpose to those
m Mucor. They are really special spore-cases, but in
further development they differ considerably from those
of Mucor. In that fungus the spore-cases were borne on
filaments standing erect from the surface, the spores being
shed into the air. In Pythium, however, the spore-cases
are submerged in the film of moisture around the decay-
ing seedling, and the different behaviour probably depends
on this. When the spore case is mature a short tube grows
from the side and swells to form a globular body with a
very thin wall. Into this the whole of the protoplasm
passes from the spore-case; it then rapidly divides into
g or 10 small rather oval masses of protoplasm, which
begin to writhe and wriggle within the thin vesicle. This
soon bursts, liberating the minute writhing spores which
swim about in the slight film of water on the soil or sur-
face of the’seedling. When examined under high mag-

63

nification each of the colourless swimming spores is’ seen
iobe furnished with two excessively fine hair-like threads
of protoplasm which, by lashing the water incessantly,
bring about the movement of the spore. The active move-
ments continue for twenty minutes or half an hour, then
the spore comes to rest, rounds off and withdraws the
whips of protoplasm. If favourably situated on a seed-
ling it sends out a fine filament which bores its way
through the outer wall of one of the cells and grows into
the interior. It has been proved that the tip of the fila-
ment is able to do this, because it secretes a substance
which enables it to digest its way through cell walls
pretty much as gastric juice renders our food materials
soluble. Once within the cells of the victim the fungus
branches and grows rapidly from cell to cell, spreading
destruction as it goes and deriving nourishment from .the
product of this destruction. In a diseased seed-bed these
processes are going on so continuously that the fungus
soon produces many thousands of the motile spores which
are able to attack new seedlings, thus accelerating the
progress of the disease. In addition, as we have already
seen, even the threads of the fungus grow across the soil
between the seedlings and directly produce new infection.

It is well ‘known that “Damping off” will recur with
even greater’ virulence in seed-beds which showed the
disease the previous season. This is explained by the
existence of yet another chapter in the life history of
Pythium, by which resting spores are formed that can
pass the winter in the soil. At a late stage in the decay
of diseased seedlings many threads of the fungus give
rise to the resting spores. These possess the power of
lying dormant over a long period, and in this resemble
the seeds of higher plants, though they only consist of a
single thick-walled cell. When, however, the temperature
is favourable and the conditions are sufficiently moist, the
thick coat bursts and a fungal filament grows out which
soon attacks any seedling that may be near, producing
spore-cases and motile spores as before. The resting
spores are produced in myriads in a diseased seed-bed ;
in fact, on one occasion, I estimated the prcsence of upwards
of half a million in a single diseased seedling observed
under the microscope. As the seedlings rot these spores
all find their way into the soil where they spend the
winter.

64

In ‘considering methods for preventing this disease it
is necessary to make use of the facts which are known of
the means of spreading of the fungus causing the disease.
We have seen that once a seed-bed is infected the fungus
rapidly spreads, first by an abundant growth of filaments
over the soil from seedling to seedling, and secondly by
means of spores which swim in the water in the soil. Any
treatment to be successful must check the vegetative
growth of fungus in the soil and also the production of
spores. It has been shown that the fungus only grows in
this way in warm, very moist, badly ventilated seed-beds.
Moreover, the effect of excessive moisture and lack of
sufficient air is two-fold. Not only do such conditions
favour the growth of Pythiuim, but these verv conditions
tend to produce weak drawn seedlings which of course
are more susceptible to attack. Very careful attention to
cultural conditions will therefore make it possible to ward
off and, in most cases, entirely prevent the disease. The
seedlings should never be overcrowded, and they should
be given as much air and as little water as possible. If
through inattention to these conditions a few of the seed-
lings in a bed should show signs of disease they should
be immediately removed and burned, and more air and
less water given to those that remain. In any treatment
it is also necessary to prevent the infection of the seed-bed
at the beginning of the season. This infection invariably
arises from the presence of resting spores fn the soil.
Naturally, then, soil known to have previously produced
the disease should not be used, but since the resting spores
are present in most garden soils it is wisest to endeavour
to get rid of them. They can be killed by partially
sterilising the soil by heating with steam or with boiling
water. It has been shown in an earlier lecture that the
See sterilisation of soil is valuable for other reasons.

y adopting the practice it is possible to eliminate the
resting cells note only of Pythium but also of other
harmful organisms.

Chapter 9.

DISEASES OF ROOTS AND TUBERS.

Fingers-and-Toes of Turnips, Cabbages, etc. Habits and life
history of the fungus. Remedial treatment. ‘‘ Liming” of the
soil, etc. Similar diseases, e.g., Black Scab or Wart Disease of
Potatoes, ‘‘Spongy Scab,”’ etc.

The disease commonly known as Fingers-and-Toes,
or Club-Root disease, attacks ali cruciferous crops, includ-
ing Cabbages, Cauliflowers, Brussels Sprouts, Kohl Rabi,
Broccoli, Turnips and Swedes. It is produced by a fungus
known as Plasmodiophora Brassicae, and causes consider-
able damage both in the gardens and fields in many dis-
tricts of England; its ravages are even greater on the
Continent, and it is also abundant in America.

The general appearance of the disease is well known.
Plants, which are attacked, Show irregular warty swellings
‘on the root, and in bad cases the whole root may simply
consist of a mass of these excrescences. The disease can
often be recognised in its earliest stages on pulling up
young cabbages for transplanting. If such seedlings are
planted they prove practically useless, for the disease
simply increases at thie expense of the roots, any leaves
‘developed being feeble and unable to provide for healthy
growth.

65

66

Seedling cabbages, which show by the nodular swell-
ings that they are attacked by Club-Root, are often
exposed for sale. Of course it is inadvisable to plant
such seedlings, for not only is it extremely unlikely that
they will produce healthy plants, but when they decay in
the ground they will infect the soil. If any plants of the
cabbage family are grow in the same soil within two or
three years of this infection, thev will almost certainly
suffer from the disease to an even greater extent.

The disease with which we are dealing should not be
confused with the gall-like swellings on the roots of
cabbages and turnips, sometimes caused by the larva of a
beetle that lives within the cavity .of the swollen tissue.
Eelworms, as well as certain bacteria, are also able to
produce small swellings on the roots of many plants, in-
cluding Crucifers. It is a little unfortunate that fequently
any swellings on roots of cruciferous plants are spoken of
as Club-Root without reference to the causal organism.
Since, however, in the vast majority of such cases the slime
fungus, Plasmodiophora Brassicae, is the cause, I am re-
stricting the name Club-Root or Fuingers-and-Toes to
the disease of which it is the cause. Whilst the life history
of Pythium and of the other fungi mentioned in the last
chapter is typical the Club-Root fungus differs somewhat
im life story and mode of nutrition.

When one of the diseased roots, say of turnip, is cut
across and examined under a microscope it is found that
the tissues of the root are altogether abnormal, many of
the cells being strangely altered. In the healthv root of
the turnip we can distinguish an outer band of softer
tissues surrounding a central core containing a certain
number of woody elements arranged like the spokes of a
wheel. Between the last-named are broad wedges of
softer cells which are packed with the reserve food material
that is stored in the root of the turnip in the form of
sugar. The root increases in thickness by the growth and
division of a layer of cells near the outer part of the
central core. When attacked by the fungus of which we
‘are speaking, the whole machinery for the growth of the
root 1s, as it were, thrown out of gear; cells which normally
would produce woody tissue simplv give rise to giant thin-
walled cells, and the result is an excessive production of
thin-walled tissues. This also occurs in the position of
the tissues which normally serve for the conveyance and

67

“storage of the food materials manufactured in the ‘leaves.
The reserve supplies are thus tapped by the fungus, with
the result that the quantity of sugar stored by the grow-
ing root rapidly diminishes. In such instances many of
the cells of the root are enormously enlarged and differs
as regards their contents from those of a healthy root. °

The healthy cells are lined by the colourless, jelly-hke,
living protoplasm with its nucleus and contain cell-sap rich
in sugar. Diseased cells differ from these in several re-
spects. They are generally much larger; protoplasm is
present as before, but it looks frothy and in it can be seen
granular masses of other slimy substance which is the
protoplasm of the fungus slowly absorbing that of its
victim. It is a remarkable fact that the early effect of the
parasite upon the cell is to stimulate it to enlarge and
even to divide, and thus obtain more food material from
the adjoining cells, and this is ultimately used by the
unbidden guest. Gradually the protoplasm of the fungus
increases in size until the whole of the protoplasm of the
cell disappears. Then the fungus undergoes certain
changes and soon the cell is seen to be filled with a large
number of tiny round bodies. These are the spores of the
fungus, and as they only measure one fifteen thousandth
of an inch in diameter they can only be seen when very
highly magnified. A single diseased cell will contain at
least 100,000 of these spores, and since the diseased part of
a turnip, for example, contains many thousands of such
cells it is easy to see that in one such root millions of the
spores of the fungus are produced.

If a very small piece of a diseased root is broken up in
water myriads of the spores are liberated from the cells.
After a few hours very remarkable changes can be observed
to take place in these. Each tiny spore swells somewhat
and then bursts producing a small hole in one side of the
colourless membrane. Gradually the living protoplasm
within squeezes its way through the aperture thus formed.
As soon as it is free it begins to wriggle and move about
as a minute speck of living protoplasm. The protoplasm
at one end is drawn out into a fine hair, and the
lashing of the water by this hair causes the movement of
the minute organism. After a time the, movements of
these specks of protoplasm become more sluggish and soon
they only creep about by slow movements, first pushing
one part of the protoplasm forward and dragging the

68

rest after it. While it is unknown how long they can
move about and live in the soil, they can certainly do so
for many days. :

In the field and garden the spores of this fungus are
liberated into the soil by the decay of the diseased tissues
of the infected roots. If diseased roots are left in the
ground for any length of time the decay takes place
rapidly and is generally accompanied by an offensive
odour. The spores pass into the soil and there remain
until conditions are favourable for their germination. If
liberated during the summer or early autumn they pro-
bably germinate at once, but if later it is likelv that they
remain in the spore condition over the winter and
germinate in the warmer spring days. The smallest
quantity of water is sufficient to allow the minute specks
of protoplasm which are liberated, to swim about. If
young cabbages, turnips, etc. are grown in the infected
ground the organism soon gains entrance to the younger
roots and sets. up the disturbances described above. When-
ever sceds of these cruciferous plants are sown in soil
known to contain the spores of this fungus the swellings
typical of the disease appear on the young roots in a few
weeks.

We have seen that this organism differs from a fungus
like Py¢hium in several important respects. At no stage
in its life history does it possess filaments as do the vast
majority of fungi. It passes through the vegetative stage
of its life as a naked speck of protoplasm living and grow-
ing inside the protoplasm of the cells of a root. Unlike
Pythium it does not kill the cells of its host plant out-
right, but rather stimulates them to enlarge. divide and
draw food supplies to them which it then utilises. | Like
Pythium, however, it rests in the form of spores in the
decaying roots and in the soil; it also passes part of its
life as a naked free-swimming speck of protoplasm, but
in order to complete its life cycle it must enter the living
cells of the root of a cruciferous plant.

In considering methods of preventing the attack of this
organism it is necessary, as in all such cases of plant
‘disease, to bear in mind the habits and life historv of the
parasite. As has already been shown, a single diseased
turmp or cabbage, if left to rot will liberate many
millions of spores into the soil. Some of these doubtless
dic, but many remain as a source of infection for future

69.

crops. Most important therefore, of all the means of com-
batting this disease are the measures taken to prevent the
spores,of the fungus reaching and infecting the soil. It
would seem to be obvious to anyone who has observed the
damage this disease can cause, that the greatest care ought
to be taken to collect and destroy by burning all diseased
roots. Again and again this disease occurs both in fields
and gardens, and in almost every case diseased roots can
be seen left lying about, to rot and infect the land. Quite
recently the writer watched a farmer carting the least
diseased portion of a crop of swedes, scarcely a root of
which had escaped attack. The most badlv diseased
plants, already putrid and rotten, were being left in heaps —
to rot on the field, and then doubtless would be ploughed
into the soil. In such cases not only does the soil of the
field or part of the garden where diseased plants were
grown become infected, but the fungus is carried on the
boots of workers, on tools, or if, as so frequentlv happens,
diseased roots are thrown on to the rubbish heap to rot,
the disease ultimately gets spread over the whole field or
garden. Diseased turnips are fed to animals, and though
there is no direct evidence, it is quite possible that the
spores pass uninjured through the bodies of the animals
and return to the soil in farmyard manure. The greatest
care ought to be exercised to destroy all diseased material.
Burning is the safest plan or, failing that. it should be
ponorest into a heap and thoroughly mixed with quick-
me.

The Club-Root disease has becn noticed to be par-
ticularly abundant on soils which are badly drained, at all
sour, or deficient in lime; the disease is practically
unknown in chalky or limestone soils. As direct methods
of treatment therefore, the drainage of the soil should be
improved, and a most drastic system of “ liming ” adopted.
Wherever the disease has been prevalent it is best to treat
the ground immediately with freshly slacked quicklime at
the rate of $ to 1 cwt. per square rod, spreading it evenly
over the ground and digging it in. Freshly prepared
guicklime is much better for this purpose than ground
lime, which varies considerably in the amount of active
quicklime which it contains. Gas lime, though valuable
as an insecticide, is a poor substitute for quicklime, since
it contains a much smaller proportion of active lime than
either of the other forms.

79

The use of certain acid artificial manures and, indeed,
also of ordinary stable manure in excess, tends to make the
soil acid, and this favours the growth of the Fingers-and-
Toes fungus. In infected soil, therefore, Basic Slag, or
other alkaline artificials, might be advantageously used
in addition to treatment with lime. ;

Needless to say, even after such treatment as outlined
above, ground known to be infected should not be planted
with cruciferous crops for at least three years. Other
vegetables, including potatoes, can of course be grown
without any danger, since they are not attacked by the
fungus. The soil of beds in which seedling cabbages
are raised should be partially sterilised as for the “ Damp-
ing-off ” Disease, and also should contain a fair sprinkling
of lime.

By strict care in disposing of rubbish, by improving
the drainage of the soil, and by regularly dressing the
ground with quicklime, it should be possible to do some-
thing towards eradicating this pest. A recent visit to some
allotments within the Manchester area afforded evidence
of this possibility. Some of the plots had not grown a
healthy cabbage for two or three years, while adjoining
plots never show a sign of the disease. The infected
plots were badly drained, the soil was sour, liming had
been tried, but in too small quantities. On the other hand
the adjoining plots had been thoroughly and ,egularly
limed, were well drained, not soured by over manuring,
and they had therefore always borne healthy crops. It is
needless to add that on the infected plots diseased
cabbages had been pulled up and the roots simply thrown
aside to rot and preve a further source of infection. It
might be well if allotment societies had some stringent
rules for dealing with the spread of such diseases via the
rubbish heap.

Brief reference must now be made to some diseases
of the potato tuber that present certain features in
common with the Fingers-and-Toes disease. The
Wart Disease, or Black Scab of Potatoes, is caused by
Synchytrium endobioticum. It is notoriously prevalent in
the districts round large towns, and the restrictions of the!
Board of Agriculture have rendered its symptoms well
known. In the early stages small warty swellings appear
in the “eyes” or buds of the potato, or thev may even
occur on the stem near the ground level. These warts

71

rapidly increase in size, and several often run together
till the potato becomes a mass of excrescences. Plants
which are attacked often grow larger and bear green.
leaves for a longer time than healthy plants. ie

The fungus causing the disease, like the Plasmodio-
phora, lives in the cells of the potato buds as minute.
spécks of naked protoplasm. In this case the presence of:
the fungus causes the invaded cells to enlarge, but renders
them incapable of further division. On the other hand
the healthy cells around are stimulated to such active
growth and division that the warts are soon produced.
Infection always occurs by motile spores from the soil,
which can only penetrate the healthy cells of a potato
in. the young condition. Later the potato forms a skin
of corky cells through which the fungus cannot
penetrate. After growing at the expense of the
mvaded cells the fungus ultimately occupies the whole
of the cell cavity, and then taking on a thick, very re-
sistant wall, forms a resting spore. With the decay of
the warty tubers these resting spores find their way
into the soil and may remain there as a source of infection
for many years. When they germinate after an
exceptionally long period of rest the thick wall bursts and
liberates large numbers of actively moving spores, each
possessed of a single whip of protoplasm to propel it.
These are the spores which infect new potato tubers. So
far no method of successfully treating this disease by
adding chemicals to the soil has been devised, but certain
varieties of potato are much less susceptible to the disease
than others, and it is advisable to grow these if the
disease is present. Indeed, it is now compulsory for every
person growing potatoes to do this and to follow certain
other stringent regulations in areas where the Wart
Disease is prevalent.*

In addition to the Wart Disease there are several other
scab diseases of potato tubers which, owing to a certain
degree of similarity, may at times be confused with the
Wart Disease. The Black Speck, or Violet Rhizoctonia
Disease, caused by Rhizoctonia Violacea, can be detected
by the minute size of the blackish warts which can be

* See Board of Agriculture leaflet 105 and Wart Disease
Order, 1914, to be obtained free from 4, Whitehall Place,
London, S.W.

72

rubbed off the surface of the tuber without causing any
evident injury. The spongy or powdery scab, caused by
Spongospora solani, a fungus akin to the Wart Disease
organism, is easily recognised by the light brown colour
of the rather powdery warts, or cankers, it produces.
Brown Scab, or Ordinary Scab, of the tuber also shows as
light brown warts, but usually evenly distributed over the
surface of the potato. It may be due to a variety of
causes, in which mechanical irritation by gritty
particles of soil and infection by definite parasitic
fungi probably play an important part. None of these
diseases, however, are so destructive as the Wart Disease;
further particulars of them may be obtained from the
leaflets issued by the Board of Agriculture.t It is quite
certain that these scab-like injuries are caused by different
fungi, but it is not surprising that the resulting diseases
present certain superficial similarities. In each case the
presence of a parasitic fungus irritates the cells of a
potato, causing them to actively divide and thus give rise
to the warts. Even in the Fingers-and-Toes disease
the result is similar, and in all the cases with which we
have dealt the attack arisés from the presence of the
organism in the soil. By, therefore, adopting precautions
to prevent the infection of the soil, and by appropriate
treatment similar to that advised for the Club-Root
disease, such diseases can, in some measure, be controlled.

+ See Leaflets 171, 137 and 232.

Chapter 10.

A DISEASE OF LEAVES, SHOOTS, AND TUBERS.

The Late-Blight disease of potatoes. Symptoms and means of
spreading. Diseased tubers and the wintering of the fungus.
Treatment of the disease, spraying and the use of resistant
varieties,

The Late-Blight of potatoes caused by Phytophthora
infestans is so common and wide-spread that it is generally
spoken of as the Potato Disease. Historically, it is cer-
tainly the oldest of the potato diseases, and it is probably
responsible for more loss to potato growers than all the
other diseases of the potato put together. The malady
first made its appearance in Barone about 1840, and the
Trish Potato Famine of 1845 was the result of a very severe
attack of Phytophthora, Whilst its ravages are seldom
so severe nowadays as they were’ in that year, the disease
is never altogether absent. In most seasons it makes a
first appearance about the middle of July, and from that
date to the end of the potato season it is often difficult
to find a field of potatoes wholly free from attack.
Farmers and growers generally, however, do not seem to
be seriously concerned so long as the tubers are not badly
diseased. It is an important fact, however, that wherever

73

74

the disease is present at all, there is a risk of a proportion
of the tubers becoming diseased; further, the attack of
the fungus on the leaves invariably results in a diminished
crop. The first outbreak of the disease in the season is
usually dependent upon the weather conditions. In this
country a few days of close, moist weather about the
middle of July are often followed by an outbreak of the
disease, and, if such weather persists, an epidemic fre-
quently results.

