exceedingly common--that of the Ivy (_Hedera Helix_), belonging to the
order _Araliaceæ_. The Ivy is an evergreen climber, fixing itself by
means of little rootlike suckers attached to the main stem and its
branches, while the lower branches trail along the ground. The leaves
are thick and glossy, usually of a deep green colour, but often
beautifully variegated. Those attached to the trailing and climbing
stems have three or five lobes, are always turned with one surface
towards the light, and are so arranged as to obtain the maximum of
light, the less exposed leaves below catching the rays which pass
between the lobes of those which are more favourably situated.
[Illustration: THE STRAWBERRY TREE IN FLOWER, WITH THE FRUITS (ALMOST
RIPE) OF THE PREVIOUS YEAR.]
The branches of the tree do not, as a rule, produce flowers as long as
they are able to climb; but as soon as they reach the summit of the tree
or wall to which they cling, or reach a situation where there is a
sufficient abundance of light and air, they change their character in a
remarkable way. They now become bushy, cease to produce suckers, and
give rise to undivided leaves that turn in all directions for light and
air. At the tip of each twig is formed a cluster of yellowish-green
flowers, arranged in a short raceme or in an umbel. These flowers have
an inconspicuous calyx which forms a border round the middle of the
ovary, and five short petals. There are also five stamens, and united
styles. The fruit is a smooth, black berry, containing from two to five
seeds.
XXIII
PARASITIC PLANTS
A number of plants extract more or less of the organic material they
require from other plants, and thus save themselves the labour of
building up this material themselves. These are termed parasites; but we
must be careful to distinguish between them and certain other plants
which, though apparently parasitic, are not really so. One plant may
climb on another, perhaps even producing "rootlets" by which it clings
to its living support, and yet it may not be a parasite in the proper
sense of the term, for it may not absorb the slightest amount of
nutritious matter except from the soil and the air. It is not at all
uncommon for the Honeysuckle to twine its stems round the trunk and
branches of a young tree, with the result that the tree becomes stunted,
and assumes a starved appearance, especially in its lower parts; and yet
the Honeysuckle is not a parasite. It has withdrawn nothing from the
tree which supports it, but has coiled itself so tightly round it as to
interfere with the circulation of its sap. The lower part of the tree is
especially affected because the strangulating coils of the climber
prevent the downward flow of the sap contained in the vessels of the
bast or inner bark, and this is the sap which holds the constructive
materials that have been built up in the leaves, under the influence of
light.
Many of the parasitic plants are of microscopic dimensions, and others
are larger species belonging to the Fungi or Mushroom group. Some,
however, are flowering plants, and these only fall within the scope of
our work.
We shall first deal with parasites which have no green leaves or
chlorophyll, and are therefore entirely dependent on outside sources for
their supply of organic material, starting with the interesting Dodders
(_Cuscuta_), which coil themselves round herbs, shrubs, or even trees,
and produce sucking organs on their stems that come in contact with
their host.
These are all smooth plants, with globular clusters of yellowish-pink
flowers, the calyx being of the same colour as the corolla. The former
is deeply divided into four or five parts, and the corolla has four or
five spreading lobes with as many scales inside its broad tube. The
ovary has two distinct styles, and the fruit is a globular capsule. The
following summary of distinguishing features will enable the reader to
identify the British species of the genus:--
1. The Greater Dodder (_Cuscuta europæa_).--A plant of a greenish yellow
colour, generally more or less tinged with red, with flowers in sessile,
globular clusters nearly half an inch in diameter, each individual
flower being about a tenth of an inch. This species is not abundant. It
may be met with in hop-fields, and is also parasitic on nettles, various
shrubs, and trees, including the elder and the ash.
2. The Flax Dodder (_C. Epilinum_).--Very much like _C. europæa_, but
the flowers are fewer in number, larger, and more fleshy. The calyx is
nearly as long as the corolla, with sharply-pointed segments; and the
corolla tube is always globular. This species is not indigenous, but is
sometimes met with in flax-fields.
