PLANT MIGRATION
All organisms, animal as well as vegetable, are at some period of their
existence provided with an opportunity of migration. In the animal
world, most land creatures have legs or wings, which allow them to roam
about freely--a freedom which is of special importance as enabling them
to obtain nourishment and to avoid disadvantageous conditions. Aquatic
animals are likewise to a great extent possessed of powers of
locomotion, but such powers are not so essential to them as to
terrestrial creatures, since the water itself is full of small
organisms, both animal and vegetable, on which they can feed; hence a
large variety of water creatures are content to remain during much of
their lives fixed to one spot, extracting from the water as it passes by
both the supply of organic food and the inorganic substances, such as
oxygen or carbonate of lime, which they require for their life
processes. These sedentary creatures, of which barnacles, sea-anemones,
and zoophytes will serve as examples, once attached, do not move from
the spot where they have settled down; but it is important to note that
not only are their eggs or young mostly liberated into the water, and by
it transported to new homes, but in their juvenile stages they often
swim vigorously, and thus achieve a wide dispersal. In the plant world,
the higher forms, with very few exceptions, spend their lives attached
to one spot, like sea-anemones, deriving their food-supply from the air
and from the soil; but they similarly are given the opportunity, after
birth, of migrating. In our familiar wild flowers, for instance, the
young plant, at an early stage of its existence, while it is still
minute, becomes covered with a coat often of very resistant qualities,
and is then cast loose by the parent in the form of _seed_, mostly in
great numbers, to achieve what travels it can before it takes root and
settles down, like its parent before it, to a humdrum existence. In the
Cryptogams, or so-called Flowerless Plants, this temporary compression
of the organism into very narrow limits suitable for easy dispersal
takes place at a different period in the life cycle, but for mechanical
purposes the results are similar. Minute bodies, or _spores_ (much
smaller than the seeds of the Seed Plants), are cast loose by the parent
often in vast numbers, and eventually settle down and reproduce the
species. In many of the lower aquatic plants these spores are provided
with means of locomotion in the form of a tail-like appendage, which by
its movement propels the germs through the water, giving them the same
advantage which is possessed by the young of many of the sedentary
animals.
The opportunity for migration thus offered to sedentary plants once at
least in each cycle is of very great importance. A plant, living on one
spot and drawing, from that portion of the soil which its roots can
reach, certain mineral salts essential for its continued growth, tends
to exhaust the available supply of these materials, and the succeeding
generation needs to reach fresh ground if it in turn is to attain
healthy development. And it is undoubtedly of advantage to plants, if
they are to continue to exist on the Earth, to be able to jump barriers
and to colonize fresh suitable habitats which may arise in the course of
natural changes, which sooner or later may render old habitats
untenable. Thus the very existence of plants upon the Earth depends on
the adequacy of seed-dispersal. This being so, the imaginative mind,
viewing the marvellous and infinitely varied contrivances of Nature,
will possibly be struck more by the want of special provision for
dispersal shown by the majority of the higher plants--their helplessness
in this respect--than by the beautiful devices exhibited by the few. In
the first place, seeds are inert, devoid of any power of
locomotion--though in some instances the last act of the parent is to
discharge them with an explosive action into the air. They are dependent
on the movements of external media--air, or water, or wandering
animals--for transportation of any magnitude, and while many possess
very beautiful devices for enabling them to take advantage of
opportunities in this regard, the majority are devoid of any special
structures. They are as inert as pebbles or grains of sand: but they
possess two attributes which form important assets--namely, numbers and
vitality. The amount of seed produced annually is hundreds, or more
usually thousands, sometimes hundreds of thousands, for each parent.
What matter if myriads perish? If one in so many thousands takes root
and grows, the species will not diminish in numbers. Vitality also
largely affects the problem. The seed can endure extremes of heat and
cold which would be fatal to the parent; it can be drowned, or scorched,
or dashed about, or in many cases eaten by animals without injury; it
can lie buried in the soil for a long period of years, yet if turned up
again and placed within reach of the requisite amount of air and heat,
will spring up vigorously.
As a matter of fact, investigation soon shows that absence of special
devices for dispersal provides no measure of the breadth of a plant’s
distribution, nor is profuse seed-production necessarily related to
abundance of offspring. Many factors come into play, and conclusions of
this obvious kind will generally only lead us astray. But that does not
render the study of each one of the factors less interesting.