The disease is easily detected by the characteristic spots
on the leaves. These often appear near the margin or tip
of the leaf and they usually present a dull, water-soaked
appearance. As the disease progresses the infected spots
increase in size, become darker in colour, and may rapidly
involve the whole leaf-blade, extending from the latter
to the petiole and even to the stem. Ultimately the Icaves
hang limp and the whole plant becomes moist and
blackened. Such diseased plants emit a peculiar, offensive
odour which is very characteristic of the disease. If the
attack is a severe one, it is generally found on lifting the
tubers that they also are diseased. If in the earliest stages
of the disease, the dark spots on the leaves are closely
examined on the under surface, a delicate, white, mildew-
like growth will be evident round the margin of the dis-
coloured spot. This growth follows the advancing
margin as the diseased area increases in size. Microscopic
examination shows the mildew appearance to be produced
by filaments of the. fungus Phytophthora, which project
from the lower surface of the leaf. The growth as a rule
has a somewhat powdery appearance produced by large
numbers of spore-like bodies, which readily fall free from
the filaments bearing them. Thin sections through a
diseased spot show that at this stage the threads of fungus
run between and within the cells of the leaf. Many of
these cells are shrivelled and dead, and the contents
having turned brown, they contrast strangely with the
taut living cells lined with colourless protoplasm and
green granules of chlorophyll.

In many places branches from the filaments of the
fungus pass out through the air pores or stomata, which
are especially abundant on the lower surface of the leaf.
These aerial filaments become branched and bear small
lemon-shaped bodies, called conidia, on the tips of the
branches. The production of such conidia enables the

75

fungus to spread with amazing rapidity; when mature
they are easily detached, and being extremely small the
slightest current of air carries them in large numbers to
the leaves of adjoining plants. The conidia are only
produced when the air is moist and sultry. Alighting
upon the healthy leaf of the potato, under such atmospheric
conditions they germinate in one of two ways. Either,
each conidium behaves as a spore-case and liberates motile
spores, or it germinates by sending out a fine filament.
By'the former method it opens at the tip and liberates
about eight small motile spores which swim about vigor-
ously in the thin film of water on the surface of the leaf.
As in the case of Pythzum, the fungus causing “ Damping-
off” of seedlings, these motile spores are each furnished
with two hairs of protoplasm, by means of which they are
enabled to move in the water. After a short period the
motile spores come to rest, withdraw the minute hairs and
give rise to a short filament which either grows through
one of the stomata into the leaf, or dissolves a way
through the outer skin of the leaf, and so starts a disease
spot. In other cases the conidium simply sends out a fine
tube and produces an infection by the same methods as
do the motile spores. After infection the fungus, feeding
on the cells of the, host, grows rapidly, in the form of
branched threads, within the tissues of the leaf. The
conidia are extremely small, being less than one-
thousandth of an inch in length. Enormous numbers are
produced from a single diseased area of a leaf, and,
further, under suitable conditions the fungus grows and
reproduces itself very rapidly. It is therefore easy to see
why, in weather conditions favourable to its growth, the
fungus spreads in a few days over whole fields of potatoes.

In the earlier chapters, it has been shown that a leaf,
such as that of the potato, is really a complicated machine
for the manufacture of food material. Many of the cells
contain chlorophyll, the green colouring matter which, in
the presence of sunlight, utilises carbon-di-oxide obtained
from the air to produce organic chemical compounds like
sugar and starch. The complex organic substances
elaborated in the leaves are passed on to the stem and
ultimately stored in the potato tubers in the form of starch.
Thus it is obvious that by injuring the leaves, the potato
disease reduces the weight of tubers obtained from a given
plant, and the earlier the attack occurs in the season, the

76

more serious is this effect on the crop. _In most years
potato plants which are attacked by Late-Blight, die down
much earlier than they would normally. .
When a field has been badly attacked by Late-Blight
the tubers almost invariably become infected. This either
takes place by spores being washed down into the soil
and directly infecting tubers near the surface, or by the
growth of the fungus down the tissues of the dying stalks
into the tubers. Potato tubers infected by this disease
can readily be recognised by the purplish discoloration
and rather sunken appearance of the skin in diseased
parts. These features are caused by the brown colour
of the cells immediately under the skin. Living filaments
of the fungus infest the discoloured cells, and if such
tubers are stored the fungus slowly invades healthy cells,
often producing the so-called dry rot of the tuber. In
addition to this, the presence of the fungus in the potatoes
renders them extremely susceptible to the attacks of
secondary rotting fungi and also of bacteria. In some
such cases the potato shows the characters of the winter
rot disease caused by Fusarium solani. The tuber
gradually shrivels and at a late stage small white tufts
of that fungus appear over the sunken parts. In other
cases the potato becomes changed into a moist, putrid
mass infested by bacteria and mites. These diseases are
extremely likely to appear in stored tubers that are, to
any degree infected by Phytophthora. Much, however,
can be done to prevent healthy tubers developing these
rots by careful attention to the method of storage. It
is well known that the favourable conditions for the
growth of most fungi are a moist, warm atmosphere
and absence of light. If, therefore, the storage clamp is
not carefully made, it is possible that very favourable
conditions for the growth of fungi will be provided. If
the tubers are stored moist, or if they are too closely
covered, and if no provision is made for adequate access
of air to all parts of the “pie,” the temperature will rise
and harmful fungi will become rampant. To avoid these
dangers then, potatoes should be stored in a dry, well-
ventilated shed ; or, failing that, the “ pie” or clamp should
be in a dry situation and well ventilated. By such pre-
cautions the conditions are rendered unfavourable for the
growth of rotting organisms; and even though a few of
the tubers are diseased the trouble is then unlikely to

77

spread to the healthy tubers. Too frequently, however,
it is found on opening a clamp that the whole store is a
putrifying mass, simply owing to neglect of the essential
facts of ventilation and dryness.

We have seen that the Phytophthora spreads through-
out the summer months with amazing rapidity by air-
borne conidia. If, however, these are to produce infection
they must germinate immediately, for being only provided
with a thin wall they cannot resist drying up or frost.
They, therefore, never serve for carrying over the fungus
from one season to the next.

The question then arises as to whether this fungus, like
Pythium, produces resting spores that are able to survive
the winter in the soil. The most diligent search of many
investigators has failed to show that such resistant resting
spores are ever produced in Nature. On the other hand,
Dr. Pethybridge, in Dublin recently, has found it possible
to obtain resting spores of this fungus by growing it
artificially on a medium consisting of cooked Quaker
Oats stiffened with a gelatinous substance called agar.
The fungus forms a dense, white growth on the surface of
this, and after some months produces the resting spores
within the medium. These are quite characteristic resting
spores and possess a thick resistant wall. Interesting as
these artificially-produced resting spores are from the
scientific point of view, the fact that their presence has
never been demonstrated in Nature, renders it necessary
to consider other means by which the fungus may survive
the winter. It has been repeatedly shown that diseased
tubers kept over winter in the open may give rise to a
growth of Phytophthora bearing spores in the warm, moist
days of early summer. During the colder months fila-
ments of the fungus in diseased tubers grow very.slowly
indeed, especially if the tubers are kept dry. If, how-
ever, such tubers are kept warm and moist the fungus
rapidly extends through the whole tuber and even pro-
duces a network of threads bearing powdery conidia on
the outside. Such conidia, carried in the air, infect leaves
of potato plants in the vicinity, and in this way start an
epidemic. It has frequently been noticed that portions
of a potato field, near to old potato pits or refuse heaps,
have been the starting points for the disease. This mode
of initial infection is, however, scarcely enough to account
for the very wide-spread frequency of the disease.

78

Recent investigations, especially in Ireland and
America, have done much to make it clear how most
epidemics of “Late-Blight” originate. We have seen
that the potato tubers very frequently become infected by
the fungus. If badly diseased tubers are used for seed
it has been found that they either wholly decay in the
ground or occasionally send up a few perfectly healthy
shoots. Recently, however, very careful field experiments
have shown that if only slightly diseased tubers are
planted a much larger proportion of them send up shoots,
and under certain weather conditions some of these become
diseased. It has also been proved that the disease arises
in such young shoots by the growth into them of the
fungus from the slightly diseased tuber. Dr. Melhus has
confirmed De Bary’s earlier observations that such
diseased shoots occasionally reach the surface where
conidia of the fungus are produced. These conidia are
then carried to the leaves of adjoining plants, which very
soon show typical disease spots. Upon these, more conidia
arise and so starting with a single diseased shoot as a
centre the disease spreads over the whole field in a few
warm moist days.

In taking measures to prevent the Late-Blight disease,
it is necessary,‘as in other plant diseases caused by fungi,
to consider the means by which the fungus spreads during
the season, and also the manner in which it is carried
over to the following year. It has been shown that this
disease spreads very rapidly throughout the summer, by
means of air-borne conidia that infect the leaves on which
they alight. If the germination of conidia can be pre-
vented then the spread of the disease will be controlled.
This may be accomplished by spraying with a fungicide,
that is, a poisonous substance which is harmful to germi-
nating spores. It has been found by experience that one
of the most powerful mixtures for preventing infection by
filaments from germinating spores of fungi is Bordeaux
mixture. This consists of a solution of Blue stone, that
is copper sulphate, to which slaked lime or lime water is
added. The lime is added to prevent the copper sulphate
injuring the leaves of the plants treated, and also to make
the mixture form a fine film on the surface of the leaves.

Bordeaux mixture should always be freshly prepared,
and the home-made article is much better than any adver-
tised preparations. It is a fairly simple matter to make

79

the mixture, and full directions for its preparation and
use are given by the Board of Agriculture in Leaflet 23.
Once the disease is evident in the field it is too late to
attempt to control it by spraying, but the first application
should be made some time before the first outbreak of
disease is expected, z.¢., about the end of June or beginning
of July in most districts. The mixture should be well
stirred before use and both surfaces of the leaves of the
plants should be thoroughly sprayed. Three or four
sprayings at intervals of a fortnight or ten days should
generally be sufficient to prevent any serious epidemic.
The cost of such sprayings is relatively small, working
out in normal times to about 25/- an acre for three applica-
tions (ze, 2d. per square rod). Apart even from the
prevention of Late-Blight spraying has been proved bene-
cial to the plant; for sprayed plants-usually retain their
leaves at least a fortnight longer than unsprayed. In
certain districts of America where spraying is regularly
adopted the increased profit is usually from £3 to £5 per
acre.

The subject of the resistance of different varieties of
potatoes to this disease has received the attention of ex-
perimental growers for many years. Darwin himself was
for some years interested in the matter, and as a result,
various species of wild potatoes growing in South America
were experimented with and used for crossing. At the
present time, therefore, something can be done to avoid
the disastrous effects of the Late-Blight disease by grow-
ing varieties known to be resistant to this disease. It must
be said, however, that though a variety is highly resistant
in one locality it may be equally susceptible in another
district where the conditions of environment differ. It is
also notorious that disease-proof varieties lose their resist-
ance after a few years, either because the plant, in time,
deviates from the original type, or because the fungus
becomes slightly modified so that it is able to break down
the resistance. Needless to say, a variety which is resistant
to Late-Blight disease will not necessarily be resistant
to the Wart disease and other diseases of the potato.
Since the Late-Blight disease usually affects the crop most
seriously towards the end of the season, it follows that
early varieties do not suffer from the disease to the same
extent as the main-crop and later varieties. By, there-
fote, selecting varieties proved to be resistant in recent

80

years in the given district, and by choosing early varieties
it is possible to avoid this disease to some extent.

It is even more important, however, to endeavour to
prevent the first outbreak of the disease in a given area.
All possible sources of infection such as diseased tubers
and stalks should be carefully collected and burned at
the end of the season. Since the disease is propagated
from year to year in slightly diseased tubers, seed potatoes
should only be used from a crop which never showed
any sign of the disease even on the leaves.

We have seen that tubers become infected either, by
the growth of fungus down the stalks, or by spores being
washed into the soil. If, therefore, the attack occurs late
in the season all haulms should be removed and destroyed
before the tubers are lifted; and infection from the latter
source may be prevented, to some extent, by seeing that
the tubers near the surface are well covered with soil.
This should be done as soon as possible after the haulm
has died down, for apart from the-risks of the Phytoph-
thora disease, tubers left too. long in the ground are
more liable to be attacked later by rots in the store.
The tubers should be stored under dry conditions in such
a way that air has free access to them, and diseased
haulms should never be used for covering the “pie” or
clamp. ; ;

; if potato plants are sprayed three or four times at
fortnightly intervals, beginning when they are about six
inches high, it. should usually be possible to prevent any
serious epidemic in a given field or garden. Much can
also be done to prevent infection by the immediate destruc-
tion of diseased tubers and haulms, and by only using
seed potatoes from a perfectly healthy crop. By such

recautionary measures, in addition to the use of varieties,
ee by experience to be resistant in the district, it
should be possible to reduce the effects of this disease to
a minimum.

Chapter 11.

DISEASES OF LEAVES AND SHOOTS (contd.).

Black-Leg or Wilt Disease of Asters. Tomato-leaf rust or
mould Leaf-blotch of Cucumber. Mildews of Roses, Goose-
berries, etc.

The Black-Leg or wilt disease of asters is extremely
prevalent wherever asters are grown. It is caused by a
species of Phytophthora, which differs in several respects
from Phytophthora infestans, the cause of the Late-Blight
disease of potatoes. The disease may manifest itself at
any stage in the life of the aster, but the initial attack is
always upon the seedlings. In severe cases the seedlings
collapse, as in the “Damping-off” disease caused by
Pythium, but more frequently they harbour the fungus
without showing any outward sign of disease at this stage.
Such seedlings may wilt and collapse when transplanted,
but many succeed in surviving even to the flowering stage
without showing external signs of injury. In the latter
cases the plants succumb quite suddenly, almost without
warning. The leaves wilt, hang limp and flaccid, and in
a few days the whole plant shrivels and dies. Even,
though affected plants show little direct sign of the disease
until the wilt sets in, they are often much dwarfed in size
and produce fewer flowering branches than healthy plants.
The fact that this disease may be present in apparently
healthy asters, which only wilt after they come into flower,

81

82

renders it all the more objectionable and difficult to com-
bat. When wilted plants are pulled up, the parts of the
stem a few inches above the level of the ground are found
to be blackened and often decaying ; these symptoms have
given the disease its popular name.

The parasitic fungus which causes the aster disease,
unlike Phytophthora infestans of the potato, never attacks
the leaves directly but enters the plant from the soil
through the root of the seedling. Once within the young
plant it may, either grow slowly not seriously interfering
with the work of the vital parts of the seedling, or it may
extend rapidly through these and cause immediate col-
lapse. Mucroscopic examination of the blackened portion
of the stem shows the distribution of the fungus in the
tissues. The cells of the rind as well as the spaces between
them are occupied by filaments of the fungus. The
former are not so rapidly killed by the fungus as are
those of the potato plant by Phytophthora infestans.
The ultimate collapse of the plant, however, is brought
about by the extension of the fungus to the vascular
cylinder of the stem, and the consequent reduction of the
supply of water to the leaves.

In moist weather the fungus gives rise to conidia on the
diseased stem. Unlike those of Phytophthora infestans
they are only produced under water, and do not become
detached from the filaments bearing them. A few hours
after their first appearance they burst at the apex liberat-
ing about fourteen motile spores which, after swimming
about for a short time, come to rest and are able to infect
other seedlings.

in studying this disease the writer has up to the present
failed to find any resting spores of the Phytophthora in
the tissues of diseased asters, and attempts to obtain such
spores by artificial cultivation have been equally unsuc-
cessful. Experience however has shown quite conclusively
that the disease originates each season from the presence
of the fungus in the soil, especially of the seed bed; and
the ease with which it may be cultivated on dead organic
substances suggests that it may be able to persist in a
vegetative condition in the rich soil used for seed-beds.
On the other hand, further research may prove the exist-
ence of resting spores.

It is often stated that this disease is caused by a species
of Fusarium, which is frequently found on the decaying

83

tissues of diseased asters. The writer,* however, has
proved that a species of Phytophthora is the primary cause
of the Black-Leg disease and that Fusarium only appears
later as a saprophyte living upon tissues previously killed
by the other fungus. The Phytophthora is always present
in the diseased asters even in the earliest stages; it has
been isolated and grown separately from all other organ-
isms, and the disease has been produced artificially from
such growths. Other organisms found on diseased asters,
including Fusarium, are unable to infect and produce wilt-
ing in healthy plants; such saprophytes only succeed after
the tissues have been killed by the Phytophthora.

The insidious nature of this disease renders it extremely
difficult to deal with, for it is usually almost impossible
to detect diseased plants until the wilting actually sets in.
Whilst sufficient scientific trials of remedial measures have
not yet been made to warrant promise of complete success
in all cases still some precautions may be indicated.
The soil of the seed-bed should be partially sterilised
by steam or hot water and asters should not be planted in
ground which produced diseased plants the previous sea-
son. All diseased material should be removed and
burned and the infested soil thoroughly drenched with
Formalin (1 pint per to galls. water) and covered with
sacking for a few days. Many growers raise aster seed-
lings on hot-beds of stable manure, but these conditions
should be avoided since they are much more favourable
to the fungus than when alkaline artificial manures are
used.

As a first example of the common diseases of leaves,
the Tomato-leaf rust or mould caused by Cladosporium
fuluum may next be considered. This is more strictly a
disease of the leaves than is the Late-Blight of potato,
but even in this case the whole tomato plant suffers
because of the attack on the leaves. The disease, which
has been known in this country for over a quarter of a
century, first appears on the leaves in the form of small,
yellowish spots. These gradually increase in size and
often run together, and the under surface of the leaf
in the diseased areas becomes covered with a rusty, velvety
growth. This is the reproductive part of the fungus pro-

** Annals of Applied Biology.” Vol. II., Nos. 2 and 3, July,
IQ15, DP. 125-137.

84

ducing the disease. The vegetative filaments ramifying
in the tissues of the leaf give rise to branches which pass
out through the stomata and stand erect from the surface
of the leaf. Food material is absorbed from the host
plant by the vegetative filaments and is passed on to the
reproductive branches. These produce large numbers of
small two-celled spores, which being readily detached are
disseminated by air currents. Under suitable conditions
they germinate immediately on the surface of tomato
leaves; each of the cells of the spore may send out
a filament, which growing through one of the stomata,
into the interior of the leaf produces a new infection.
The careless watering of slightly diseased plants may
carry spores to healthy leaves and thus spread the disease
which is highly infectious.

The disease only occurs in this country on tomatoes
grown under glass, and though the spores described above
are the only kind known, it is certain that they are able to
survive the winter in the greenhouse and give rise to infec-
tion the following season. If the houses are kept sufh-
ciently well ventilated the disease seldom assumes serious
importance; neglect of this precaution may, on the other
hand, prove disastrous. It is possible to prevent any bad
outbreak by regularly spraying with a Bordeaux mixture
of half the usual strength, until the young fruit is begin-
ning to set, when, owing to the poisonous character of
Bordeaux mixture, Liver of Sulphur (1 oz. per 4 galls. of
water) should be substituted. In order to prevent the
disease recurring, all diseased leaves should be picked off
immediately and dropped into a vessel containing a
solution of copper sulphate; at the end of the season
remains of plants should be burned and the greenhouse
disinfected in the manner to be described in connection
with the Cucumber-leaf disease.

Several other diseases of the tomato plant, which are
readily distinguished from the Leaf-rust described above,
may be mentioned. The Sleepy disease caused by
Fusarium lycopersici, is a wilt disease somewhat similar
in symptoms to the Black-Leg of asters although caused
by a very different fungus. |The Black-Stripe disease
shows itself on the fruit and also sometimes on the stem
as.a.dark, velvety growth of fungus. This disease, caused
by Macrosporium solani, should not be confused with the
Bacteriosis of the tomato in which blackening of the parts

85

attacked is also produced. In the latter case, Bacellus
solanacearum is the cause. The Septoria disease of the
leaves produced by Septoria lycopersici is the only other
malady likely to be confused with the leaf-rust, but in
this case the spots are always small and concentric, and
the spores are produced in minute black bodies scattered
over the patches.