3. The Lesser Dodder (_C. Epithymum_).--A more slender plant, with
thread-like stems, and flowers in small, compact, globular heads, with
red calyx and cylindrical corolla. This species occurs principally on
sunny heaths, where it is parasitic on shrubby plants, such as thyme and
ling. It is much more common than the foregoing.
4. The Clover Dodder (_C. Trifolii_).--Very much like the Lesser Dodder,
of which it is sometimes regarded as a variety. Its calyx is of a very
pale colour, and is almost as long as the tube of the corolla, which is
cylindrical in form. It is rare, but sometimes appears in undesirable
numbers in clover fields.
All the species produce their flowers in August and September, but _C.
europæa_ may often be seen in bloom very early in July.
The seeds of the Dodder fall from the opened capsules during late summer
and early autumn, alighting on the soil, or on the decomposing foliage
that covers the ground, or on the rough barks of the tree that served as
a host for the parasitic plant. The seeds of many other plants fall
about the same time, but those of the Dodder do not begin to germinate
until about a month later than the majority of these, in the following
season, and consequently the young Dodder plants do not appear before
their future hosts have had time to grow sufficiently large to support
and nourish them. Perennial plants, too, which are attacked by the
Dodder, have also produced strong shoots and leaves from their roots or
underground stems by the time that the parasite begins its search for
ready-made organic food; and it is clear that if the Dodder seeds
germinated earlier in the season, the young plants would starve for want
of suitable herbs to give them support and nourishment.
[Illustration: GREATER DODDER, ON NETTLE--A COMPLETE PLANT.]
When the seed germinates it sends out a filament which penetrates into
the soil and fixes the seedling firmly. The other end grows upward,
carrying up with it a little swollen mass of food-reserve, sufficient to
support the growing seedling until it has had some chance of reaching a
suitable host. The upper end of the seedling now sends out a filament
which rapidly elongates, and, growing upward, searches for some stem on
which to climb.
All this time the little mass of food-reserve is being rapidly
exhausted, and if the young seedling fails to reach a suitable plant on
which to climb it soon dies, for its lower extremity is unable to absorb
sufficient food material from the soil; and the plant itself, having no
chlorophyll, cannot decompose carbonic acid gas and build up organic
material to add to its substance.
[Illustration: THE CLOVER DODDER, WITH A SEPARATE CLUSTER OF FLOWERS
REPRESENTING THE NATURAL SIZE.]
Again, should the young plant fail to reach a favourable support, so
that it is of necessity compelled to trail along the ground, the
filaments which would soon produce suckers when attached to a living
plant have no power to form any structures capable of extracting food
material from a damp soil.
Circumstances being more favourable, however, the upper filament
eventually finds a stem, and immediately begins to twine itself round
it, making a few close coils in a clockwise direction. Should the
support prove to be a dead stem, little wartlike swellings are produced
at points where the two touch, and these serve as a means of attachment
for the climbing filament, but no suckers are formed. If, however, the
filament surrounds a living stem, each of the swellings gives rise to
suckers that penetrate into the tissues of the latter, and withdraw the
organic food necessary for the continued existence of the plant.
The Dodder now grows rapidly, giving off branches which search in all
directions for additional supports, sometimes climbing from one plant to
another, and producing new suckers whenever a favourable situation has
been reached. The plant has now all it requires both in the way of
mechanical support and nourishment, and its lower part, thus rendered
useless, soon withers, breaking all connexion with the soil on which the
seed originally germinated. New branches continue to form, each one
producing additional suckers for the extraction of food from the host or
hosts, until a tangled mass of clinging stems is the result. Then the
globular clusters of little flowers appear, followed by balls of small
capsules which throw off their lids when ripe, allowing the seeds to be
shaken out by the wind. The Dodder plant now withers, leaving, in the
autumn, its dead tangles of climbing filaments still attached to the
withered herbs on which it fed, or to the branches of the tree which
served as its host.