This matter of seed-dispersal is of prime importance in our study of
familiar British plantscapes, for our vegetation is the expression of
the past and present efficiency of its particular rôle in the
ever-changing drama of Nature. We shall do well to spend a little time
in considering it.
First of all, as to the nature of the seeds with which we have to deal.
These are, as already pointed out, young plants, already a long way
advanced from the egg stage, neatly tucked up and enclosed, in most
cases along with a supply of food material, in a tight, strong skin,
which is mostly of a particularly impervious character, protecting the
young plant from injury by bruising, from attacks of small animal
enemies, from extremes of heat and cold, of moisture and dryness. The
young plant, too, is in a peculiarly resistant physiological condition.
For instance, its breathing--or absorption of oxygen--is exceedingly
slow, and it is not suffocated by burial, sometimes even for years, in
the soil. And while the mature plant is killed instantly by immersion in
boiling water or by exposure to a very low temperature, some seeds, if
boiled for a quarter of an hour, are quite uninjured, while others,
subjected experimentally to even the temperature of liquid hydrogen
(-260° C., or 436 degrees of frost on our more familiar Fahrenheit
scale), remain unaffected. Many seeds are liberated from the parent
plant enclosed by or attached to appendages of various sorts (when they
are called by the botanist _fruits_) which sometimes greatly aid
dispersal, as in the Dandelion (_Taraxacum_), and sometimes appear to
hinder it; in any case, while the young plant itself is usually quite
small, it may, when surrounded by its food-supply and enclosed in its
wrappings, be a bulky object--as is seen in the Cocoanut or Horse
Chestnut. In the British flora, to which we may confine our attention, a
crab-apple (containing a number of seeds), a hazelnut, and an acorn
(each containing a single seed), are the largest _units of dispersal_
with which we have to deal. But these are quite exceptional in size, and
the average seed (using that term in its original sense of the natural
unit of dispersal) in the British flora does not exceed the size of a
pin’s head. This remarkable reduction of size alone aids dispersal
greatly.
The migrations of plants are effected mainly during the seed stage,
these tiny, tightly packed portmanteaux being much better fitted for
travel than the bulky and fragile organisms to which they give rise. But
before we consider the adventures of seeds it must be pointed out that a
considerable, if slow, migration of plants takes place by mere
vegetative growth. The stems of many species are not erect, but
prostrate; creeping upon or below the ground, they may in time cause a
plant to spread far beyond its place of origin. A whole field, or for
that matter a whole hillside, of Bracken (_Pteris Aquilina_) may quite
possibly have originated from a single wind-borne spore. Among Sedges
and Grasses this mode of growth is common--as we know to our cost in the
case of the Couch-grass (_Triticum repens_)--and it is found in varying
form in many kinds of plants, as in the suckers of trees, the offsets of
bulbs, the runners of the Strawberry (_Fragaria_); it is especially
characteristic of marsh and water plants. Its effect is to produce large
colonies, such as the great beds of Reeds (_Phragmites_) or Reed-mace
(_Typha_) which fringe our lakes, the groves of Bent (_Ammophila_) on
sand dunes, and the beds of Anemones (_A. nemorosa_) or Broad-leaved
Garlic (_Allium ursinum_) of our spring woods. In all these cases the
whole colony may be the result of the continued growth of a single
individual. It should be noted, however, that such migration is possible
only so far as favourable soil conditions extend. A slight barrier--a
streamlet, a patch of ground too wet or too dry, will arrest further
progress, and the plant must fall back on seed-dispersal in order to
conquer further territory.
A vegetative device which, so far as its method and value in dispersal
are concerned, approaches those of seeds, is found in the bulbils with
which some plants are furnished. These are small buds--congested
shoots--borne on stems, or on leaves as in the Lady’s Smock (_Cardamine
pratensis_), or among the
[Illustration: FIG. 7.--CORAL ROOT (DENTARIA BULBIFERA).
_a_, Upper half of shoot, 1/2; _b_, creeping stem, 1/2; _c_, bulbil,
2/1.]
flowers as in many Leeks (_Allium spp._). These usually fall from the
parent when mature, and being comparatively small and possessed of
considerable vitality, they may achieve a considerable dispersal before
they send out roots and fasten themselves to the soil. An example is
figured (Fig. 7). In this plant (_Dentaria bulbifera_, the Coral Root, a
rather rare native of England) the bulbils resemble not the smooth
flower-stems of which they are axillary branches, but the curiously
knobby underground stems from which the leaves and flowering shoots
arise.