The Leaf-Blotch of the Cucumber caused by Cercospora
melonis is another destructive disease of leaves. It was
first described in this country by Dr. M. C. Cooke in 1896,
and since then it has become so wide-spread that many
horticulturists have been compelled to cease growing
cucumbers. Once the fungus appears in greenhouses it is
extremely difficult to eradicate. The leaves are most often
attacked, but the fungus frequently spreads to the fruit.
An outbreak of the disease is usually first indicated by
the appearance of pale, scattered spots on the leaves.
These spots gradually increase in size, become brown,
and the leaves are so rapidly killed that death of the
plant may soon result.

Microscopic examination shows that, in the region of
the spots, the tissues of the leaf are occupied by filaments
of the fungus, that the chlorophyll bodies are pale
in colour and many of the cells of the leaf are shrivelling
and dying. From the fungus within the leaf stiff branched
filaments grow out and stand more or less erect from the
surface. These aerial threads are dark in colour, and
bear numbers of large conidia which fall free as they
mature. Each somewhat spindle-shaped conidium is
divided into about seven or eight cells, and may germi-
nate in a warm, moist atmosphere by sending out fila-
mentous germ-tubes from any of the cells. The germ-
tubes may then produce new infections by growing through
the stomata into the healthy tissues of the leaf. This
spreading by means of conidia that germinate imme-
diately, takes place very rapidly under favourable con-
ditions. In addition to producing large conidia on the
leaves the fungus is said to grow as a saprophyte on decay-
ing leaves and damp soil producing myriads of smaller
spores which also rapidly spread the disease. If the con-
ditions are unfavourable to the germination of the spores,
and especially at the end of the season, the large conidia
persist alive as resting spores. Filaments of the fungus
are also able to pass into a resting condition in the soil,

86

only to begin active growth with the production of spores,
when the conditions are once more favourable. In this
way the disease survives in greenhouses from one season
to the next, and once a house is infected the disease is
almost certain to recur year after year unless precautions
are taken to thoroughly disinfect the soil and all parts of
the house.
The practice of disinfecting greenhouses with burning
sulphur is largely employed in some districts. Whilst
this is an excellent preventative of insect pests and
of certain mildews, it 1s useless against the Leaf-Blotch
of cucumbers. The writer recently established this with
certainty. A large house which had been badly infected
with the disease was cleared out and thoroughly disin-
fected with burning sulphur. A few fragments of
diseased leaves and fruits were then collected from the
soil of this greenhouse and brought into the laboratory.
Spores of the Cercospora, taken from this material, ger-
minated in water in a few hours and cultures of the fungus
were readily obtained on suitable media. From this
experiment it is clear that the spores and resting filaments
of Cercospora melonis remain uninjured in houses dis-
infected by burning sulphur alone. Probably more certain
results would be obtained by spraying the house
thoroughly, and also drenching the soil with Formalin
(1 pint per 20 galls.of water). ‘The Board of Agriculture *
-recommends the use of Jeyes fluid (1 oz. per gall. of water)
for this purpose, but the writer has no experience of this
as a disinfectant against parasitic fungi. Needless to say
all diseased material should be destroyed by burning. It
is possible to control this disease to some extent by spray-
ing with Liver of Sulphur (potassium sulphide) two ounces
in three gallons of water, to which two ounces of soft soap.
is added to facilitate the sticking of the solution to the
leaves, which should be thoroughly wetted by the spray.
The disease is only prevalent where cucumbers are forced
under glass, but if the ventilation is carefully regulated
much can be done to reduce the possibility of an epidemic.
A most effective way of combating it is by growing
disease-resisting varieties of the cucumber, of which there
are a number on the market. The most reliable of these
have rather coarse, hard foliage, but unfortunately the

* Leaflet No. 76.

87

fruit is not so highly valued as that of some of the more
susceptible varieties. At the same time it ought to be
quite possible to produce, by crossing, a variety which
combines the qualities of disease-resistance and those most
acceptable in the fruit.

Many diseases of leaves belong to the class spoken of
as mildews and are caused by fungi belonging to the
family Evysiphacee. The Rose mildew, caused by
Sphaerotheca pannosa, is one of the most familiar of these
diseases and is typical of the class. The diseased leaves
become covered with a white, powdery growth of the
fungus, which causes them to curl up and die. The
fungus grows mainly on the surface of the leaves,
swollen branches from the filaments acting as sucker-like
organs of attachment, while other branches penetrate the
outer walls of the epidermal cells and swell out within the
surface cells in the form of bladder-like haustoria. These
absorb food material from the cells occupied as well ‘as
from the adjoining cells. |The substances thus absorbed
from the living leaf are passed on to the vegetative fila-
ments of the fungus outside which is thus enabled to grow
and multiply. Erect threads arise from the creeping fila-
ments on the. surface and bear single chains of colourless,
thin-walled, oval spores or conidia. Myriads of these are
produced on the surface of mildewed leaves and give to
the latter the characteristic powdery appearance. Being
extremely’ light they are easily spread by the wind to
healthy leaves and produce new infections throughout the
summer. Later in the season, the production of conidia
gradually gives place to another means of spreading. The
fungus on the shrivelling leaves and also on the twigs
assumes a brown colour and gives rise to minute dark
bodies about the size of a pin head. This is the winter or
resting stage of the fungus. Each of the minute dark
bodies or perithecia is furnished with a thick wall made
up of a number of closely interwoven fungal threads: and
within this resistant coat a club-shaped spore-case (ascus)
containing eight oval spores, gradually develops. When
mature the perithecium splits across and the ascus is
squeezed out. The latter then opens at the apex and the
oval spores are forcibly ejected. These are able to infect
leaves with the mildew and serve to start the disease afresh
each spring. As has been stated above, many destructive
mildews are caused by fungi having a life history closely

88

corresponding to that of the Rose mildew. The mildews
of apple, chrysanthemums, peas, hops, strawberries, and
gooseberries are all common in this country. The
American Gooseberry mildew is the most destructive of
these, and growers of gooseberries have to observe certain
restrictions prescribed by the Government in regard to this
disease. Full particulars of the symptoms and treatment
may be obtained from the Board of Agriculture.*

Preventive measures against most of these mildews are
similar. The main facts to consider are the means of
spreading during the season and the method of carrying
over from autumn to spring. Sulphur, or one of its com-
pounds, is the most effective fungicide for use in combat-
ting mildews. Plants may be dusted with flowers: of
sulphur or sprayed with liver of sulphur. Sprays, con-
taining lime and sulphur, are now extensively used with
success, especially in America, against the mildews of
hops and gooseberries.t| The treatment with sulphur, or
its compounds, however, is only effective against the
summer stage of the fungus and other measures must be
taken to destroy the resting fungus. The only satis-
factory method of dealing with this is by the removal
and immediate destruction of all branches and leaves
bearing perithecia. Otherwise no amount of spraying will
prevent the disease recurring year after year.

* Leaflet No. 105.

+ Eyre and Salmon have recently recommended the use of
ammonium sulphide against American Gooseberry Mildew.
“Journal Board Agric.,” Feb., 1916.

Chapter 12.

DISEASES OF LEAVES (contd.).

Rusts. The Wheat Rust. Life Story of the fungus, relation to
the Barberry. The Mint Rust and Hollyhock Rust as examples
of other Rust diseases.

_ The Rusts constitute a very important group of plant
diseases, which includes some of the most widely dis-
tributed and highly destructive of parasitic fungi. The
grain-producing plants and grasses of our fields, the
plants of our gardens and greenhouses, and even forest
and fruit trees are attacked by members of the rust
family of fungi. Of all plant diseases they are in many
respects the most remarkable, as they are also the most
difficult to combat. Whilst the nature and the life history
of the fungi causing them have only been known for a
relatively short period, the blighting effects caused by out-
breaks of these maladies are referred to by writers of the
remotest antiquity. Throughout the centuries wheat rusts
have been responsible for great damage wherever wheat
has been grown, and at the present time the loss caused
to the wheat crop alone throughout the world amounts to
many millions of pounds annually.

89

go

Since the life story of the fungus causing the Black
Rust of wheat (Puccinia graminis) is typical of many
rusts, it will be described in some detail. The disease
usually appears in the summer on the leaves and stalks of
the growing wheat in the form of yellowish streaks,
at first shining through the epidermis. The orange
patches on the leaves consist of the developing uredo-
spores of the fungus causing the disease. As this pro-
gresses, the epidermis is ruptured and the bright orange
uredospores are liberated as a fine powder. These spores
bring- about the rapid spread of the malady from one
plant to another throughout the summer months, so that in
a short space of time whole fields of wheat may be rusted.
As the disease pustules increase in number and size the
leaves lose colour and become paler day by day and a
badly diseased field may thus give the appearance of
premature ripening.

With the advance of the season the streaks on the
plants gradually change colour from orange almost to
black, and although for a time some uredospores continue
to arise in the pustules, mingled with them are now dark,
brown spores of a different appearance. Finally, in the
autumn, none but the dark spores are produced in the
pustules. These spores are the teleutospores which serve
for carrying the fungus over to the next season. Before
considering in detail the form and behaviour of the uredo-
spores and teleutospores, reference must be made to an
opinion which prevailed at least a hundred and fifty years
ago with regard to the wheat rust.

It was strongly held by farmers that the presence of
bushes of the common barberry near to wheat fields, bore
some relation to outbreaks of rust, but no very definite
reasons were given for this belief. The farmers of Massa-
chusetts were so convinced of the connection between the
barberry plant and the wheat rust, that a law was passed
in 1755 compelling the destruction of all the barberry
bushes. About a century ago Sir Joseph Banks suggested
that a certain bright yellow fungus common on the bar-
berry might be the same as that causing the rust of wheat.
This fungus on the barberry, however, when examined
microscopically was so unlike the fungus on wheat that
for a long time the relationship was not understood.

If a leaf of wheat is cut across through one of the
yellow streaks and examined under the microscope, it is

gi

found that the pustule is formed by the rupture of the
epidermis of the leaf. Large numbers of the orange
uredospores arise under the skin and burst through to the
outside. The torn edges of the epidermis thus act as a
boundary to the pustule. The filaments of the fungus
ramify in the tissues of the leaf often sending special
branches called haustoria into the living cells from which
nutriment is thus absorbed. The reproductive branches
of the fungus grow outwards and accumulate in rows
under the skin of the leaf. The round, orange uredo-
spores are produced on the tips of such branches, and
are liberated when the epidermis bursts under the pres-
sure. Each uredospore is a single, oval cell about a
thousandth of an inch in length, and readily falls
free from the stalk bearing it. The wall of the spore is
studded with minute warts and has four thin round pores
near the middle part. The uredospores are able to ger-
minate, immediately they are liberated, in a film of water
or in damp air. Through the pores mentioned above two
or three fine filaments grow out; one of these usually out-
strips the rest and may become a long, branched, wavy
filament. If in water alone, or indeed apart from a living
leaf of the wheat plant, this filament is only able to survive
until the small store of reserve food material in the spore
is exhausted. If, however, the spore germinates on the
leaf of a wheat plant the germ tube grows to one of the
stomata, and passing through the pore enters the tissues
of the leaf. Here the fungus absorbing food material
grows and produces a new pustule with uredospores in
about a fortnight. In this way very large numbers of
uredospores are produced during the summer months and
hence the disease spreads rapidly.

The teleutospores, like the uredospores, arise from
branches of the filaments of the fungus in the leaf and
they appear towards the end of summer in the same
pustules as the uredospores. With the advance of the
season, however, the pustules give rise to teleutospores
only. Microscopically, these are longer than the uredo-
spores, are more spindle-shaped, but are also borne on
stalks. Each spore consists of two cells and is furnished
with a thick, resistant wall. Unlike the uredospores the
teleutospores do not germinate immediately when placed
under moist conditions, but they require to rest for a period
of months. If, however, teleutospores which are produced

g2

in the autumn are allowed to remain exposed to the
weather through the winter, they will, under suitable con-
ditions, germinate in a few hours in March or April. In
this case the process of germination of the spore differs
from any of those dealt with in previous chapters. Each
of the two cells of the spore sends out a delicate tube,
which, after growing to two or three times the length of
the spore, becomes divided into four cells or segments.
Then a delicate peg-like branch is put out from each of
these segments, and a minute, oval spore is formed at the
tip of each peg. The question now arises as to what
becomes of these teleutospores and the small oval spores
or sporidia they produce. For many years this was not
understood, although trials again and again proved that
the teleutospores or their sporidia were unable to cause
new infection on the wheat plant.

The clue to the problem was discovered by De Bary
in the old belief of farmers that outbreaks of rust were
in some way connected with the presence of barberry. He
first made a careful study of the yellow fungus which
occurs on barberry, and then proved the connection of this
with the rust on wheat. If one of the diseased areas on
the barberry is closely examined it is found to arise on
a swollen part of the leaf, and on the under side a number
of small, yellow, cup-like bodies are produced. From
these cluster-cups yellow spores, known as aecidiospores,
are liberated as a fine dust. A section through the diseased
area shows that the cluster-cups arise as round masses of
fungal filaments beneath the skin of the leaf. Within
the ball of fungus, aecidiospores are produced in chains,
and when this growing ball bursts the skin of the leaf, it
opens at the apex and the edges turning back give the
structure the appearance of a minute cup or bowl. The
aecidiospores arise from threads at the base of this, and
as they ripen and are set free new spores are produced.
Now De Bary discovered that when sporidia from the
teleutospores on wheat are sown on barberry leaves in the
spring, infection takes place and the cluster-cups just
described are produced in two or three weeks. Not only
did he prove this, but he also showed that when the
aecidiospores from the cluster-cups are sown on the leaves
of wheat, they germinate, and like the uredospores send
germ tubes through the stomata and thus infect the leaf.
Pustules containing uredospores and later teleutospores

93

are produced from this infection. The connection between
the fungus on the barberry and that on wheat was thus
established, and it was shown that, although the a pear-
ances of the fungus on the two host plants differ so
materially, each represents a stage in the life history of
one fungus.

Since De Bary’s remarkable discovery very many
rust fungi, showing this type of life history, have been
studied. The rust which frequently appears on the leaves
of the pear, for example, is the aecidiospore stage of
Gymnosporangium which produces swellings on the
branches of juniper, where the teleutospores arise. The
Blister-Rust of the Weymouth or white pine, a disease
which is very prevalent on the continent of Europe, is the
aecidiospore stage of a fungus which forms its uredospores
and teleutospores on the leaves of currants. Similarly,
the Gooseberry rust is the cluster-cup stage of a fungus
which forms the uredospores and teleutospores on the
leaves of sedges. Many other similar cases occur as
diseases of cultivated plants and trees.

When the facts became known for the wheat rust it
was thought that in order to eliminate the disease from a
given district all that was necessary was to destroy the
barberry plants in the district. It was soon found, how-
ever, that in some countries, for example, Australia, where
the rusts of wheat are most destructive, the barberry is
almost unknown. In such cases it seems likely that the
rust fungus is able to maintain itself without passing
through the barberry. It probably does this by means
of the uredospores, which have been shown to be capable
of resisting a mild winter.

Apart from the destruction of the barberry other means
of combatting the wheat rust had to be devised. Up to
the present this has proved a very difficult problem. The
most satisfactory progress has been attained along the
lines of breeding disease-resistant wheats. It has long
been known that certain wheats are much more resistant
to rust diseases than others. The chief difficultv lay in
the fact that the most resistant forms, e.g., certain sem1-
wild wheats, were almost valueless as crops. In recent
years, however, much has been done, especially by Pro-
fessor Biffen, of Cambridge, to produce by crossing, wheats
which combine qualities of rust-resistance with good
cropping and milling capacities. The subject is neverthe-

04

less still beset with difficulties, both in regard to the
permanence of the rust-resistance, and to the fact that
a wheat, which is resistant to one form of rust may be
equally susceptible to another. Further, wheats which
are resistant, say in England, are not necessarily so in
India or Australia. It has been essential therefore for
each country to establish its own rust-resistant varieties
of wheat.

Many of our common garden plants are liable to
attacks by different rust fungi. The Mint rust caused
by Puccinia menthe is one of the most prevalent of these
in some districts. In this example the three forms of
spore appear in succession, and there is no intervention of
a second host plant in the life cycle of the rust. In the
spring diseased plants send up shoots which are often
swollen and distorted, and bear the cluster-cups of the
fungus. ‘lhe aecidiospores are liberated as a bright yel-
low dust and infect the leaves of healthy shoots so spread-
ing the disease. The pustules produced by this infection
are brown in colour and are scattered, as minute dark '
spots, over the leaves. Uredospores arise from these
pustules throughout the summer, and towards the end of
the season teleutospores are produced from similar disease
spots. The teleutospores remain in a resting condition in
the soil for some months, but germinate in the early
spring giving rise to sporidia, which infect the young buds
on the underground stem. Such infected buds are not
killed outright by the fungus, but grow out to produce
the distorted shoots bearing the aecidiospores, described
above. It has also been shown that once the underground
stem is infected, the fungus lives there perennially and
grows into the young buds as they are formed. This
renders the disease all the more difficult to eradicate.
Indeed, the best plan is to destroy all infected material
and only use for planting, rhizomes known to be free from
the fungus. Care should also be taken to prevent infec-
tion by teleutospores from soil which has grown diseased
plants.

As an example of a rust caused by a fungus with a
simpler life history, the Hollyhock rust produced by
Puccinia malvacearum may be considered. This disease
occurs on a large number: of the members of the Mallow
family; it is widely distributed throughout the world and
is abundant in this country, both on wild mallows and on

95

the cultivated Hollyhock. The fungus, which is a native
of Chile, was introduced into Europe about 1875, and for
some years its ravages were so severe that it was scarcely
possible to grow the Hollyhock free from rust. Even at
present the disease causes considerable trouble in some
districts.

The disease pustules chiefly occur on the leaves, but the
stem and even the flower buds and fruits are often
attacked. As in the rusts already described, the develop-
ing cushion of spores bursts through the epidermis. The
filaments of the fungus ramify in the tissues of the host
sending haustoria into the living cells, which are slowly
depleted of nutritive substances. The pustules arise on
both surfaces of the leaf, are small and circular in out-
line and produce teleutospores only. | These germinate
zz stfz under suitable conditions, immediately they
mature, giving rise to sporidia as in other rusts. The
sporidia, falling on to the surface of any part of a
hollyhock plant germinate and penetrate the epidermis
producing an infection. From this region a new pustule
of teleutospores is produced in about fourteen days. The
Hollyhock rust thus omits both the aecidiospore. and
uredospore stages from its life story.

‘The fungus probably passes the winter by teleuto-
spores which fail to germinate owing to unfavourable
conditions. In addition it is likely that it also is carried
over in the few radical leaves which generally survive the
winter in this country. The writer has frequently observed
incipient disease spots on such leaves in the winter, and
it is probable that these develop much more slowly in the
cold weather than those produced in the warmer months.
By destroying diseased leaves as soon as the spots appear,
it is usually possible to restrict the damage caused by this
tust.

From the examples described in this chapter it will be
evident that, although the fungi causing the rust diseases
belong to one group, yet they present certain differences
from one another.. A large number, like the rust of wheat,
produce aecidiospores on one host, and the uredospores
and teleutospores on an entirely different species of plant.
Others, however, like the rust of mint pass the whole life
cycle on one host and give rise to aecidiospores, uredo-
spores and teleutospores in succession on the same species.
Still others like the Hollvhock rust, in addition to living

y6

wholly on one host plant, altogether omit some of the
spore forms from the life story. _ Whilst showing these
variations, however, the rust fungi are all highly
specialised parasites; they cannot grow apart from living
plants, and with rare exceptions, a specific rust fungus is
restricted to a single species of piant.