Other parasitic plants possessing no chlorophyll, and therefore
incapable of building up organic compounds for themselves, derive their
food from the roots of trees and shrubs.
Among these is the Toothwort (_Lathræa_), which is carnivorous as well
as parasitic, and is described in our chapter (XXIV) dealing with
carnivorous plants, so that we need only refer here to its habit as a
parasite.
The seed of this plant germinates on the damp ground to which it falls
in early summer. The young root penetrates into the soil, deriving its
nourishment entirely from the food reserve that was stored up in the
seed, and soon sends out lateral branches in search of the roots of a
suitable host. If it fails to attain this end by the time that the
reserve is exhausted, it dies; but if it succeeds in reaching the root
of an Elm, Hazel, Hornbeam, Ash, Poplar, or other tree, it fastens
itself to it, and develops suckers which penetrate into the substance of
the root to extract its sap. The parasite now grows very rapidly,
producing its underground stems, with their fleshy, overlapping scales,
as described on p. 352.
The Broomrapes of the same order (_Orobanchaceæ_) are very similar in
their parasitic habits to the Toothwort, and, like the latter, they
possess no chlorophyll. The seeds germinate on the damp soil, producing
a long, narrow embryo that grows downward into the ground until it
reaches the root of some herb or shrub. It then gives off suckers which
penetrate into the root, and, with the aid of the organic food thus
obtained, forms a tuberous swelling on its surface. Flowering stems are
afterwards produced, and these, rising above the soil, bear terminal
spikes of lipped flowers, followed by capsules containing many seeds.
[Illustration: THE GREAT BROOMRAPE.]
There are several British species of this genus (_Orobanche_), and their
flowering stems, which are usually unbranched, produce scale-like leaves
of the same colour as themselves. Each flower of the spike is in the
axil of a bract resembling the scales of the lower part of the stem; and
in some species there is a pair of smaller bracts close to the base of
the calyx. The corolla is either tubular or bell-shaped, and more or
less distinctly lipped. Each flower has four stamens, arranged in pairs,
and a two-lobed stigma. The following outline of leading features will
serve for the identification of the common Broomrapes:--
1. The Great Broomrape (_O. Rapum_).--A plant from twelve to eighteen
inches high, of a pale yellow colour at first, but afterwards turning to
a dull purple brown. Stem thick, especially below, and unbranched.
Scales lanceolate. Flowers sessile, whitish, with only one bract,
forming a spike from six to nine inches long. This species is moderately
common, and is parasitic on the roots of Furze and Broom. Time of
flowering--May to July.
2. The Clove Broomrape (_O. caryophyllacea_).--Very similar to the Great
Broomrape in colour, but usually smaller, and easily distinguished by
the sweet clove-like scent of its flowers. Spike not so dense as in the
last species, and the corolla tube not so broad. The plant is not
uncommon in the southern counties of England. It is parasitic on the
roots of the Great Hedge Bedstraw, and flowers from May to July.
3. The Tall Broomrape (_O. elatior_).--Also much like the Great
Broomrape, of which it is perhaps a variety. It retains its original
yellowish colour for a longer period, and is parasitic on the Great
Knapweed, flowering from June to August.
4. The Least Broomrape (_O. minor_).--A yellow or pale brown plant, from
six inches to over a foot in height, more slender than the preceding
species, with smaller flowers. The flowers are whitish, but more or less
tinged with purple, and bloom from June to October. It is parasitic on a
number of different plants, including the Ivy, Clovers, Hawkweed, Wild
Carrot, &c., and is found in many districts in South and Central
England.
* * * * *
We have now to consider those parasites which bear leaves possessing
chlorophyll granules, and are therefore able to build up a portion of
the organic compounds necessary for their development. Most of these, at
least as far as the British flowering species are concerned, have also
true roots which grow into the soil and absorb mineral food, like those
of the non-parasitic plants allied to them, so that it is difficult to
understand why they should require the additional nourishment stolen
from the roots of neighbouring plants. One, however, the well-known
Mistletoe, grows on trees at a distance from the ground, and therefore
obtains the whole of its food, with the exception of carbonic acid gas,
direct from its host.