Since seeds themselves possess, as already stated, no power of
locomotion, they have to rely on external agents for their dispersal.
These may in general be summed up as (1) Action of the parent plant, (2)
water, (3) wind, (4) animals.
1. _Action of the Parent._--The Ivy-leaved Toad-flax, or
Mother-of-Thousands (_Linaria Cymbalaria_), is a pretty little plant,
native in central and southern Europe, naturalized and common on old
walls in this country. Its Snapdragon-shaped purple flowers are borne on
short stalks which curve towards the light, placing the blossoms in a
conspicuous position, where they may be the more readily visited by
insects, and thus pollinated. But when flowering is over, and the little
round fruit is ripening, the stalk twists so that the fruit is turned
towards the wall and finally pushed into any convenient crevice: when
the capsule opens, the seeds, instead of dropping to the base of the
wall where on germination the young plants would be smothered among
stronger growths, find themselves lodged in niches in which the young
plants may develop successfully. Many water plants have flowers which
rise into the air, following on which the flower-stem curves and the
seed is ripened below the surface, free from the dangers of weather, of
feeding water birds, and so on.
A very common type is that in which the seed-vessel opens at the top
when the seed is mature. Gusts of wind, or passing animals, bending the
stem, cause the latter to spring back, casting the seeds out. When the
seed-vessel opens widely, as in the Columbine (_Aquilegia_), the seeds
may be cast to some small distance. The efficacy of the arrangement is
not so obvious when, as in the Poppies (_Papaver_) or Bell-flowers
(_Campanula_), the openings are small (Fig. 8), but it is clear that
these plants do not suffer from lack of dispersal, in view of their
abundance and wide range.
[Illustration: FIG. 8.--FRUIT OF GIANT BELL-FLOWER (CAMPANULA
LATIFOLIA). 3/4.]
But the assistance which the parent plant gives is often of a more
active and even dramatic character, though in these cases it is usually
effected not by a movement of living tissue as in the last case, but by
mechanical changes taking place in tissues already dead or dying. If we
stand by a bank of Gorse (_Ulex_) on a warm day we may become aware of a
snapping sound, and may possibly feel on our faces the impact of small
bodies. These are gorse seeds in process of being distributed by the
parent. In this shrub the fragrant flowers are succeeded by short tough,
hairy pods, formed of two valves joined together by their edges. (In
reality the pod is a modified leaf folded down the middle, the two edges
thus brought together being joined--see p. 129.) When the seed is ripe
the pod dries, and owing to unequal shrinkage of the valves stresses are
set up which at last tear the pod suddenly asunder along its edges,
flinging the seeds violently out into new ground, where they will have a
better chance of life than if merely dropped into the middle of the
parent bush. A similar arrangement is found in the Vetches and many
other Leguminosæ. In the Cranesbills (_Geranium_) a very ingenious
catapult device may be examined. The fruit is of peculiar structure. We
might make a rough model of it by taking five single-sticks and tying
them to a broom-handle--firmly at the points, less securely
elsewhere--and slipping a tennis-ball into each basketwork handguard
before turning its open side in against the broom-stick, so that the
ball cannot fall out. Imagine now that unequal drying on the part of the
sticks tends to make each bend into a semicircular form, which is
hindered by the fastenings at either end. The stress will eventually
tear the weak fastenings at the base: the lower end will fly up, bearing
with it the ball (representing the seed), which will be projected
[Illustration: FIG. 9.--FRUIT OF GERANIUM.
_a_, Mature; _b_, ditto, with pouches raised ready to discharge nuts;
_c_, in act of discharging.]
out through the open side. In the Cranesbills the jerk is so violent
that seeds may be flung to a distance of twenty feet. One of the most
efficient of all devices of this kind is found in the Sand-box Tree
(_Hura crepitans_), a native of South America. By sudden rupture and
twisting of the carpels of the woody sub-globular fruit, the large seeds
of this plant are thrown to a distance of thirty yards, the explosion
being accompanied by a report like that of a pistol-shot. In the common
Dog Violet (_Viola Riviniana_) (Fig. 10) the fruit is a three-valved
capsule, which on ripening divides; each valve assumes a horizontal
position and its edges contract till it is shaped like an open boat, the
seeds lying in a row down the middle. The sides as they dry close in
tighter and tighter on the seeds, which are in turn pinched out, and
fly off with a little snap to a distance of many feet. It is an
interesting experience to watch these tricks of Nature--much more
interesting than merely to read about them. If plants of Vetch, Gorse,
Dog Violet, Storksbill, Wood Sorrel, Touch-me-not (to name a few),
bearing unripe fruit, be brought home and placed in water in a
sitting-room, the click of the bursting fruits will be distinctly
audible, and by spreading a white sheet the efficiency of the devices
may be tested.