Further information on the subject of diseases of
plants caused by parasitic fungi may be obtained from
the following books :—-

Ward, H. M. “ Diseases of Plants,” S.P.C.K., London.
Duggar, B. M. “ Fungous Diseases of Plants,” 1909, Ginn and

Company.
Massee, G. “ Diseases of Cultivated Plants and Trees,” Duck-

worth, 1910.

Chapter 13.

INJURIOUS ANIMALS OTHER THAN INSECTS.

Injurious and beneficial Animals. Birds and the need for
scientific investigation with reference to their food at different
Seasons of the year. Eelworms and more especially the Stem
Eelworm and the Knot Root Eelworm. Pulmonate Molluscs
(Snails and Slugs). The Black Currant Gall Mite,

All who engage in agricultural or horticultural pur-
suits sooner or later have to concern themselves with some
of the forms of animal life which are associated with their
plants. Very frequently certain of these animals - are
directly injurious to the operations of man, but there are
others which, on the contrary, are distinctly beneficial in
their effects. It is, therefore, a matter of considerable
importance to be able to discriminate between these two
classes, for it is obviously bad policy to devote time and
money in destroying organisms which are beneficial in
their action. The animal kingdom comprises a_ vast
assemblage of different forms, but fortunately for our
present purpose we need only concern ourselves with a
relatively small proportion of their number. These in-
clude certain birds, species of Nematodes or Eelworms,

97

08

Oligochaeta or Earthworms, Pulmonate Molluscs or Slugs.
and Snails, land Isopods or Wood-lice, certain Acari or
Mites, and the great class of the Insects.

Dealing first with INJURIOUS ANIMALS. BIRDS (1),
(2), *(3} merit some amount of attention, but the fact can-
not be emphasised too strongly that we possess extra-
ordinarily little reliable knowledge concerning the food
of some of our very commonest birds. Both the British
Association and the Board of Agriculture recognise that,
before any effective legislation can be recommended, a
very full scientific enquiry is needed. It is necessary,
for instance, to examine and tabulate the contents of the
crops of certain common birds in each month of the
year so that an opinion may be formed of the benefits or
imjuries caused by them at the different seasons. It is.
further necessary that some estimate should be made of
the available food in the district where the birds were
feeding when killed, in order to decide whether the foods
disgovered in the crops were selected from choice or
necessity. Much information is also desirable as to the
food of nestling birds. Fortunately some progress is
being made towards supplying this much needed informa-
tion, and the Department of Economic Zoology in this
University is performing a useful part in the work on
behalf of the Board of Agriculture. Certain species of
wild birds may be direct-injurious by feeding upon or
injuring plants or parts of plants; others are indirectly
harmful in that they may feed upon forms of animal life
which are in themselves beneficial. Fortunately there
are very few species of birds which we may declare to
be wholly destructive and, of these, the House Sparrow
and. Wood Pigeon are the most important. The Black-
bird also appears to have very little utility in the eyes.
of man, and is a most persistent devourer of fruit. On
the other hand, there is a large number of birds whose
role is doubtful; in many cases we lack adequate knowl-
edge to judge fairly as to their feeding habits, but they
all appear to have a good deal of utility in their favour.
Among these may be cited the Song Thrush, Great and
Blue Tit, Greenfinch, Chaffinch, Rook, Robin, Linnet,
Yellow-hammer, Skylark, Starling, Woodpeckers and

* The numbers in brackets refer to the literature which is
listed at the end of this series of lectures.

99

others. The Rook, for instance, is a bird concerning
which it is at present impossible to say whether it is a
beneficial or harmful species. It consumes a vast amount
of grain but, on the other hand, during the summer it
devours a great many injurious insects of various kinds,
including both leather-jackets and wireworms. The
Starling also devours a large amount of grain during
certain times of the year, but this appears to be com.
pensated by the great quantity of injurious insects which
the bird consumes at other periods. As many as 197
leather-jackets, for instance, have been found by Mr.
Leigh in the crop of a single bird. The Chaffinch is by
no means as destructive as is commonly believed. It
consumes large quantities of seeds of such troublesome
weeds as, dock, knotgrass, hawkweed, and_ especially
chickweed. Mr. Leigh informs me that although he ,
found grain in the crops of 41 per cent. of the birds which
he examined, it appeared to have been taken in most cases
from manure or ricks,in farmyards and not from culti-
vated land. It must be further added that evidence
points to the fact that the majority of species of birds
feed their nestlings on soft-bodied insects and other
invertebrates. | Consequently even the most destructive
birds may perform a useful function during that stage in
their life.

EELWORMS (4), (5), belong to the group of the Nema-
toda. They are always small in size and have thread-like
bodies, the two ends being more or less pointed. They
can be readily distinguished from the Oligochaeta or
Earthworms by the total absence of body rings or seg-
ments. Those which are plant parasites are microscopic
forms living free in damp soil or inside the tissues of
plants. Others live in decaying vegetation, and both the
parasitic and saprophytic forms can be recognised by the
presence of a spine which can be protruded through the
mouth and serves to penetrate the cell-walls of plants.
The Eelworms spread from one plant to another by
wandering through the soil; and when they leave the dead
plants they le near the surface of the ground. Fre-
quently when these animals are numerous it is useless to
grow susceptible plants in the same patch of soil during
successive seasons, and then as long an interval as possible
should elapse between the growing of two crops of the
same plant. The STEM EELWORM (1 ylenchis devastatrix,

100

Kuhn) attacks a great variety of plants including straw-
berries, onions, beans and peas, hyacinths, and also feld
crops. It is an extremely slender species, about 1-25th
of an inch long, and the males and females closely
resemble one another. Strawberries when attacked decay
away at the level of the soil or just below it, and the
crowns and roots rot away. A remedy is to pull up and
burn the affected plants and dress the soil with either
lime or sulphate of potash in the proportion of 1 cwt. to
the acre. The KNoT RooT EELWORM (Heterodera radici-
cola, Greef) differs from the previous species in the male,
being thread-like, while the female is greatly swollen
except at the head end. It also goes through a more com-
plex life-history. This species renders its presence evident
by forming knot-like swellings or galls upon the roots of
the affected plants. It is a great enemy of cucumbers and
tomatoes grown in glass houses, but also attacks vines,
potatoes, lettuces, and many other plants in the open.

As a temporary measure to save a growing crop, one
part of permanganate of potash to 200 of water applied
at intervals of ten days is recommended in the Kew
Bulletin. \t does not harm the plants, but since it does
not destroy the eggs of Eelworms, no permanent value
can be ascribed to it. Treatment of the soil with one part
of carbolic acid to twenty of water, with a dressing of
sulphate of potash, 3 cwt. per acre, intimately mixing the
soil with gas lime or naphthaline, are among the remedies
that are recommended. When applying remedial measures
the soil must remain unused for at least six weeks for any
permanent benefit to be derived. This species, however,
is extremely difficult to eradicate owing to the fact it
produces vast numbers of eggs throughout the year, and
the young Eelworms are thus constantly being liberated
into the soil. Furthermore, most of the above methods are
not lasting in their effects, owing to the fact that fre-
quently a number of eggs remain over undestroyed, and
serve to start the infection afresh. When a glass house
is infested with Eelworms, it is often necessary where
possible, to remove the soil bodily and treat it by one
of the methods already mentioned. In the case of plants
grown under glass the horticulturist soon finds that the
conditions encourage a host of other living things. In
addition to Eelworms, Woodlice, insects of various kinds,
and fungi often enforce their presence, and under the

Tol

warmth and moisture that is provided they are liable to
multiply exceedingly. Experiments carried out at the
Rothamsted Station have shown that we can very con-
siderably reduce this undesirable population by partial
sterilization of the soil by means of steam. In cases of
very bad infestations of Eelworm this method is said to
be the only effective remedy at present available.

SLUGS and SNAILS (6), (7), belong to the class of the
Mollusca, which is a large assemblage of animals includ-
ing such divers forms as Oysters, Whelks, Scallops, Octopi,
and the familiar fossils which are known as Ammonites
and Belemnites. Both Slugs and Snails differ from other
Molluscs in being land and not aquatic animals. They
are always provided with a pulmonary chamber, which is
a kind of lung enabling them to breathe in the air. In
aquatic Molluscs this pulmonary chamber is almost always
absent, respiration taking place by other means.

SNAILS or Helicidz are provided with an external
spiral shell into which the animal can withdraw itself, and

ere are three species which are commonly met with. The
Garden Snail (Helix aspersa, Mull.) is the largest and
its shell: measures about 14 inches in diameter. It is well
enough known to need no description, being easily recog-
nisable by its brown shell marked with pale irregular
lines. The Strawberry Snail (47. rufescens, Pen.) has a
shell which seldom exceeds half an inch in diameter and
is more flattened in form. It also varies in colour from
dirty grey to brown or reddish-brown, often with a num-
ber of transverse streaks of a darker tint. The Wood
Snail (H. nemoralis).has an extremely variable shell being
white, grey, pinkish, yellow or brown, and is marked with
one to five or more conspicuous brown spiral bands. It
is, moreover, considerably larger than the Strawberry
Snail.

SLuGsS or Limacide are naked and only possess a
vestigal shell, which is placed near the hinder end of the
‘body or buried beneath the skin of the back; all the
injurious species have the shell in the latter position. The
situation of the shell is clearly marked externally and
the area of skin covering it is known as the shield or
mantle. Closely related to the margin of the latter, on
the right side of the body, is the respiratory pore—a well-
defined aperture leading into the pulmonary chamber.
Slugs secrete an abundance of mucous, which serves to

102

lubricate the skin; it is very tenacious and capable of
being drawn out into a thread which is used as a means
of descent from trees and bushes. The most injurious
species are: (1) The Black Slug (Avion ater, Linn.)- not-
withstanding its name this species varies greatly in colour
and may be either black, grey, reddish, or reddish brown.
When at rest the animal can be further recognised by its
contracted and almost hemispherical form. (2) The Grey
Field Slug (Agviolimax agrestis, Linn.) is perhaps the
most injurious species we have in this country. It is ashy-
grey in colour with a yellowish or reddish tinge, and occa-
sionally specimens have a mottled appearance; longitu-
dinal markings are entirely absent. (3) The Black Striped
Slug (Limax maximus, L.) 1s the largest of the three and
may attain a length of over six inches. It is usually some
shade of grey, with longitudinal markings of a darker
colour, frequently black. Individuals inclining to brown
or dull yellow are also not infrequently met with.

It is well known that both Slugs and Snails confine
their operations to night, and are seldom evident during
the day except after rain. It is consequently useless to
apply remedial measures during the warm parts of the
day, or in very dry weather, the evening and early morn-
ing being the most suitable times. The mucous secreted
by Slugs enables them to resist the action of obnoxious
substances in the powered condition, they have the faculty
of crawling out of their mucous investment, and in that
way léave the powdered material behind them. This
mucous, however, cannot be secreted indefinitely, and if
two or more dressings are applied with an interval of
about fifteen minutes between each application the Slugs
are usually killed. A mixture of lime and soot applied
two or three times is an effective remedy, but the lime
should be quite fresh and very finely powdered. Accord-
ing to Theobald the most effective substance is hydro-
oxide of calcium, a I to 2 per cent. solution in water.
Snails, on the other hand, are more difficult to destroy
from the fact that they retract themselves into the shell
and can close the mouth of the latter. In this condition
they can remain completely dormant for several years.
Dressings of soot is a well-known remedy against Snails,
it acts as a deterrent making the plants and surface of
the soil obnoxious to these animals. Nitrate of soda is
an effective dressing for use on a large scale against both

103

Slugs and Snails and,.moreover, is beneficial to the plants.
Natural enemies are {lso an important factor: thrushes,
blackbirds, starlings, and also ducks and fowl render help
in keeping down an excess of Slugs and Snails.

Among other injurious animals WOODLICE (8) and
MILLIPEDES (7) were also referred te but, owing to the
limited space at my disposal, I must pass over these and
deal with the Acari or MITES. They are classified as a
group of the Arachnida, which also includes Spiders,
Harvestmen, and Scorpions. All can be recognised by
the presence of eight pairs of legs, the absence of feelers
or antennae, and the fusion of the head and thorax into
one compact region or cephalothorax. Acari are further
distinguished by the abdomen not being definitely marked
off from the rest of the body. The Red Spiders or Trom-
bididae belong to this group, but the most important for
our purpose are the Eriophyidae or Gall Mites. £7zo-
phyes ribis (4) or the BLACK CURRANT GALL MITE is
responsible for the “ Big-Bud” disease which has spread
throughout the country. Its presence can be readily
detected by the swollen and distorted appearance of the
buds which harbour the Mite. Badly infested buds sel-
dom develop into shoots, they remain unopened and, after
retaining their green colour for a time, become brown and
die off. The damage is caused by the jaws of the Mite
cutting through the epidermis of the delicate young leaves,
followed by the inserting of the sucking tube which
extracts the sap. Throughout the winter the Mites feed
and shelter in the galled buds. Migration takes place from
the infected buds, which open from about the middle of
April until well on in June. The Mites then crawl out
in great numbers in order to find new and succulent buds
to serve for their future sustenance. This migration 1s
aided by the habit the Mites possess of often attaching
themselves to passing insects wandering over the twigs.
By this means they become distributed to other branches
and to fresh bushes. Strong winds are also a factor aid-
ing their dispersal. Having entered new buds the Mites
commence laying their eggs and thereby multiply rapidly
until the end of the summer. Shoots examined during the
end of August and in September, exhibit the “ Big-Bud ”
appearance, and are filled with the new generation of the
Mite, which will carry on infection for the next season. A
certain number of eggs are to be found all the year round

104

but are most abundant in the summer. Our knowledge
of the life-history of this Mite is incomplete, we still
require definite information as to whether the species can
pass the winter elsewhere than in the buds—whether it can
survive under the bark, in the roots, or beneath the soil.

With regard to remedial measures, so far as I am
aware, no completely successful methods of treatment have
yet been devised. Instances are known where all diseased
bushes in a plantation have been cut down, the stumps and
root stocks subjected to treatment, and yet the young
shoots came up infested with this Mite. It is of first
importance to cultivate from perfectly clean stock, and
cuttings taken for setting should also be selected from
such plants. Hand picking of the infected buds at the
end of winter is valuable, and all buds collected should
be burned as soon as possible; with badly infected shoots
extensive pruning is necessary. When the bushes are very
badly infected there is no remedy beyond taking up and
burning them, followed by replanting with clean new
stock. Spraying or dusting with a mixture of lime and
sulphur during the migratory period has been recom-
mended, but often the results are unsatisfactory. An
efficient spraying mixture still remains to be discovered.
Some varieties of currant are claimed to be less severely
attacked than others, and among them may be mentioned
the Boskoop Giant, Lee’s Prolific. and Edina. Varieties
claimed to be immune have been placed on the market, but
whether they will remain so time alone will determine.
There is a possibility that careful selection and inter-
crossing of likely varieties along scientific lines may lead
to the production of resistant stock.

Chapter 14.

INJURIOUS INSECTS.

General observations on Insects and their feeding habits.
Insecticides and their use. Earwigs. Destructive Caterpillars
of Cabbage Butterflies, and the Currant, Winter, and Codling
Moths. The Gooseberry Saw-fly. The Pea Thrips. Wireworms.

The next class to be considered is that of the INSECTS
(9), (10),* and, viewed from our present standpoint, they
are of greater importance than the whole of the rest of
the Animal Kingdom. Insects can be readily recognised
by the presence of a pair of antennae or feelers, six pairs
of legs, and the division of the body into head, thorax
and abdomen. Either one or two pairs of wings are almost
always present in adult Insects. The most remarkable
feature in their life-history is the fact that they pass
through a series of changes which we term metamorphosis.
On hatching from the egg, the first stage is the larva,
which is succeeded by that of the pupa, and from the
latter emerges the perfect Insect. In many Insects, how-
ever, the pupa is absent, and then there is a gradual growth
from the larva to the perfect Insect. Larve are variously
known as caterpillars, maggots, or grubs, and, being
exclusively concerned with feeding, they are as a general
rule more destructive than the perfect Insects. The pupa
or chrysalis is purely a resting stage, no food is taken,
and during this period the organs and tissues of the future
‘Insect are gradually built up. It is of great importance

* The numbers in brackets refer to the literature given at the end
; of the final lecture.

105

106

to understand the method by which. an Insect feeds,
whether it be in the larval or adult condition. Almost all
remedial measures have to be based upon this factor. We
can recognise three methods by which Insects feed: (1)
By means of the biting action of their jaws. (2) By means
of sucking, and in this case the mouth organs are modified
to form a suctorial apparatus. (3) By means of a com-
bination of both of these methods.

Various chemical substances are used for destroying
Insects and are known as Insecticides. Biting Insects are
mainly destroyed by poisoning their food, while sucking
Insects can usually only be destroyed by using contact
insecticides—those which kill by means of surface contact.
This method is adopted for the simple reason that we
cannot poison the food when it consists of the internal
juices of plants, as is the case with sucking Insects. When
using insecticides the grower should proceed with caution
until experience has been acquired. Also contact insecti-
cides are liable to injure the foliage under certain con-
ditions. Many of the insecticides that have been recom-
mended are inefficient, while others need scientific testing
to fully determine their value. Insecticides are artificial
wetbeels of control, but it 1s necessary to point out that
the utilisation of zatural methods of control should not be
neglected. Natural methods consist in the preservation
and increase of those organisms which are directly bene-
ficial to man, in that they destroy the injurious forms of
animal life. This latter method will be dealt with in a
subsequent lecture.

The first order of Insects that I shall deal with is the
Ortho piera, and the only member thereof that concerns us
is the common Earwig (Forjicula auricularia) (10). It is
a good example of an Insect which undergoes incomplete
metamorphosis. The adult Earwig lays its eggs in a
group either beneath stones or in the soil. During the
incubation period she guards the eggs until they hatch,
exhibiting in this respect a rudimentary instance of
parental care. The young larve are minute white
creatures, with very slender forceps and no traces of wings.
After they have passed through several moults rudiments
of wings appear, and ae ye growth chiefly consists
of an increase in the size of the Insect and the gradual
development of the wings. No pupa or resting stage is
passed through. The Earwig is almost exclusively noc-

107

turnal in its habits, and has been very seldom observed
to use its wings in flight. During the day Earwigs hide
away beneath the soil, among vegetation, under stones,
bark, and in other dark situations. They cause some
amount of harm to cultivated plants, particularly dahlias,
but their diet may include animal matter also. Earwigs
can be most readily got rid of by means of traps. Plant
pots filled with straw or dead moss placed in an inverted
position upon the ground, or upon stakes, are usually
effective. The pots should be examined frequently, and
the Insects shaken out into boiling water, or the straw and
other rubbish containing them burnt.