This plant--the Mistletoe (_Viscum album_), of the order
_Loranthaceæ_--is attached to the tree on which it grows by a thick stem
that becomes woody when old. Its branches are of a yellowish-green
colour, and are repeatedly forked in such a manner as to form a dense
tuft that often reaches a diameter of two feet or more. The leaves are
of the same colour as the branches, and are rather thick and fleshy. The
flowers grow in the forks of the branches, on very short stalks, and are
imperfect, the males and females being on separate plants. The former
are in clusters of about three or four, in a cuplike, fleshy bract, each
flower having four thick, triangular petals with an anther on the
middle. The females are either solitary or in clusters of two or three,
with a similar bract, and very small petals. The fruit is a white,
glutinous berry, almost transparent, with only one seed.
The Mistletoe grows on a variety of trees, including the Apple, Pear,
Black Poplar, and Oak; and thrives most luxuriantly on those which have
a soft tissue beneath the bark. It is found principally in the southern
and western counties of England, and flowers from March to May.
[Illustration: MISTLETOE.]
There is no doubt but that the seeds of the Mistletoe are distributed
from tree to tree by the agency of birds, especially the thrushes, which
devour the berries in large numbers. The seed of the berry is protected
by a covering which remains quite untouched by the digestive fluids of
the bird, and consequently it is expelled intact with the excrement, and
frequently drops to a branch of the tree, where it lodges in a crevice
of the bark, and is securely fixed in its place by the slimy excrement
in which it is embedded.
Here the seed germinates, sending out a little rootlet that always turns
towards the bark on which it rests, and subsists for a time on the
food-reserve that it contains. When the young root reaches the bark it
becomes flattened against the surface, and spreads out, forming a disc
that holds the seedling firmly to the tree.
A projection (the _sinker_) is then sent inwards from the disc, and this
penetrates the bark, reaching the wood beneath, but does not enter the
latter. This terminates the growth of the seedling for the first year,
but as soon as the warm weather of the following spring commences, the
sinker begins to spread over the surface of the outer ring of wood,
while at the same time a new annual ring of wood begins to form
outside, thus surrounding and banking in the sinker. It would appear, on
making a section of the tree, as if the sinker had actually pushed its
growth through the outer ring of wood, whereas it does not penetrate the
wood at all, but is only banked up by the new wood that grows round it.
This is repeated year by year, until the sinker is at last quite deeply
set in the branch, being surrounded by the wood of several annual rings.
[Illustration: A YOUNG MISTLETOE PLANT ON THE BRANCH OF A TREE. The
branch is cut longitudinally to show the suckers.]
During the second year's growth the sinker sends out little roots which
run up and down the stem, beneath the bark, and these give rise to new
sinkers that grow down to the surface of the wood, and become, in turn,
embedded in the new layers of wood that form round them. And while the
young Mistletoe plant is thus securing a firm hold on its host, and
withdrawing ready-made organic compounds from its sap, the outer green
stem develops, and soon gives rise to the first pair of leaves.
If food is obtained in abundance, as is the case when the host is a tree
of a soft and sappy nature, the growth is rather rapid; but otherwise
the development is comparatively slow. In any case the age of the
parasite may be ascertained by counting the number of annual rings of
wood that lie outside the deepest sinker; and by this means it has been
found that the Mistletoe may attain an age of over thirty years.
We have now to consider a group of plants, the parasitic habits of which
would scarcely be suspected by an ordinary observer. They are green
plants, with well-developed foliage leaves, and true roots which absorb
mineral food from the soil. Their seedlings grow in the same way as
those of non-parasitic species, deriving no nourishment from
neighbouring plants, but obtaining all their food from the air and the
soil, and building up all the organic compounds required for their
growth by the agency of their own chlorophyll.