The next order of Insects which concern us is the
Lepidoptera. Their larve are known as caterpillars, while
the perfect Insects are recognised as Butterflies and Moths.
Butterflies can be readily distinguished from Moths by
their feelers or antennae terminating in a club or knob,
while those of Moths taper off to a point. Furthermore,
Butterflies are diurnal while most Moths are nocturnai.
Lepidoptera are only injurious in the caterpillar stage;
the adults feed entirely upon the nectar and_ juices
of flowers which they imbibe by means of a flexible
sucking tube, and never pierce or injure the tissucs of
plants. Four wings are present and they are closely
covered with microscopic scales which easily rub off,
revealing the transparent wing membrane beneath. The
eggs of Butterflies and Moths are almost always laid on
or near the leavesof the plants which are to serve
as food for the future caterpillars. Very few Butterfly
larve are injurious, and only two species need con-
cern us, viz, the Large and Small Cabbage Whites
(Pieris brassicae and P. rapae) (10). The larve otf
the former species are partial to the outer leaves of
cabbages, while those of the smaller species also attack
turnips. Both kinds frequently devour the leaves of
“nasturtiums” and other plants. When very abundant
hand picking of the larve is the best method. The pupe
occur on palings, walls, and similar objects in the imme-
diate vicinity, but a quick eye is needed to distinguish
them, and for this reason their destruction is not likelv to
very materially reduce their numbers. The eggs of the
Large Cabbage Butterfly are pale yellow and laid in
clusters on the under sides of the leaves of the food plants.
Every cluster destroyed means the reduction of a whole

108

brood of the larve. The eggs of the Small Cabbage
Butterfly are laid singly and, consequently, their detection
and destruction is too laborious to be worth while.
Insecticides are of very little value against these two
species. Among Moths, the larve of the common Currant
Moth (Abraxas grossulariata) (4) are very destructive in
that they defoliate currants and gooseberries. The Moth
is conspicuously spotted with black on a white ground,
and is on the wing during July and August. The larva
is similarly conspicuous, being deeply spotted with black
on an ochreous-white ground, with an orange coloured line
along each side. It is prevalent at the end of the summer
and hibernates during the winter among dead leaves, in
chinks of walls, under bark, etc. During the spring it
recommences feeding and turns to the pupa in May or
June. The pronounced colouration of this larva renders
hand picking a very easy and effective measure. In exten-
sive infestations spraying with lead arsenate at the end
of the summer kills large numbers of the young larve
through poisoning their food. If they are still abundant
during the following spring the operation should be
repeated. Since lead arsenate is a poison it must not be
applied later than four weeks before the fruit is to be
gathered. The grower will do best to utilise Swift’s
arsenate paste rather than prepare his own compound.
From 8 to Ioozs. of the paste mixed in 10 gallons of
soft water is a suitable strength; weaker solutions, how-
ever, are often equally effective. The Winter Moth
(Cheimatobia brumata) (4) does immense damage to the:
foliage of apple, pear, plum and cherry trees and is univer-
sally common in this country. The male is a thin-bodied
brown Moth, measuring I4in. across the expanded wings.
The female is wingless and spider-like in appearance.
The Insect occurs from October until the beginning of
January, the eggs are laid on or near the bases of the
buds, and the green larve belong to the type commonly
known as “loopers.” They commence feeding upon the
leaf buds, and then the flower buds, which they spin
together with the leaves to form shelters. Later on they
attack the foliage and even the fruit. During June they
are fully fed and pass to the soil, where they change to
the chrysalis a few inches below the surface. The most
effective measure is “grease banding” the trees. Strips
of grease-proof paper, 6 to 8 inches wide, and sufficiently

109

long to encircle the trunks, should be tied tightly with
string above and below, and placed on the trees during the
first week in October. The most suitable height is from
2 to 4 feet from the ground. The paper is to be well
smeared with cart grease, which must never be allowed to
become dry. To ensure this, three applications during the
season are usually sufficient. “Tree tanglefoot”” may be
used instead of cart grease, and has the advantage of not
requiring renewal during the whole winter. By means of
this device the wingless females are trapped in large num-
bers as they crawl up the tree trunks from the soil. If
the grease bands be retained until the end of March, large
numbers of females of the destructive March Moth
(Anisopteryx aescularia) (4), which are likewise wingless,
also meet with a similar fate. If the grease bands are
neglected many of the female Moths succeed in making
their way up to the buds.to lay their eggs. When the
larvee are very abundant the only measure is to spray with
lead arsenate, using an ordinary knapsack sprayer, except
for very large trees, which demand a more powerful instru-
ment. The spray should be distributed as a fine mist,
as all that is needed is to render the leaves poisonous.
It is not advisable to spray during blossoming, and spray-
ing with winter washes is useless. The Codling Moth
(Carpocapsa pomonella) (4) is one of the most important
of apple pests, attacking many varieties besides the Cod-
ling, and also pears. Those types such as the Russet and
Nonpareil, in which the “eyes” are more or less closed,
are less susceptible than the Blenheim Orange and many
others. The perfect Insect is a pretty brown Moth with
coppery reflections, and measures about #in. from tip to tip
of the expanded wings. It flies during June and July,
laying its eggs singly on the young fruit, but occasionally
it may select the leaves. They hatch just about the time
when the petals have fallen and the fruit set. The young
larva are whitish, pale yellow, or often pink, with the head
and the shield immediately behind dark brown. They
make their way to the calyx end of the fruits and
gradually eat their way to the core. The entrance hole
can always be detected, and through it the larve ejects
particles of excrement to the exterior, thereby avoiding
contamination of its burrow. About midsummer they eat
their way out of the fruit, and if the latter are still on the
tree the larve crawl down until they reach the trunk. In

FIG

the case of fallen fruit the larvae make their way back to
the trees and crawl up the trunk. In either case when
the trunk is reached they spin cocoons among loose bark,
moss or lichens, and there remain dormant until the fol-
lowing spring, when they turn to the pupa and shortly
afterwards give rise to the next generation of Moths. In
a few instances two broods have been noticed to occur in
one year. The attacks of this Insect cause the fruit to
fall prematurely or decay rapidy when stored. As a
remedial measure all loose bark, moss, etc., should be
scraped off the trunks, and artificial shelters in the form
of one or more bands of loose straw or old sacking should
be tied round the trees, not very far from the ground. It
is safest to do this in June,,and the bands can be examined
at leisure during the winter and burnt. By this means
large numbers of the cocoons containing the larve are
often destroyed. Fallen apples should be cleared away
as soon as possible. Lofts and rooms utilised for storage
should be well swept out, and the walls, floors, shelves,
and window frames lime-washed. In severe attacks spray-
ing the fruit-bearing portions of the trees with arsenate
of lead is advisable, and should be carried out a few davs
after the petals have fallen. The larve have to eat the
coating of this mixture in order to make their way into the
calyx, and are poisoned thereby,if the application has been
successful.

The order Hymenoptera is characterised by the presence
of (1) two pairs of transparent wings provided with rela-
tively few veins, (2) biting and sucking mouth organs, and
£3) complete metamorphosis. The Sawflies are the only
group that directly concerns us, and they may be easily
separated from other Hymenoptera by the absence of a
“waist,” or constriction of the body. The Gooseberry
Sawfly (Nematus ribesiz) (4), (10), is very destructive to
red currants and gooseberries, but seldom harms black
currants. The perfect Insects appear in April and May;
they have yellow bodies marked with black, and measure
about #in. in wing expanse. The eggs are laid in neat
rows along the veins on the undersides of the leaves of.
the host plant. They hatch into bluish-green caterpillars
spotted with black, and also marked laterally with blue
and yellow. Unlike Moth caterpillars they possess ten
pairs of feet, and when fully grown measure about ?in.
long. The bushes may be very quickly stripped of their

III

foliage by these larva, and the fruit are not exempt from
attack. About the beginning of June, they enter the soil
beneath or near the bushes in order to spin their brown
papery cocoons within which the pupal stage is passed.
From ten to twenty-one days, according to the tempera-
ture, are passed berteath the ground until the flies emerge,
and there are three broods during the year. The autumn
larvee pass the winter in their cocoons, turning to pupe
early the following spring. Leaves bearing the eggs of
this Insect should be destroyed whenever met with, while
hand picking is an effective means of getting rid of the
larve if done thoroughly. In the autumn wholesale
removal of the surface soil beneath and around the bushes
to a depth of five inches is valuable. It needs to be buried
in a deep hole dug for the purpose. By this means the
winged Insects are buried beneath the earth and perish on
emergence from the pupe. Fresh soil and manure should
be placed round the bushes. Spraying with arsenate of
lead is an effective poison for the larve, and can be applied
any time they are abundant after the fruit has been
gathered.

The Thysanoptera form a very small order of Insects,
comprising only those minute forms which are known as
“Thrips.” They are provided with four strap-like wings
with long “ fringes” all round, and are entirely suctorial
in their feeding habits. The Pea Thrips (Kakothrips
vobustus) (11) is a dark brown Insect, about z,in. long,
attacking edible peas and broad beans, often causing much
damage. The adults occur from May until August, and
the eggs are laid within the flowers on the stamen sheath
or on the young developing pods. The larve resemble
the adults with the exception of having no wings; when
fully fed they descend to the ground, penetrating to a
depth of 3-12in. They remain in the soil until spring,
when the adults emerge from the pupe, there being thus
only one brood in the year. Both the larve and the adults
are injurious, and in bad attacks no pods are formed or
are curled and undersized. The terminal buds and shoots
may also be infested, and damage is stated to be most
severe in light soil. This Insect sometimes spoils a whole
crop, and no varieties appear to be immune, but it has not
so far been found on sweet peas or scarlet runner beans,
though they are mentioned as host plants in France. Con-
trol is difficult to achieve, but early sown plants are less

II2

severely attacked. Spraying is useless when the Insects
are in the flowers as it does not reach them in that situa-
tion. When they feed openly on the pods in large num-
bers, spraying by means of contact insecticides is then
likely to be effective. A mixture of 1 lb. soft soap in
1o gallons of water is a cheap remedy and worthy of a
trial, or better still 3 lbs. tobacco powder (or 1 Ib. of Voss’
tobacco extract), $ lb. soft soap in 10 gallons of water.
Treatment of the soil during the winter does not offer much
promise on account of the depth to which the Insect
descends. :

Coleoptera or Beetles are characterised by the anterior
pair of wings being modified to form horny sheaths which
usually cover the upper side of the abdomen. They are
exclusively biting in their feeding habits and pass through
a direct metamorphosis. Wireworms (12) are the larve
of Click Beetles amily Elateride) and are known to
attack almost any kind of crop. They are more especially
pests of the agriculturalist, though tomatoes, strawberries,
potatoes, and other vegetables are liable to suffer injury
when grown in gardens or allotments. Three years and
even more are believed to be spent in the larval stage and,
owing to their resistant coats, these larve are notoriously
difficult to destroy, no effective remedy having so far been
discovered. Lures in the form of slices of potato, carrot,
or beet buried an inch or more beneath the soil when Wire-
worms are prevalent, often attracts considerable numbers
which can then be readily destroyed. The lures should
be examined twice a week and the spots marked with pegs.
In bad infestations crude powdered naphthaline dug well
into the soil in the autumn or early spring is worthy of
trial. Gas lime, lime, or salt are of very little use. In
the case of a badly infested potato crop there is no remedy
beyond digging it up. Infested soil should be well turned
over, exposed, and broken up. Birds then have easy access
to the Wireworms and material benefit is often derived by
adopting this measure.

Chapter 15.

INJURIOUS INSECTS (Continued).

Crane Flies; the Pear Midge; the Celery Fly; the Cabbage
Root Fly; the Onion Fly; the Narcissus Fly; the House Fly.

In this lecture we are concerned with the order Diptera
which comprises the true Flies. These Insects can be
recognised by the presence of a single pair of wings, the
hinder pair being absent, and only represented by curious
knobbed organs known as halteres or balancers. The
larva of the Diptera are devoid of true limbs and are
‘commonly known as maggots. A pupa stage is always
present and, in a very large number of species, the skin
of the larva is retained, forming a hardened case or
puparium enclosing the true pupa. Although no adult
Flies are directly injurious to vegetation, certain kinds
such as Mosquitoes, Sand Flies, and Tsetse Flies are
injurious to man. They pierce the skin in order to suck
his blood and thereby act as carriers of the organisms of
some of the most virulent diseases.

Some of the most familiar of the larger Flies are the
“Daddy Long Legs,” or Crane Flies (Zzgula oleracea and
allied species) (13). Although they are commonly pests
of our meadows and cereal crops nevertheless they not in-
frequently injure turnips, peas, beans, cabbages, hops,
dahlias, carnations and other garden plants. Their larve
are commonly known as “ leather jackets,” and when fully
grown they attain a length of 1$in. In colour they are
dull grey or brown and are not unlike fragments of small

113

114

dark twigs. They live exclusively beneath the soil and,
although they devour a considerable amount of dead
vegetable matter, their staple diet seems to consist mainly
of the roots of various plants. They are specially common
in damp parts of meadows, wherever there is rank herbage,
Soccally grass. Leather Jackets feed mostly at night
when they often come to the surface of the soil. When
fully fed they turn to elongate pupa, which force their
way to the surface of the soil, where they may be often
seen projecting for about half their length vertically out
of the ground. The Crane Flies issue late in the spring
and lay their black spindle-shaped eggs on or near the
surface of the ground. These eggs give rise to the Leather
Jackets which eventually transform into a second brood
of Flies appearing in great numbers during August and
September. The late brood of Crane Flies is always more
abundant than the spring one, and their eggs develop into
larvee which remain beneath the soil all through the winter.
They are often abundant in garden lawns in low-lying
districts, and it is advisable in such cases to roll heavily
and keep the grass closely cut.. Rolling at the proper
seasons crushes the pupz and if done regularly after dark
a large number of the larve would probably also be
destroyed. When present in large numbers thorough turn-
ing of the soil in the autumn and winter renders the larve
accessible to rooks, starlings, and other birds which prey
upon them in large numbers. A good soil dressing 1s
1 to 2cwts. of nitrate of soda to the acre, and although
Leather Jackets are susceptible to its effects, they are by
no means always eradicated. Gas lime is only doubtfully
effective. Theobald advises the use of traps of partially
buried turf as a device for enticing the Flies to lay their
eggs, and also to attract the larve from the adjacent soil.
To arrest local attacks in parts of lawns and beds 3 oz. of
carbon bisulphide to each square yard injected by means
of a Vermorel injector, or other suitable instrument, to a
depth of about 6 inches is usually quite effective.

The Pear Midge (Dzdlosis pyrivora) (4) is one of the
worst enemies of pear growers. All varieties appear to be
attacked by this Insect, but it is not known to affect any
other kind of fruit. The adult Midge is only about }in.
long and is blackish-grey or black in colour; the female
can be distinguished from the male by the abdomen ter-
minating in a long pointed egg-laying instrument or

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ovipositor. The Flies or Midges first appear during April
just about the time when the pear blossoms commence to
show their petals, and are to be found up to about the
middle of May. The eggs are laid within the blossoms,
and when the latter are unopened the petals are stated to
be pierced by the ovipositor and the eggs deposited on the
anthers. In opened flowers the pistil is pierced and the
eggs inserted therein. The larva are very minute white
or pale yellow maggots and only attain a length of about
+in. when fully grown. They feed within the develop-
ing fruitlets eating out their centres and leaving behind
them a mass of decomposing tissues. As many as ten,
twenty, or even thirty of these maggots may be found
within a single fruitlet. The attacked fruits usually swell
more rapidly than sound ones, and can be readily recog-
nised on the tree by being often twice the size of the latter
and more or less distorted. _ When mature the maggots
leave the fruitlets either before or after the latter have
fallen. In either case they crawl out from their shelter
and exhibit curious jumping movements until they bury
themselves in the ground beneath the trees. Here they
spin delicate cocoons of a dirty creamish colour, and
hibernate therein throughout the rest of the year until the
following spring. Unfortunately there is no universal
measure for dealing with this Insect. All infested fruit-
lets should be collected and destroyed before the larve
leave them. In very bad infestations it is better to gather
and destroy the whole crop. [If an orchard be well stocked
with poultry in the spring when the Flies appear and also
in June when the maggots reach the earth, material benefit
is very often attained. Removal of the surface soil con-
taining the pupze is scarcely a practicable measure. In
America the application of Kainit is recommended. If
well spread at the rate of half a ton to the acre around the
trees it is stated to destroy the larve and pup in the soil.
When the larve are leaving the fruitlets 5 cwt. to the acre
is said to be sufficient to destroy them. In this country
Kainit has been very little used, and reports as to its value
are conflicting, nevertheless, it fully merits a fair trial.
The Celery Fly (Acidia heraclet) (17) is a pretty
brownish Insect with mottled ornamental wings. It
may appear at the end of April but is commonest in
June and there are several broods in the year. The eggs
are laid on the leaves of celery and also parsnips. The

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larva are white or greenish maggots which mine the leaves
of those plants. ‘There may be several larvae in a leaf and
by their devouring the middle layers of tissues transparent
patches result, covered only by the upper and lower
epidermis. After a while these patches become brown and
the functions of the leaves are very greatly reduced. The
larve: turn into yellow or yellow-brown pupa shaped some-
what like minute barrels, and are found sometimes in the
leaves but mostly in the soil. The Insect winters in the
pupa buried a few inches beneath the ground. This fact
1s of value with regard to preventive measures and deep
trenching in winter between the original rows and burying
the soil therein containing the pupe will destroy many
of the Flies which would otherwise emerge. The mixing
of gas lime with the soil adds to the effectiveness of this
measure. Screening the young plants with cheap muslin
when first put out protects them from the Flies until they
are well established, and less liable to suffer severely from
the Insect. Picking off and burning the mined leaves will
destroy the larva, but in bad attacks the depletion of the
foliage by this method would be too great. All infected
celery tops should be burnt and not cast aside on refuse
heaps. heobald recommends spraying with nicotine; a
useful formula is 4 0z. of 98 per cent. nicotine, and } Ib. of
soft soap to 10 gallons of water. Various preparations of
nicotine are obtainable and so long as the above propor-
tions are maintained it matters very little which is used
(15). It is best to spray in the evening and when the
foliage is not too dry, the spray is said to soak through
the epidermis and kill a large number of the larve.

The Cabbage Root Fly (Chortophila brassicae) (7) is
one of the worst pests of cabbage and cauliflowers, and
may also attack radishes, turnips, swedes and _ stocks.
Growth of the affected plant is checked, the leaves flag and
discolour, the roots are largely destroyed, and the plants
die. The Fly is an ashy-grey Insect not unlike the House
Fly in general appearance and measures about jin. long.
(he winter is passed through in the pupa stage and the
first brood of Flies appear in April or the beginning of
May and there are most probably three generations in a
year. The eggs are visible to the naked eye, and are laid
close to, or on the plant, usually just below the surface of
the soil. The larve are typical Fly maggots, white or
pale yellowish, and about }in. in length when mature.

117

They commence injury by gnawing the outer layers of the
young roots, afterwards making tunnels inside the main
root; they may also invade the lower part of the stem.
The pupe are about 4in. long, oval in form, a light or
dark brown and are found in the soil close to the plants.
As regards preventive measures early plants have the best
chance of success as they become well set before the bulk
of the Flies appear. Earthing the soil around growing
plants is valuable as it causes the development of fresh
rootlets, which serve to replace those already destroyed
by the maggots. A cupful of paraffin well mixed with
each bucketful of sand, sprinkled round the plants once
a week until good growth is made, is to be recommended
and it acts as a deterrent, driving the Flies elsewhere.
Dusting the young plants with soot is said to be effective
and is well worth trial. In America tarred felt paper discs,
slipped round the stems of the young plants and pressed
flat on the ground are strongly recommended. They are
said to afford efficient protection to young plants against
the Flies laying eggs thereon. Experiments are being
conducted under my direction to test the value of these
discs, and if they prove satisfactory, their low cost and
the simplicity of the method will argue strongly in their
favour. When the crop is infested much benefit is derived
by pulling up and burning all infected plants as soon as
noticed. Furthermore, all cabbage stumps should be up-
rooted straight away and not left to decay; by these
means large numbers of larvae are destroyed which would
otherwise escape into the soil to pupate. In very severe
infestations I would strongly advise discontinuing grow-
ing cabbages for one year, and replacing with peas and
beans or other distantly related crops. Unless some such
course be taken bad infestations may continue for several
years in succession, owing to the large number of pupz
the soil contains during the winter, after the season is over.
Such pupee are very hard to get rid of as soil dressings
such as lime or gas lime are of very little value. Digging
over the soil exposes considerable numbers of pupz to
the attacks of insectivorous birds, while over large areas
deep ploughing might possibly effectively bury a large
proportion of them.

The Onion Fly (Hylemia cepetorui) (17) 1s closely
related to the Cabbage Fly and is a common pest wherever
onions are grown; it is a greyish Insect very like the

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Cabbage Fly. The Flies are common in April and May
laying their eggs on the necks of onions or on the leaves
just above the soil surface. After about a week, the eggs
hatch into larvae which become fully grown in three to four
weeks, and are then about jin. long. They turn to brown
pupz in the soil, though a few may remain in the bulbs,
and the Flies commence to appear about fourteen days
later. There are several broods in the year though the
exact number has not been determined; the winter is
passed as pupz which give rise to the early Flies of the
next year. In the earliest indications of an attack the first
leaves become yellow and, then whitish, followed by other
leaves behaving in a similar manner. Very young plants
are usually nearly eaten through just above the forming
bulbs, and the larvae migrate through the soil to attack
fresh plants. As the onions increase in size each may
shelter a number of maggots which devour the interior
and render it rotten. As regards remedial measures
insecticides are useless against the maggots on account of
their burrowing habits. _ All infected onions should be
pulled up and burnt. Earthing up the young plants is
valuable as it protects the forming bulbs. Early sowing
is also to be recommended in order to get the plants well
started before the Flies appear; or the seeds may be sown
under glass February and planted out in April. Trench-
ing and burying the soil containing the winter pupee as in
the case of the Celery Fly, should not be neglected.
Various substances are also advised in order to deter the
females from laying their eggs, and one or other of the
following methods are useful. Watering, or better still,
spraying the bases of the plants with an emulsion consist-
of two to three pints of paraffin and 1 lb. of soft soap
dissolved in one gallon of boiling water. To this pour
seven to eight gallons of soft water. Add the paraffin
while the soap solution is hot and churn the mixture very
thoroughly by syringing it back into itself so that no free
paraffin remains on the surface. A mixture of one bushel
of soot to two of finely powdered lime is also recom-
mended. In America they advise a mixture of carbolic
acid and lime. Three pints are slaked with a gallon of
water and a tablespoonful of crude carbolic is added after-
wards. This should be well watered round the bases of
the plants.

Growers of bulbs often lose many plants from the

11g

Narcissus Fly (Merodon equestris) (14), which is a large
hairy Insect marked very like a small Bumble Bee. It
appears in spring and early summer up to July, flying in
the sunshine over the beds. The eggs are laid on or near
the leaf bases or on the necks of the bulbs if the latter are
exposed. Narcissi and Hyacinths are the bulbs chiefly
attacked. The method by which the young larve enter
the bulbs is doubtful; they either penetrate between the
scales of the neck or crawl outside to the base-of the
bulb and then gnaw their way within, or possibly both
methods may occur. When fully grown the larvae measure
din. long and in this stage they pass the winter, usually
turning to the pupa in the spring, either in the soil or some-
times in the bulbs. The life-history is said to occupy two
years, but this is probably erroneous. The presence of
these larvae can be usually detected by gently pressing
the bulbs, the infected ones being less hard, but it is often
impossible to be quite sure without cutting open the bulb.
Before planting, or just after lifting, all infected bulbs
should be burnt, and any bulbs, concerning which there is
the least suspicion, should be steeped for an hour in water
at 110 deg. F.; if this temperature be not exceeded no harm
should accrue to the bulbs, while any maggots present are
said to die afterwards as the result. Possibly a lower
temperature is effective and growers are advised to deter-
mine this point for themselves. Any bulbs that fail to
appear or undergo very little growth in spring should be
dug up and destroyed as soon as possible.

The House Fly (Musca domestica) (16) although quite
harmless to vegetation is of great importance economically.
Its eggs are usually laid in fermenting accumulations of
horse manure, but may be also deposited in decaying
vegetable refuse, ash-pit contents and other substances.
The female Fly lays upwards of 120 to 150 eggs at a time,
and each is capable of laying five to six such batches.
The length of the complete life cycle depends very largely
upon temperature, and during hot weather it may only
occupy three weeks from the time the eggs are laid up to
the time when the resultant Flies emerge. Certain of the
late autumn Flies survive the winter and give rise to the
maggots in the following spring; the Flies appear in their
greatest profusion during August and September. The
House Fly is injurious to man in acting as a carrier of
disease germs, and it is specially concerned with the spread

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of infantile or summer diarrhcea and typhoid fever. It is,
therefore, of prime importance to give the Insect no oppor-
tunities for breeding and thereby providing a check upon
its abundance. All accumulations of manure and refuse
should be removed uz least once a fortnight, or more often
if possible. “he adoption of closed ash-bins excludes the
access of the Flies to their contents, and are most effective
in this respect. | Accumulations of farm manure provide
nutriment for enormous numbers of House Fly maggots.
Experiments on a large scale are being conducted both in’
this country and America for the purpose of rendering
manure heaps repellant to the Flies and their maggots
and, at the same time, still retaining their valuable fer-
tilising properties. When troublesome in houses the House
Fly can be readily poisoned by using one teaspoonful of
formalin added to a teacupful of water poured into a
soup plate. The mixture should be sweetened with a little
sugar or should contain about 25 per cent. of milk. If
placed at night large numbers of the Flies will partake
of it in the early morning and are poisoned thereby. The
mixture also has the advantage of being too weak to be
harmful to human beings or domestic animals.

Chapter 16.

INJURIOUS ANIMALS (Continued).

Aphides and their life-histories; preventive and remedial
measures. The Apple Sucker. Scale Insects. Greénhouse Pests
and Fumigation Methods.

The present lecture is devoted to a consideration of
certain injurious Hemiptera. Members of this order of
Insects are characterised by the presence of a jointed
rostrum or beak, enclosing two pairs of stylets used for
piercing the tissues of plants and imbibing sap therefrom.
Nearly always four wings are present, the young re-
semble the adults in general form, and a pupa stage is
almost always absent. The family of the Aphididae is
of great importance, including as it does the “ Green Fly ”
or “Plant Lice.” Aphides may draw the sap from all
parts of plants, even the roots, and the injuries they
cause are often great. They bear near the end of the
body a pair of tubes, which secrete a substance commonly
termed “honey dew.” This accumulates on the leaves,
blocking up their stomata, and also provides nutriment
upon which various Fungi develop. Aphides undergo a
remarkable life cycle. In the autumn we usually find
the fertilised winged females. The eggs laid by them
develop the following spring into wingless females.
These latter breed with great rapidity by parthenogenesis
—i.e., without the agency of the males, none being
present. Eggs are not laid but living young are brought
forth, and this goes on for several generations until the
summer. Winged females then appear but there are still
no males, and living young continue to be produced.
After a variable number of generations of this kind,
winged males and winged females become evident, and

121

122

the eggs laid by the latter give rise to a similar cycle in
the following spring. Some Aphides are restricted to a
particular species of plant, while others have alternate
hosts. Thus the Hop Aphis winters on damson and flies
to hops in the spring. F he Elm Aphis goes to the roots
of Rzbes, the Mealy Plum Aphis to rushes and aquatic
grasses, and the Bean Aphis to mangolds, poppies, dock,
etc. Some of the most destructive Aphides are the Bean
Aphis (Aphis rumicis), Currant Aphides (Rhopalosiphon
vibis Linn, and Myzus ribis Linn.), Hop and Damson
Aphis (Phorodon humuli), Apple Aphides (ApAts pomz
De Geer., Aphis sorbi Kalt., and Aahis jitcha Sand.),
Plum Aphis (4 p4is pruni Réaum.), Cabbage Aphis (A phzs
brassicae Linn.), Turnip Aphis (A fAis rapae Curt.), Rose
Aphides (species of Szphonophora), Woolly Aphis (Schz-
zoneura lanigera), and others. {n dealing with these pests
it is important to remember that when using insecticides
contact insecticides only are of use. Insecticides fre-
quently do not destroy the eggs and, notwithstanding
spraying, fresh broods appear from the unaffected eggs.

hen the leaves of the host plant are curled insecticides
are of little value, as they do not reach the Aphides
within. Frequently the undersides of leaves alone shelter
the Aphides, and spraying must be adjusted so as to
reach them. The earlier measures are applied after the
appearance of Aphides, the better the chances are of
success, before the latter become numerous.

The Bean Aphis (17) is usually black and very con-
spicuous; it especially attacks broad beans. It appears
when the beans commence to bloom and attacks the heads.
It breeds rapidly, covering the plants with a black sticky
mass which gradually extends downwards over the stems
and leaves. A simple and effective’ measure is to pick
off the infested tops as soon as any Aphides are seen
thereon, and drop them straight away into a pail of lime.
Jn bad infestations spite spraying with a knapsack
sprayer is necessary. [wo applications are desirable and
are usually successful. The ‘ee Aphis (17) may also
infest swedes and turnips. It usually appears about
May, but evidences of injury are not generally noticeable
until June, when the leaves begin to show blister-like
areas on the upper surface, while the Aphides are to be
found in the corresponding hollows on the undersides of
the leaves. The leaves become yellow and discoloured,

123

and in late summer the plants may swarm with the
Insects. In the early stages of attack benefit is derived
by cutting off the blistered or yellowish leaves and
destroying them. Later on dusting with soot is worthy
of a trial, and spraying is practicable on a small scale in
garden plots. All methods of cultivation tending to
produce vigorous growth are serviceable, and copious
watering in dry weather, which favours Aphid multiplica-
tion, is of great value. Aphis pomi and A. sorbi (4) which
attack the apple, puncture the leaves and cause them to
curl and become discoloured, while A. fitchzz attacks the
blossoms and buds, but does not usually cause leaf
curling. For all thtee species it is important to spray:
about the middle of April, when the eggs have hatched
and the young are most vulnerable. The Rose Aphides
are familiar to every gardener, and there are three species
commonly met with. Siphounophora dirhoda Wik. is
stated to migrate to grasses, Polygonum and _ wheat,
S. rosae Reau. to teazles, while S. rosarum Wik. appears
to have no alternate host. If only the latter species be
present one or two early sprayings are sufficient, but with
the other two species additional applications are some-
times necessary owing to fresh infections from their
other plant hosts. Paraffin should never be used on rose
trees, and only 4lb. of soft soap should be mixed to Io
gallons of water in the quassia wash referred to further
on. When only affecting a few twigs here and _ there,
Rose Aphides can be readily killed by momentary immer-
sion in a vessel of water just too hot to keep the hand
in, without injury to the plant (10). Of the Currant
Aphides (4), &. 726is produces reddish blister-like galls
on the surface of the leaves, while M/. rvibis causes the
leaves to curl up especially those on the terminal shoots.
Both species as they become numerous are difficult to deal
with, as they are protected in the hollows of the blisters
on the undersides of the leaves, or within the curled up
leaves. One of the effects of their presence is the fre-
quent falling of the fruit before maturity. Both species
deposit their eggs under the broken rind or upon it,
chiefly on twigs of the previous year’s growth. Both
currants and gooseberries are attacked, and M. rzbzs
especially frequents black currants. Early spraying in
April is the best measure, and care must be exercised in
order that the fluid reaches the undersides of the leaves.

124

The Plum Aphis (4) appears before the buds open, and
the parent form may be found in March. They are dull
fat purple Insects, while the young are olive green at
first, becoming purple later on. The latter attack the
young unfolding leaves and soon cause them to curl,
not only those of plum and damson, but also allied fruit
trees. Spraying should take place as soon as the Aphides
are noticed and before the buds are open, if possible, as
the Insects are then most readily destroyed. Further
spraying late in September and in October helps to kill
off the egg-laying females.

The Woolly Aphis, or American Blight (4) is a univer-
sal pest of apple in this country. By constant sucking of
the sap it lessens the vitality of the trees; their punctures
in the bark and young wood cause abnormal growths of
soft tissue which form characteristic rounded swellings.
Later on these swellings split and from them arise large
rugose deformities often ascribed to “canker.” These
wounds further predispose the trees to the attacks of fun-
gus enemies. fades ground this Insect further causes
gall-like swellings on the roots. As a result of Wooly
Aphis attacks, young trees may die, stunted trees often
result, and the fruit is deficient and of poor quality. The
parent wingless Aphides are reddish or purplish-brown,
and are invested with a white wooly substance. Living
young are produced and become similarly invested with
this white material, forming conspicuous objects on the
branches of the trees. The wingless egg-laying females
and winged males occur in autumn. Each female is very
small and lays a single egg near the foot of the tree,
and the egg hatches the following spring. Winged
parthenogenetic females appear to be rare and are seldom
met with. In the winter the Insect lives in the adult
state on the bark, or in the roots below ground, and also
in the egg stage. The possibility of resistent varieties
of apple is worthy of attention, especially as in Australia
the roots of apples grafted on to the Northern Spy and
Majetin-(an English apple) are said to be proof against
this Insect. Spraying with soft soap and quassia is an
efficient summer treatment, but the solution must not be
spared, and force is necessary or the wooly covering of
the Aphide will not be wetted. The only way of getting
rid of the root forms is by injecting bisulphide of carbon.
For an average-sized tree four I oz. injections into the

125

soil 2 feet away from the trunk are sufficient. The fluid
must not reach any of the larger roots and, moreover, it
is very poisonous and highly inflammable. If summer
spraying is not sufficient, a winter wash, as detailed in
Leaflet 34 of the Board of Agriculture, is advisable.
Banding the tree trunks in early spring is said to give
encouraging results, and séveral cases have been reported
of large numbers of £ peace migrating from the roots,
being trapped on these bands as they were ascending the
trunks. “Tree tanglefoot” is best for this purpose.

As regards preventive and remedial measures (4, 15)
against Aphides there are three seasons of application.
(a) In autumn to kill off the egg-laying females. The
leaves are then of much less value, and it matters very
little if they are injured. Thorough spraying with a
mixture of 1 pint paraffin and 14lbs. soft soap added
to 10 galls. of soft water, which should be made up in
the manner suggested in the previous lecture for dealing
with the Onion Fly. In the case of Currant Aphides
heavy pruning is valuable, as the eggs are present in
large numbers on the shoots. All prunings should be
taken away and burnt. (b) Winter measures: these con-
sist of using sprays, which have the effect of sealing up
the eggs with a coating through which the young Insect
is unable to make its way. A good mixture recommended
by Theobald consists of 1 cwt. of fresh lime, which
should be gradually slaked, and mixed with 100 galls.
of water in which 3o0lbs. of salt have been dissolved. The
addition of 5lbs. of water glass is stated to be an advan-
tage, though not essential. Failure in obtaining satis-
factory results are usually to be traced to want of care
in starting with freshly-burnt lime, or in slaking this. It
is best used as a late winter wash, as its effects wear off
owing to weather action if it remains on the trees all
through the winter. This wash is useful in sealing up
the eggs of the Apple and Plum Aphides, and may also
be used for the same purpose against the Currant Aphides
if pruning has not been He ‘(c) Spring spraying, which
should be done as early as possible. A useful mixture
for most Aphides is made up by boiling for 2 hours rlb.
of quassia chips (which must be quite fresh) in just
sufficient water to keep liquid. This solution should be
strained, and then well mixed with 10 galls. of warm
water, in which 1]b. of soft soap has been previously dis-

126

solved. Washes containing parafiin are liable. to injure
the delicate spring foliage, and if adopted should be
used in weak strength. When the leaves commence to
curl it is waste of most insecticides to use them; nicotine
and soft soap compounds are the only ones which offer
any prospect of partial success. Summer spraying is not
to be recommended except in the case of the Woolv Aphis.

Another family of Hemiptera, viz. the Psyllidae, or
“Jumpers,” includes the well-known Apple Sucker
(Psylla mali) (4), which is a not distant relation of the
Froghoppers, or “Cuckoo Spit” Insects. The adult Apple
Suckers are small, greenish-yellow, four-winged Insects
about 4in. long. They are to be found flying and leaping
about apple leaves from May until the autumn. They lay
their eggs from late September until early in November,
usually on the bark of one-year shoots below buds or
around leaf scars. The eggs are orange, darkening to
orange-red, and hatch in April. The young larve are
very minute, fiattened, dirty-yellow Insects with red
eyes, and they secrete a waxy substance from the hind end
of the body. As soon as the buds open they congregate
within, while the older larvz and nymphs are to be found
on the undersides of the leaves. Damage is caused by
the larvae and nymphs piercing the young leaves, which
become brown as if frost-bitten, and wither. In this way
floral and leaf buds are destroyed wholesale. The
adults cause a relatively small amount of injury. Un-
doubtedly the most vulnerable period in the life-history
of the Insect is when the young larvz emerge from the
egg; for various reasons, however, spraying at this time
presents difficulties. Owing to the waxy substance which
the larvze exude, sprays should contain a wax ‘solvent
which, however, is liable to damage the developing
leaves. Furthermore, the larve very soon enter the buds,
and then spraying is of little value. The larvae emerge
from the eggs during several weeks, and the time appears
to vary in different kinds of apple; for this reason
several sprayings are necessary. The best period for
dealing with this species is apparently February and
March, 2-3 weeks before the buds open. A wash of
lime and salt recommended by Theobald appears to be
an effective measure. It should be used on dry days, and
is made by taking 1-14 cwts. of best quality lime, slaking
it gradually, and mixing it with tco galls. of water in

127

which 30-40lbs. of salt have been dissolved. The mixture
should then be strained through sacking or other material
before being used. Lime washes are useful in other
ways and beneficial to the trees. This mixture coats the
eggs and prevents them from hatching, and also seals
up the buds protecting them from any larve that may be
hatched. In the autumn, spraying with paraffin is also
valuable—it should be done as soon as the fruit have
been gathered. so as to kill the females before the eggs
have been laid. Paraffin 4pts, soft soap i4lbs, and
water Io galls., forms a suitable mixture, but a stronger
proportion of paraffin can be used at this time of the year
if desirable. Heavy spraying is necessary, not only on
the leaves but also on the clouds of Apple Suckers which
are disturbed and take to the wing.

“The family of the Coccida, or Scale Insects (4), in-
clude some highly injurious members. The females are
degenerate, and spend their life hidden beneath a scale-
like covering formed by the cast skins of the larvez, cuti-
cular secretions, and other means. The males live under
smaller but similar scales, and when mature issue as
minute winged Insects. The Mussel Scale (Lepidosaphes
ulmi = Mytilaspis pomorum) is the commonest and best
known species, and is an abundant pest of apple trees.
During the spring and summer it sucks the sap, and
passes the winter in the eggs which are hidden beneath
the parent female’s scaley covering which still remains.
One or other of the Woburn washes (4,15) have given
good results in destroying the eggs of this Insect. They
can be used any time during the winter, and the 10b
formula given by Pickering and Theobald (15) is as good
as any. The Brown Scale (Lecanium persice) often
attacks currants and gooseberry, and the Woolly Currant
Scale’ (Pulvinaria vitis) both currants and vines. For
use against these two Scale Insects the wash 1ob referred
to above, is generally recommended and should be well
sprayed over the bushes during January. With regard
to Scale Insects on vines different treatment 1s necessary,
and the same applies to Mealy Bug and to the Green-
house White Fly. The latter belongs to an allied family
of the Hemiptera, viz. the Aleurodidz, and is a minute
moth-like white Insect with four wings. Its larve are
green and resemble young Scale Insects, and are destruc-
tive to tomatoes and other greenhouse plants. For

128

Insects in greenhouses which attack the leaves of plants,
and do not live in the soil, fumigation with hydrocyanic
acid gas is the best general remedy. It is made up as
follows :—2 ozs. cyanide of potassium, 4 ozs. sulphunc
acid, and 8 ozs. of water. Estimate cubic space of green-
house and use this preparation for each 1,000 cubic feet
of space, doubling the quantity for 2,000 cubic feet, and
so on. First close up all windows and doors and any
holes should be securely filled up. One window should
alone be left open for purpose of introducing the mix-
ture, which is extremely poisonous to man and all forms
of animal life, but does not injure plants. Use at dusk
when the plants are dzy, and the temperature of. the
greenhouse should xot exceed 60 F.—the best temperature
is 50 F. Do not use when there is a bright light (day-
light). Pour the water into a jar first, and then add the
acid slowly. Wrap the cyanide in a piece of blotting-
paper and drop into the jar with a suitable instrument
from outside through the window of the greenhouse.
Then close the window and leave for at least 1 hour.
Afterwards open all doors and windows from the outside,
but do not enter the greenhouse until another hour has
elapsed. Cost about 6d.