It is difficult to understand why these plants should afterwards produce
suckers on their roots in order to obtain nourishment from other
species, but they do this, and experiments have proved that the food
thus obtained is more or less essential to their development. Some of
them die while still young if grown apart from other species, and the
others, under similar conditions, though they reach what we may term the
adult stage, remain somewhat weak and stunted, and produce but few
flowers and fruits.
Most of the plants referred to belong to the order _Scrophulariaceæ_,
and among them we may mention the Eyebright (_Euphrasia_), the Yellow
Rattle (_Rhinanthus_), the Cow-wheat (_Melampyrum_), and the Lousewort
(_Pedicularis_). They generally appear in large numbers close together,
often in such abundance as to determine the general colour of the ground
on which they grow, and yet they do not apparently cause much damage to
the grass and other plants which they rob.
These green parasites are described in various chapters, according to
their habitats and their flowering seasons; so we shall do no more here
than to briefly refer to their parasitic habits.
The Eyebright (p. 274) grows on heaths and downs, where it
derives organic food from the roots of the neighbouring grasses. The
Lousewort, too (p. 118), which grows in marshes and moist
meadows, is parasitic principally on the roots of grasses, apparently
without affecting the latter. The last-named species is a perennial, the
roots of which have to find hosts that are capable of supporting it year
by year. If the host of the present year should happen to die in the
autumn, the suckers that were attached to its roots soon die, and the
parasite has to seek a new source of supply. This it does by extending
its roots until it reaches a new host, and then producing new suckers.
Thus we are able to understand the origin of the long roots so often
seen on the Lousewort, and also the reason why these roots never grow
downwards into the soil, but always horizontally, just beneath the
surface. Further, since the roots extend themselves in search of food at
times when the supply is temporarily diminished or stopped, it is clear
that some reserve is necessary for the elongation referred to. Such a
reserve exists in the older, thick portion of the perennial root, near
the base of the stem.
In the case of the Cow-wheat (p. 146) no suckers are produced
until the lateral branches of the root of the seedling reach a moderate
length; but in order to increase the chances of finding a suitable host
these branches are developed in large numbers, and extend themselves in
all directions. The suckers produced on them cling very firmly to the
root-fibres of the host, which they almost completely embrace.
The suckers of the Yellow Rattle (p. 118) are globular, often
nearly one-eighth of an inch in diameter, and partly surround the
root-fibres of the plants to which they are attached.
XXIV
CARNIVOROUS PLANTS
Quite a number of plants, belonging to different orders, are provided
with the means of capturing small animals, and of digesting their prey
and absorbing the nutrient matter thus obtained into their own systems.
In this way they are enabled to obtain nitrogenous material which, in
the ordinary way, is absorbed in the form of mineral solutions, from the
soil, by the agency of the roots. The greater number of these
carnivorous plants are to be found in tropical lands; but a few are
British, and are of such an interesting nature that we propose to devote
a short chapter to a description of their peculiar structure and habits.
The plants to which we refer are often spoken of as insectivorous
species; but although in nearly all cases the animal food consists
almost entirely of insects, it is not entirely derived from this one
group of animal life, and therefore the term carnivorous is rather more
appropriate.
In pools we sometimes meet with floating plants that have no true roots,
at least at the time of flowering, but consist of a tuft of long,
rootlike, submerged branches, bearing much-divided leaves, and sending
leafless stalks of yellow flowers above the surface of the water. These
plants are the Bladderworts (_Utricularia_), of the order
_Lentibulaceæ_, and are so called because they have little air-bladders
either attached to the leaves or supported on leafless branches.
The leaves are divided into numerous very narrow segments, thus
presenting a proportionately large amount of surface to the water for
the absorption of dissolved gases required by the plant; and the flowers
consist of a deeply two-lobed calyx; a spurred corolla, with its mouth
closed or nearly closed by means of a convex 'palate'; two stamens; and
a one-celled ovary that ripens into a globular fruit.
[Illustration: THE GREATER BLADDER-WORT.]