Chapter 17.

BENEFICIAL ANIMALS, &c.

Birds. The Testacella Slug. Earthworms. Centipedes. Bene-
ficial Insects. The Isle of Wight Bee Disease. Literature bearing
upon Economic Zoology.

Beneficial animals are on the whole less widely known
than injurious species and, unlike the latter, they should
be encouraged so far as may be possible and under no
circumstances be destroyed. BIRDS (1, 2) occupy a very
high place as benefactors of the farmer and _horti-
culturalist. There are a number of kinds which cause
litthe harm, and are in many cases, directly beneficial.
Among these may be cited the Fieldfare, Hedge Spar-
row, Wrens, Long-tailed and Coal Tits, Wagtails,
Pipits, Swallow, Martins, Swift Cuckoo, Plover, and
many others. Tits, for instance, are particularly partial
to Scale Insects, as well as Aphides and other Insects.
The Willow Wren devours large numbers of various In-
sects, and Newstead records one whose crop was filled
with larvze of the Winter Moth, another with Aphides,
and three other individuals contained large numbers of
Fly Maggots. Pipits, the Cuckoo, and Swift are also
prominent devourers of Insect life. Occasionally, how-
ever, one or other species may be observed devouring
fruit, the Mistle Thrush, for instance. but I believe in
such cases the small amount of harm they cause is neglig-
able compared with the benefit they confer. In this
country much could be done along Continental lines to-
wards encouraging beneficial Birds, especially by means
of nesting-boxes, which help to ensure their presence in

129

130

desired areas (19). The Department of Economic
Zoology in this University has achieved some good re-
sults in the larch plantations of the Manchester Corpora-
tion catchment area around Lake Thirlmere. Here the
Larch Saw-fly is most destructive, and by fastening on
the trees large numbers of nesting-boxes suitable for Tits,
which prey on the larve of this Insect, direct benefit has
resulted. ;

Among Mo.Ltuscs the carnivorous Slug Testacella
alone is valuable. It is a dirty white or yellowish form
with a small shell situated at the hinder end of the body.
In injurious Slugs the remains of the shell is always very
far forwards and in close relation to the respiratory
pore. It is local in this district, but has occurred in
several localities. Testacella feeds upon other Slugs,
Worms, and dead animal matter, and causes no harm to
vegetation.,

EARTHWORMS are true segmented worms and_ differ
from Eelworms. In the course of burrowing Earth-
worms let in moisture and air, the subsoil becomes
loosened, and direct benefit therefrom is derived. Large
quantities of earth are swallowed by them, which they
pass out of their bodies in the form of “ worm casts,”
commonly seen on lawns and flower-beds. In this
manner fresh soil is constantly being brought to the sur-
face, and at the same time Earthworms draw numerous
leaves and other kinds of vegetation into their burrows
which they consume in appreciable quantities. Humus is
partly due to the activities of Earthworms—the bringing
of soil to the surface and the burying of vegetable
material is an important factor in the humus formation,
which adds to the general fertility of the land. Darwin
calculated that as much as ten tons of soil per annum
passes through the bodies of Earthworms and is brought
by them to the surface, over each acre of good land.

MILLIPEDES (7) belong to the class of Myriapoda
which are more closely related to the Insecta than to any
other group. Like the Insects, Myriapods are provided
with a single pair of feelers or antennz, but they always
possess more than six pairs of legs, usually a large
number, and never acquire wings. The two main groups
of the Myriapods are the Centipedes or Chilopoda and
the Millipedes or Diplopoda. The former, which are
beneficial rather than injurious, have a somewhat flattened

131

body and a single pair of legs to each segment. So far
as known they feed upon small worms, slugs, insect
larvze, etc., and also upon dead animal matter. ‘The Milli-
pedes may be readily recognised by the possession of two
pairs of legs to each body segment. They occasionally
cause injury to potatoes and other root crops, and are
often known as “ false wireworms.”

Among INSECTS (10) many kinds are beneficial and
may be divided into two groups. (i) Predaceous Insects
which attack other Insects directly and devour them, and
immediate benefit is derived from their action. Among
these may be mentioned Ground Beetles, Lady-Birds and
their larve, the larve of Hoverer Flies, Wasps, Robber
Flies, and others. (2) Parasitic Insects which deposit
their eggs in the bodies of other Insects or in their
immediate neighbourhood. They pass the greater part
of their life within their hosts, whose death they sooner
or later bring about. Parasitic Insects amount to tens of
thousands of species and constitute Nature’s most effec-
tive method of control over the excessive multiplication
of Insect life. They are almost exclusively confined to
the orders Hymenoptera and Diptera. One or two of the
most important families of beneficial Insects may be
briefly mentioned.

GROUND BEETLES, or Carabidz, form a very exten-
sive family, comprising a large number of British
species. They can be recognised by their hard convex
bodies, long thin legs and slender feelers, and their very
active running habits. The majority are beneficial, de-
vouring other Insects, Molluscs, and dead animal matter.
Some few, however, are known to be injurious, but it is
a safe rule never to destroy these Insects when seen, with
the exception of those frequenting strawberry beds, which
usually pertain to a harmful species. The larve of
Ground Beetles have very similar feeding habits to the
adults. They are elongate fleshy creatures, with a
definite hard, brown head and first segment, three pairs
of legs and two horn-like processes, either long or short,
at the hinder end of the body. They frequent the soil,
can run freely, and their pupz are found buried some
inches below the surface. ADY BIRDS are beetles be-
longing to the family of the Coccinellide. We have a
number of species in this country, and_one of the com-
monest and best known is the Seven-Spot Lady Bird

132

(Coccineila septempunctata). Both as larve and adults
Lady Birds devour great numbers of Aphides and Scale
Insects, and for this reason they should never be
destroyed. The females deposit their eggs as a rule on
Aphid infested plants so that their larvee may not have
far to wander for their food supply. _ The Beetles are
all very similar in shape and are mostly black and red,
or black and yellow in colour. They hibernate during
the winter beneath bark of trees, under rubbish and in
outhouses, etc. In the following spring they lay their
cream-coloured eggs closely packed together in groups.
The larve are black or leaden-coloured, marked as a rule
with yellow or orange. They crawl freely about the
plants and consume great numbers of Aphides and other
Insects. The pupz are attached to the upper or under-
sides of the leaves and are broad black objects marked
with cream-colour or yellow. The adult Lady Birds
appear early in summer and are common objects of the
field and garden throughout the season. It is note-
worthy that the destructive Scale Insect [cerya purchasi
which devastated the orange groves of California
has been almost entirely destroyed and checked by the
importation into America of an Australian Lady Bird
Novius cardinalis. The Scale Insect has thus remained
permanently controlled, and the Nowiazs Beetle is now a
regular resident in California. The orange Scale Insect
has been controlled by a similar measure in Florida, New
Zealand, Portugal, Cape Colony, Formosa, Egypt, France,
and other countries. HOVERER FLIES belong to the family
of the Syrphide. They are often brilliantly coloured,
being black with yellow bands, and have the appearance
of small Wasps. They hover in the air, remaining
stationary, except for their vibrating wings, over one spot
for several minutes and then, darting away suddenly,
hover again over a fresh spot. They only fly in sunshine,
and rest on leaves and flowers in dull and wet weather;
they feed mainly upon nectar. Most species of Hoverer
Flies lay their eggs among colonies of Aphides, and
their maggot-like larvae on emerging feed voraciously
upon the latter. When fully fed the pupz are to be
found enclosed in membraneous puparium on the leaves
and stems of plants close to where the larve lived. The
greater number of the members of this family are, there-
fore, beneficial Insects.

133

ICHNEUMON FLIES and their allies are parasitic Insects
belonging to the order Hymenoptera, and are related to
the Bees and Wasps. They can be recognised by the
presence of four transparent wings, and by the posses-
sion in the female of a long needle-like ovipositor
or egg-laying instrument. Almost every species of In-
sect 1s preyed upon by some other species of Ichneumon
Fly or its ally. By means of their ovipositor these
Insects pierce the skin of other Insects and lay their eggs
within the body of the latter. The host Insects are not
killed by this operation, but continue feeding. The
larvee of the parasite hatch out in due course and
gradually devour the blood and tissues of the host, avoid-
ing, however, the vital organs until the very last. The
parasite turns to the pupz either within or outside. the
host Insect, and just about the same time the latter dies.
Some Insects are greatly destroyed by parasites and thus
kept in check naturally—the common Hawthorn Scale
and the Gypsy Moth are familiar examples. In America,
however, the Gypsy Moth has got introduced artificially,
and has now spread with alarming rapidity and causes
millions of dollars damage annually. Unfortunately the
natural enemies of this Moth were not imported, and the
U.S. Department of Agriculture recognise that the only
way of ridding the country of this Insect is by importing
its natural enemies or parasites from Europe. This work
is now steadily going ahead on a very large scale, but
it is too early to know the final result. In Italy mulberry
cultivation has been threatened by a Scale Insect. Dzasgzs
pentagona. In 1891 the pest assumed such serious pro-
portions that the italian Government passed a legislative
measure compelling mulberry cultivators to use all avail-
able means for coping with it. This, however, proved of
little efficacy, and it was not until Professor Berlese, of
Florence, introduced great numbers of a minute parasite
from America into Italy that much headway was made.
From all later reports it appears that this parasite has
acclimatised itself to Italy and is proving of great value
in destroying the Mulberry Scale.

Of all beneficial Insects the Hive Bee is the most use-
ful species. It is desirable here to make some reference
to the Isle of Wight Disease (18), which is causing much
mortality among apiaries all over the country. The
epidemic is due to a minute Protozoan organism (Nosema

134

apis), and the disease is spread by the distribution of its
spores among unaffected bees. The disease is primarily
one of the digestive system, and aftected Bees are, as a
rule, unable to fly more than a few yards without alight-
ing. As the disease progresses the Bees can only fly a
few feet from the hive, and then drop and craw! aimlessly
over the ground. They may often be seen crawling up
grass stems or up the supports of the hive. Diseased
Bees frequently lose their power of flight, their abdomens
become greatly distended, and often the wings are “ out
of joint,” the hind wings protruding upwards and outside
the anterior pair. In bad attacks large numbers of
diseased Bees are to be found clustered together within
the hive, or on the alighting board and ground. Some-
times the symptoms resemble those of “ Bee paralysis” or
of ‘‘dysentry,” but nevertheless the disease is quite dis-
tinct from either of these complaints. The spores of the
disease are spread in various ways; thus water near the
hives becomes infected with Bee excrement, containing
the spores, which are liable to be imbibed by the next
Bee which visits the same spot. Honey, pollen and wax
also become infected with the spores in a similar manner,
and are very fertile sources for spreading the disease.
Infection from one hive or apiary to another can be
effected by the sale of diseased swarms, by the robbing
of a diseased colony by healthy Bees, and by swarms
occupying hives which were formerly infected. Bad
weather also encourages the spread of the complaint, as
the bees do not then take cleansing flights. There is
evidence to indicate that partial immunity of stocks
happens. Such stocks, however, might be hard to recog-
nise, and at the same time would act as sources of infec-
tion for susceptable colonies. As regards measures
against the disease, healthy stocks should be removed
from the neighbourhood of diseased hives. Clean water
should be readily accessible, changed daily and protected
from contamination. The usual drinking places should,
if possible, be done away with. All dead Bees should
be ‘burnt and diseased colonies, including the queens,
destroyed. The ground around the hives should be dug
over and treated with quicklime. Infected hives and all
utensils should be charred inside and out with a painter’s
lamp. In place of charring a very thorough application
of tania or carbolic acid may be used, and the hives

135

afterwards properly aired in strong sunlight. No cure
for the disease has so far been discovered. The problem
of hereditary infection is of great importance, but I am
not aware that any evidence thereon is yet forthcoming.
If the queen is capable of transmitting the Nosema
parasite to the eggs, the young brood would thus be born
infected, and the disease be passed from one generation
to another, as happens in the case of the Nosema which
causes Pebrine in silkworms. Investigations along these
lines are greatly needed.

LITERATURE BEARING UPON ECONOMIC ZOOLOGY.

(1) NewstTeaD, R.—The Food of Some British Birds. Supfle-
ment to Journal Bd. of Agriculture, vol. xv, 1908 (4d.).

(2) CoLtincE, W. E.—The Food of Some British Wild Birds.
Dulau & Co., 1913 (4/-).

(3) LercH, H. S.—Interim Report on The Feeding Habits of the
Rook. ‘London: His Majesty's Stationery Office, 1915.

{4) THEOBALD, F. V.—Insect Pests of Fruit. Published by the
Author, Wye Court, Wye, 1909 (£1 Is. ?).

{5) RUSSELL, E. J., and PETHERBRIDGE, F. R.—Partial Steriliza-
tion of Soil for Greenhouse Work.—/ournal Bd. of Agri-
culture, vol. xviii, 1912, p. 8009.

(6) THEOBALD, F. V.—Injurious and Beneficial Slugs and Snails.
Journal Bd. of Agriculiure, vol: xi, 1905, pp. 504 and 650.

(7) THEOBALD, F. V.—Second Report on Economic Zoology.
London: British Museum (Natural History), 1904.

{8) Contince, W. E.—The Economic Importance of Wood-Lice.
Journal Bd. of Agriculture, vol. xxi, 1914, p. 206.

(9) CarRPENTER, G. H.—lInsects, their Structure and Life. Dent
and Co. (4/6).

(10) Matt, L. C.—Injurious and Useful Insects. Bell & Sons.
(3/6).

(11) Witiams, C. B.—The Pea Thrips. Annals of Applied
Biology, vol. i, 1914-15, Pp. 222.

{12) Leaflet, No. 10. (Wire Worms). &d. of Agriculture.
‘Gratis on application.)

(73) THEOBALD, F. V.—First Report on Economic Zoology.
British Museum (Natural History), 1903.

136

(14) Narcissus Flies. /Journai Bd. of Agriculture, vol. xvi, 1914,
p. 136.

(15) PICKERING, S., and THEOBALD, F. V.—Fruit Trees and their
Enemies with a Spraying Calendar. Simpkin, Marshall,
Hamilton, Kent & Co., 1908 (1/6).

(16) Hewitt, C. G.—House Flies and how they Spread Disease.
Cambridge University Press, 1912 (1/-).

(17) ORMEROD, E. A.—Manual of Injurious Insects. Simpkin,

: Marshall, Hamilton, Kent & Co. (5/-).

(18) Imus, A. D.—The Isle of Wight Bee Disease. Journal Roy.
Agricultural Society, vol. Ixxv, 1914, p. 62.

(19) HiksEMANN, M.—How to Attract and Protect Wild Birds.
Witherby & Co., 1912 (1/6).

(Nos. 9 and 10 are good elementary works on Insects; No. 4
is the best book of reference on fruit pests, while No. 15 is an
elementary outline of the same subject; Nos. 7 and 13 are reports
on various species of injurious Insects and other animals; No. 17
is a general book on pests of fruit and vegetables and, although
out of date, contains useful information; Nos. 1 and 2 deal with
the economic status of Birds, and No. 19 with the encouragement
and protection of the beneficial species. The remainder of the
references deal for the most part with special subjects referred
to in these lectures.)

INDEX.

Abraxas grossulariata
Acari

Acid manures—effect on fungal
POWER. sizanw nendcndavmrwoasinenmeegaenes 70
Acid soil ........ ay
Acidia heraclet .....cccee - 115
Aecidiospores on barberry 92
Agriolimax agrestis ......... 102
Aleurodide wo... , Ja?
American Blight ....... eee 124

American felt-paper discs for
Cabbage Root Fly... 117
American gooseberry mildew... 88
Anisopteryx aescularia . 109
AMNUAIS: sssccssvcssiisisren 2
Antennae 107
APHIGeS: croscvessecowenonnsineues g2.3 121
insect enemies of ............ 132

Aphides, spraying methods
against
Aphis brassicae
fitchii

Sordi _....
Apple Aphides
Apple, insect enemies of ...... 108,
122, 124, 126,
Apple, self sterile varieties
Apple sucker... eee
Arion ater
Arsenate of lead . 108, 109, 110,
Arsenate paste 108
Artificial selection —
Ascent of sap
Asters—wilt or Black-Leg disease... 81
Australia, wheat rusts in 93

Bacillus solanacearum .......cceeceseeeee 84
Bacteria, nitrifying 12
Batteriosis of tomato 84
Bacterised peat .......... 13
Banks, Sir Joseph ............. 90
Barberry, aecidiospores on 92
Barberry and wheat rust 92
BEY? 1G: vices sisi spitmandaiiiten'n 92
Bean, Aphis: saseaviuvouypssrecns everson. 122

PAGE

Bee Hive .... een
DISCBSES, Ol: —svicscsiwurirrwneamaninadainaiy

Beetles ....... 112, 131, 132
B@BONIS:. esas stsviaysnnsamnmsaucien sticisipaa connie

Beneficial animals 129 et seq.
Biennale sessuseimecenonsmawsnmerrezerrs

Birds in relation to other animals
98, en as 114, 117, 129

Black Currant Gall M 103

Black currants, ineeet a of

103, 123
‘“Black-Leg ”’ of Asters ............ 81, 83
Black Seab of potatoes 70
Black Slug ow... .. 102
Black Striped Slug _....... .. 102
Black stripe of tomatoes . 84
Blackbird. cccsvaasorcgsscopenne 98, 103
Breeding of plants . 10
Blister rust of pine 93
Blood-sucking Flies 113
BLES AU. susnssissasasaianqiyeatasietesteeSiuaasuceretee i 98
Biffen, Prof., and disease resis-
RAISCE: a inasersumgsinankcmmunranieneniananasiniy
“Big bud”
Bone Meal
Books dealing with Economic
LOOOEY session vtec cteamanedtseemssmnenians 135

Bordeaux mixture as a Fungi-

2 i DIANE Rea na erence mere omen e en , 84
Bottomley, Professor 13
Bracket fungi ........... 61
Breathing of plants it

of roots 7
of germinating seeds ... 43
pores of shoots ............ 7

Brown Scale
Bud variation

Bulb, ripening of 8
Bulbs, insects attacking

Bulbil cece
Burning sulphur as a disin
Burning of refuse
Butterflies

Cabbage Aphis
Cabbage Butterflies
Gahbenne and cauliflowers ‘ Club

oot ”

65

PAGE

Cabbages, insect enemies of... 107,
116, 122
Cabbage Root Fly... 166
CAChi. muses cesnadescunmbenaascumencaapaadctmess 23
California, beneficial insects in ... 132

Carabide
Callus)...
Carbolic Acid,

worms
CerOus Acid, for sterilisation of

Carbone Acid and Lime, remedy

for Onion Py oo... s-5sen onsdint: 118
Carbon Bisulphide, remedy for
Leather Jackets ....0.0.ec ee 114

Carbon _ Bisulphide,
Root Form of Woolly Aphis
Carbon nutrition
Carnation, layering of
Carnivorous Slug
Carpocapsa pomonella
Cauliflowers, insect enemies of .. 116
Celery Fly es

remedy for
.. 124
20

Cells, diseased by fingers-and- 67
Celosta cristate ...........cceeeeeerseeeeeeees 50
Cercospora melonis .. 85
Chaffinech.............. 98, 99
Cheimatobia brumata _....... i» AOS
Cherry, irisect enemies of 108
Cherry, pruning of ....... 35
Chilopoda ............... 130
Chortophila brassicae 116
CRY SBS, sicccndensizevinamsycnaargcunirinn 105
C lutosporium fulvum on tomato . 83
Clay: SOUS). scmcvensausnarenacyetscieaassaaens
Click beetles

“Climbers ......

Coal, Tit; sisgevcctmegreseosrinnag

Coccinella septem-p

Codling Moth. sescesceesseseccaseenusioary 109
Coleoptera —.........eeeeeeeee 112, 131, 132
Conidia of cercospora melonis ...... 85
Conidia of Phytophthora 74
Contact, sensitiveness to 6
Copper sulphate ........... 84
CORTON - secaiduons (aroeetuctot niece awouceroal staictkesk 29

Crab apple, use for pollination ... 39

Crane flies ......... 113
Crested varieties 49
CEOCUS: xeie, seseacsetgentncseedead 29
Cross pollination, effect of ......... 37
Cruciferae—Club Root disease 65, 66
CUCKOO. gexjcevaerescenomianuanee meeeeitent 129
“Cuckoo Spit’. insects — ............ 126

Cucumbers attacked by Eelworms 100
Cucumber, disease resistant varie-
HES! scirmcasiannnnstresseunarnnitatenlzeidiiaanssnaas a Ee
Cucumber, leaf disease
Currant Aphides

oo
rants, insect ae of ... 10
ae 110, 122, 123, 12
Currant Moth ..........:cccccceeeeeeteeees
Currants, pruning of
Currant Scale Insects
Cut-leaved varieties
Cuttings

“Daddy Long Legs’
Dahlias attacked ataewie
“Damping off’’ disease i
“Damping off” methods of pre-
vention
" Demuiny 6 off ’’—symptoms
Damson Aphides
Darwin, Charles
Darwin quoted
Desert plants
Destruction of diseased plants 69,

, 88
De Vries’ mutation theory ......... 45
Diaspis pentagona ...........0 133
Diplopoda .............. . 130
Diplosis pyrivora oc. ceeccecceeeeeeeeeeeee 114
Diptera: sen. ence sau neti cincnoomeene vet 113, 132

Discs, felt-paper for Cabbage Root
MEY. soared tem anasnd Reger cinaeas seni stelecatepetiNs 117

cumber
Diseases caused by fungi
Diseases of Bees ............
Diseases of Silkworms
Diseases of Man caused by Insects
113, 119, a

Dominant characters
Double flowers ...........
Double-fruited orange
Drainage of soil
“Dysentery ”

in Bees

MAECHWOLINS!.xesncssnecnnacemensencmensuenion 130
Earwig ...... .... ». 105
Eelworms ap BS
Elateride ..... wa 11
Elm Aphis _..... 121
Environment, effect of . . 24
Epidemics of late blight 78
ETVODhYys TIS -pesemwenoasvenrscanceseotens 103
Eriophyidae ..... 103
Erysiphace® ....... 002.0... 87
Ether vapour for forcing 36
Evening primrose .......... . 4
Eye of potato ..0.0......ccceceeeeeeeee oe 27

‘False Wireworms”
Farmyard manure .....
Fasciation

139

ae PAGE
Fertilisation 00.0... 37
Field Fare 129
Blies ow... 113
Hoverer .......... 132
Flowers, production of 34
Flower, structure of 36
.Fluctuations oc . 45
Fog; effect on vegetation 55
Food materials in turnip 67
Forficula auricularia .....0...... 106
OQGING | sancisncesgnascstuntaranenanascanpasiont 35

Formalin, remedy for house flies 120
Formalin as soil disinfectant ... 83, 86

Forsythia, pruning of 35
“Frog hoppers” . 126
ioee effect of ....... 53
Fruits, ripening of . 40
Fumigation Methods . 128
Fungi as parasites. 60
Fungi as saprophytes 60
Fungi, nutzition of 60
Fusarium oo.ccceccvceeccees 82, 83
Fusarium lycopersict 84
Fusarium SHG sascssonsurarcsarn 76
Gall: Mites: conmswgar ag aesensnanox 103
Garden Snail ow... . 101
Gas Lime for Helworms . 100

for Wireworms .... 112

for Leather Jackets ...... 114

for Cabbage Root Fly ... 117

Germination ..............ccccceseeeceeeees 42
GIAGIOLUS aie sasssssarcrvatierevssauanctuenstens 29
Gooseberry, American mildew ... 88

Gooseberry, layering of
ooseberry, rust on
Gooseberry Sawfly

Gooseberries, insect ee of

110, 127

Grafting, i
Grass, insect enemies of
Grease banding a
Great Tit ..... . 98

Green Fly i
Greenfinch .
Greenhouse Pests
Greenhouse White Fly
Grey Slug
Groundsel
Gymnosporangium
Gypsy moth, methods of canal 5
in America

Hand picking for “ big bud” .. 104

Hand picking for cabbage cater-
pillars 107

Hand picking for currant cater-
pillars 108

PAGE
ar eed picking for gooseberry saw-

'y 1
Hand nicking for bean Aphis ... 122

Hardening off seccssasossoarsesarsaneisees 24
Healing of wounds 52
Hedge Sparrow uu... ... 129
FRG] G1 Bet getsnctoccnsssdopecrmncsepetqe cin . 101
Helix aspersa 101
remoralis ... 101
PULOSCONS -sasaictnnansitectownssatemcitaiiva 101
Hemiptera... o. 121, 126, 127
Heterodera radicicola .occeeccceesceceee 10
Hive, Bee oo... 133-34
Hoeing, importance of ............... 12
Hollyhock rust... 94, 95
Honey agaric oo... ceccceeeeeeeees 61
Hop Aphis 122
Hot- water bath for forcing ......... 36
Hot Water, remedy for Narcissus
HIGS. screneaniaeapygeranvndennamenany tataaaets 119
Hot Water, remedy for Rose
Aphis) sacawcineenansren wea wris 123

House Fly as a disease carrier
119, 129

House Sparrow

Hoverer Flies

Hydrocyanic A Gas

Hydro-oxide of Calcium
Hylemia cepetorum
Hymenoptera

Leerya purchasi
Ichneumon Flies
Infection of asters
Insect pollination
Insects
Methods of feeding
Beneficial insects
Insects as carriers of disease in

IMEAny: sasctncaceouatasmiemdnn. «cetains au 119, 120
Insecticides and their uses ...106,
122 125

Isle of Wight—Blue disease 1133. 34
Italy, control of Mulberry Seale in 133

J erusalem artichoke
Jeyes’ fluid

‘Jumpers ”’ (Psyllidea)
Juniper

Kainit
Kakothrips robustus
Knot Root Helworm

1406

PAGE

Lady Birds wo... 131, 132
eta Sawfly » Lol
sedanucaninnceumeeneencnetys . 105

Lae “plight of persis . 7
Hee oms ... : 74

gansietievenamaheene 74, 75

ead

Late blight» Aw einen oni”

tions
Late blight and potato tubers
Law against barberry
TA VORIN GS siosescaum beau rer ation leremeneiensce 2
Lead, arsenate of . 108, 109, 110, 111
Leaf blotch of cucumbers 85

Leaf-cuttings oo...
Leaf mosaic 2... eee
Leaves, absorption of water by ... 23
arrangement of ............ 18
needle-shaped » 2
pores Of siswes = ee
wilting of . 22
structure of . 2
work of _...... . 20
‘‘Leather Jackets” . 113
Lecanium persicae » 127
Leguminous plants i; AS
Leigh, quoted ......... 99, 135
Lepidoptera _...... . 107
Lepidosaphes ulmi ......... » 12f
Light, effect of, on plants 5
Lightning, injury by 53
DSL Y* seemosanwasesscneaaetteoentins 29
Liracias sisleuias 101
Limax Maximus ..e.cccceccecceecees 102
Lime for Cabbage Root Fly _...... 117

Lime and Carbolic Acid, remedy

for Onion Fly
Lime and Salt spraying .......
Lime and Soot, remedy for ‘Slugs 102
Lime and Sulphur for “ big bud’”’ 104
Liming of soils ca

Liming of soil against ‘‘ Club
Root” disease oes 69
GGIATIOG? adc sieeve stb ccnvdinntsinonacmonednnieee 98

Literature bearing upon Economic

Zoology
Liver of sulphur...
Longevity of seeds
Long-tailed Tit

'

Macrosporium solani
Malformations ..
Mallow, rust .....
Manure, farmyard
March moth
Martins

Mealy Bug
Mendel, Cesar
Merodon equestris
Michaelmas daisy
Mildew, American gooseberry

PAGE
Mildew of roseS ou... tenes 87
Mildews, treatment of .. 88
Mildews of leaves ......... v.87
Millipedes _........... 103, 130
Mint rust vowx OA
Mites ........ .. 103
Mollusea_......... ...- 101, 130
Moorland plants - 2
‘‘ Mosaic disease ”’ 51
Mosquitoes 113
Moths _.......... 107
Motile Spores .n..cacs ox 63
Mould on bread and dung 59
IMUGCOR) - dencsacmansieccnieenad paupesneae ta 59
Mucor—spores—germination _...... 59

Mulberry Scale, control of, in
GOA“ dasvacendiaahcn- asian asad uwekunnts 135

Musca domestica .....102-.:00006 . 119
Mushrooms and toadstools . 8
Mussel Seale... ee. , at
Mutations we 6
Myriapoda _............. .... 130
Mytilaspis pomorum .... 127
Myzus ribis wee 122, 123
Napthalene for Eelworms _......... 100

for Wireworms ... 112
Narcissus Flies ata secicmscstas te acienih ater 119
“Nasturtiums,”’ enemies of ...... 107

Natural selection
Nematus ribesii
Nesting boxes
Nicotine spraying
Nitrate of soda
Nitrate of soda, remedy for Slugs
and Snails 102
Nitrate of

soda, remedy for

“Leather Jackets” .............. 114
Nitro-bacterine ............... vex, DS
Nitrogen, importance of me
Nosema apis .....00 cece 133-34
Novius cardinalis we 132
Nutrition of fungi... 60
CRT BIN oo cdconesnheadaciommaoinecanendoers 117
Orange Scale and methods of

eradication in California ......... 132
Orthoptera ooo... 106
Overwatering — ..........eeeceeee 7
Ovipositor ........... 114, 133

Oxygen, need for

Paraffin, remed for Cabb Be
Rect > vy ag

Paraffin and Soft Soap amuses
118, 125, a

“ Paralysis” in Bees
Parasites

Pea Thrips
Peas, insect enemies of ...............
Pear, insect enemies of ...
Pear Midge ..............
Peat, bacterised
Pebrine in silkworms
Peony
Perennials 3
Phosphorie Acid, importance of... 14
Phytophthora compared with
Phythium
Riaiephitora, conidia
disease in Asters 81, 83
growth from diseased

tubers ... 77
hibernation ............ 77
PRJEMENE — sinsveressereree 73
initial infections ... 77
resting spores on

artificial media 77
10

Pieris brassicae and P. rapae ...... 7
Pines PUSt sunisrcanimvcsmmaremcsntanian 93
Pipit ... 120 .
Plant TACO cosevsnosncnpiageniagowinoncoee signs 121
Plasmodiophora Brassicae ......... 65, 66

Plasmodiophora compared with

Plum Aphides
Plum, insect enemies of
Pollination
Potash, importance of
Potato

hak 108, 122
37

cut for “sets”

disease, resistant varieties 79
disease eet . 13
grown in the 27
attacked by Wireworm 112

attacked by Millipedes.... 131
Black Scab or Wart
disease 70
Black Speck or Violet
Rhizoctonia disease 71
control of scab diseases
Brown Scab or Ordinary
Scab ...... 72
mechanical irritation
Spongy or Powdery Scab... 72

Potatoes,

StOTASE assinccancasnanawerincien ae 76
Precautions against late blight
Gisease getwigeswenereraesnermyonncemereness 78
Predaceous Insects ........... 131, 132
Primrose, Jack-in-the-Green _...... 51
Pruning cisiivoviesacwemsserwesesrsverare 34
Pruning against “big bud”’ ...... 104

Pruning against Currant Aphides 125
Pythium compared with Plasmodio-
phora
Puceinia graminis
menthae
Aecidiospores in

PAGE

Puccinia menthae
Teleutospores iN we... 94
Uredospores in 94
Pulvinaria vitis eet
PUPS. suwitamuceis . 105
Puparium . 113
Psylla madi oe ccessecseesesseceeseesseeees 126

Pythium compared with Phyioph
TAGE 6 guasnannniraenncisanite
Pythium de Baryanum
Life History ... 61,

Pythium, resting spores
reproduction

Quassia and Soft Soap spraying ... 125
Quicklime as a fungicide 0

cae 69,

Radishes, insect enemies of:
Recessive characters
Red currant, layering of
Red Spider
Reproduction, vegetative
Respiration of plants, see breath-

ing
Resting filaments of
melonis
Resting spores
Retarded crowns 3
Rhizoctonia violace@ ... ccc 71
Rhopalosiphon ribis

DACING. © sig scacieoracinisinsonagypenenisaregeaksyiris 16
Ringing 30
RODIN, sssiseaanrenainecnreece 98
ROOK. sreecavangiisnaavgrars 98, 99, ar
Root, absorption by...

cuttings... a
hairs... 10
pressur2 9
PTUDING secspacvn cers sveys . 34
tips, sensitiveness of 4
tubercles .. 12
Rose Aphides » 122, 123
Rose mildew ............:ceeccececeeeseeeeees 87
Rotation of crops. ........... 15
Rotation of twining stem 19
RUNMNETS: — sesasweenvensmaersanany 29
Russell, E. J. es
Rust of wheat. ......... . 90
Rust on gooseberry ..... . 93
Rust resistant wheats 93
FRUIBES! cscccuencusilge. soit 89
Ruston, A. (Ge. scnisinsciwanranoninncssicsnete 56
oaeheeseen Lal al auc eamenia enleeiandainpaaiaesomet a

APTOPhytes. .acseesessvieis ceomverersieeres

BawAy srisssacwwers 110, 130

Scale Insects ...
Enemies of
Scattering of spores

142

PAGE
x

Schizoneura laniger@ — ..ecccecccccceeceeee 22
Sedge and gooseberry rust » 93
Seed coat occ 42
Seeds, germination of . 42
maturing of . 40

Seed potatoes ........... 78, 80
bepee deen “a

Seeds, vitality of ....... 41
Seedlings ‘ oping off” 61
Seedlings 44
Selection, natural 44
Self-fertile flowers 38
pollination ....... 37
sterile flowers 38
Septoria lycopersici ....... 84
Seven-spot Ladybird _.... 131
Silkworms, Pebrine in _............... 35

1
Siphonophora (Rose Aphides) 122, 123
Skylark 98
Sleepy disease of. tomatoes
Slightly diseased tubers, effect of

DANCIN ise suse tes ctaadednedeneenletnntend
SlUgS: _ siete eercrsesmnravqrencun 101, 130
natural enemies of ............ 103
Slug, carnivorous _................ ... 130
Smoky atmosphere, effect of 54
Sais. ereuctnsanssurecimtesonvucctadsun 101
natural enemies of 103
Sparrow, hedge ..............4. . 129
house .......... 98
Sphaerothect pannosa . 87
Spongospora Solani —wicccceeeeeesseeeeee 72
Spores of Cladosporium fulvum ... 84
Plasmodiophora _......... 67
Sporidia from teleutospores 92
Sports: sesamiae 45
Spraying of potatoes ................ 79
with lead arsenate... 108
109, 110, 111

with nicotine
with soft soap
Spring spraying
Soft soap spraying .....
with quassia
Soil, acidity o
bacteria of .....
Soils, chemistry of
Soil disinfectants

Soil, partial sterilisation of its bad
physical condition of _........ "10
spores cf fungi 63,64, 68, 82

Soot for Cabbage Root Fly ......... a4
and Lime for Slugs . ue

for Onion Fly
Starling 98, 99, 103, iit
Steam, for sterilising soil 14
Stem HEelworm

function of .
Sterilisation of soil
Stocks, insect enemies of

Stomat:
Bierce of potatoes 76
Strawberries, Eelworms oct a 100
Insect enemy of .., 131
1

Strawberry snail ..........0... 01
SODSOLL, eenwcsiespnneticecenaeonueawisitncans 15
Sulphate of Potash for Eelworms eH
Swallow wsnwdsenpoaasnneuavasncmninanteansvs

Swedes, ‘‘ Fingers-and-Toes”’ in %

Swedes, insect enemies of

Sweet Pea ............0. 39
Sweet Peas ciossed . 47
Swilt: — srasascsomcnnagaviernes . 129
Swift’s arsenate paste . 108
Swimming spores ............. 63
Symptoms of wilt in Asters 81

Synchytrium endobioticum
Syrphidae

Teleutospores of Puccinia
malvacearum 95
Hi i 91
a

of wheat rust

Tendril
Testacella
Theobald quoted
Thrips
Thrush, Mirtle
song...
Thysanoptera © ....
Tipula oleracea
Tits
Toluene, for sterilising soil
Tomato, leaf rust
pollination of
Tomatoes attacked by Eelworms... 100
by White Fly... 127

Transpiration
Transplanting
Traps for Earwigs
for Leather Jackets
for Wireworms
“Tree Tanglefoot ”

WRENCHING: sc cscsdivsniesacaindtonsanaieonciiuns 15
Trombidide . 103
Tsetse flies . 13
Tubers _....... 4, 26
EDD: scpracsisarsne nastics eeesedniewtaataatny seaieaity 28

9)
Turnip A his
‘ingers-and-Toes ”
structure of healthy root 66
Turnips, insect enemies of ... 116, 122
Turf Traps for “Leather Jackets 314
Twiners 18

Tylenchus devastatrix ...........00cc00 99
Uredospores of wheat rust ..... 90, 91
Variation in plants ... ........... . 44

PAGE PAGE

Variegated leaves 0.0... eee 50 »=Wilting of leaves eee 22
Vegetative filaments of fungus .. 58 Winter Moth ou... .. 108
Vegetative reproduction ............ 3,25 Winter rot of potatoes sda awee 76
Vermorel injector ........00.. ... 1144 Winter Spraying .... 125, 127
Vine, Scale insect attacking .........127 Wireworms .........ccccceseseeeesereeee 112
Viola, layering of ............. we §=30 “False Wireworms” .. we. 1ST
Vitality of seeds ........... .. 41 Woburn Washes _........... ve. 127
Voss’ Tobacco extract... 112 Wood Snail ......... .. 101
Wood Pigeon won 98

‘ Woodlice —......... s. 103
Wage) scenic narceginssnnaer eines 129 Woodpecker o..eeeeceeeecccceeeeeeeeeeeee 98
Wart Disease of potatoes . 70 Woolly Aphis .... 122, 124
Watering 22 Woolly Currant Scale ............. 127
Weeds. ........- 2 ,Wren oo ........ . 129

Weigelia, prun ng
Weymouth pine-rust
Wheat rust ........... sed
White Bly. jccwmamanseraiamranienenesss 127) = Yellow-hammer ...seeeeseeeeeeeeseeereeee 98

35 Willow Wren

NOW READY. Price 1/- net. Postage 2d.

A POCKET SYNOPSIS

OF THE

Families of British Flowering
Plants

(Based upon the system of Engler)

BY

W. B. GROVE, M.A,

Lecturer in Botany at the Birmingham Municipal Technical
; School.

Manchester
AT THE UNIVERSITY PRESS.

LONGMANS, GREEN & CO.,

London, New York, Bombay,
etc.

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