[Illustration: FIG. 10.--FRUIT OF VIOLA. 3/4.
_a_, Mature capsule; _b_, capsule open ready to discharge seeds; _c_,
capsule after seeds are discharged.]
A very interesting case, in which the seed is actually buried in the
soil by movements of its appendages (portions of the parent plant which
remain attached to it), may be watched in the case of the Storksbills
(_Erodium_), Several species of which are British plants of frequent
occurrence. Here the young fruit much resembles that of its allies the
Cranesbills. The long rod-like axis at the lower end of which the seed
is enclosed contracts unequally in drying, so that the upper half
assumes a position at right angles to that of the
[Illustration: FIG. 11.--FRUIT OF STORKSBILL (ERODIUM). 2/3.
_a_, Mature, twisting beginning; _b_, separate fruit, fully twisted.]
lower half, which when dry is much twisted, like a rope (Fig. 11). The
covering of the seed itself is furnished with stiff short hairs pointing
upwards. The whole structure when mature is cast off by the parent. The
curiously twisted appendage is hygroscopic, and readily responds to
wetness by untwisting and to dryness by twisting. Should it be thus
caused to untwist when the upper end is free from obstruction the latter
will revolve slowly like the hand of a clock. But should it meet with an
obstacle in the course of its revolutions, such as a blade of grass, the
motion is transferred to the lower end, which revolves like an auger,
and, lengthening as it untwists, forces the seed into the ground. Should
dryness supervene, the backward-pointing hairs on the seed-envelope
prevent its being drawn out again when retwisting and consequent
shortening take place. These _Erodium_ fruits are among the most
interesting in the British flora, and are well worth experimenting with.
2. _Water._--Water, which forms the most frequent and the most serious
barrier to plant migration, under certain circumstances is a very
efficient agent of dispersal. At the same time, its powers in the latter
direction are strictly circumscribed. As regards fresh water, seeds
which float may be wafted across lakes. Rivers are more effectual, as
seeds may be transported long distances in their currents and thrown up
finally on their banks or over flooded areas. When we consider the sea,
we realize that there is here a possibility of almost unlimited
dispersal provided that the seeds are not injured by salt water, and
that they can remain afloat. It is on the latter point that the whole
efficacy of water dispersal turns. This was long ago recognized, and
investigations have been made by many naturalists to determine the
buoyancy of seeds of all kinds. The results show that, taking the seeds
of the plants of any country as a whole, not more than about 10 per
cent. are capable of floating for more than a short period, while most
of them sink at once in either fresh or salt water. So one’s vision of
seeds transported in myriads over hundreds of miles of sea is rudely
dispelled; and the fact that many seeds can survive prolonged immersion
in sea-water uninjured is of little account. The 10 per cent. of our own
flora which produce buoyant seeds are mainly riverside and seaside
plants; and no doubt their dispersal is to a great extent due to streams
and tidal currents. But the majority of the hundreds of thousands of
seeds which a river transports annually find their last resting-place in
quiet backwaters or on the floor of the sea.
It is different, however, with the flora which fringes beaches in the
Tropics. Here many of the plants possess large fruits of great buoyancy,
which are still afloat and alive after months of tossing on the waves,
and if cast up germinate readily. These bold wanderers are a familiar
feature of Tropic plant life, and their successful voyaging accounts for
the uniformity of the beach flora on innumerable islands. Even our own
inhospitable shores sometimes receive these waifs of warmer seas,
brought from the West Indies by the Gulf Stream and the prevailing
south-west winds. Of these the most frequent are the large bean-like
seeds of _Entada scandens_, a Leguminous plant, which are originally
enclosed in gigantic pods several feet in length, and the more globular
seeds of the Bonduc (_Guilandina bonducella_), another species of the
same order. But the most famous of all floating fruits is the Double
Cocoanut, or Coco-de-mer, a huge nut weighing 40 or 50 lb. and
containing several seeds a foot and a half long. It is the product of a
Palm (_Lodoicea Sechellarum_); cast up on the shores of India, it was
known centuries before its place of origin in the Seychelles was
discovered, and fantastic legends grew up regarding it.
3. _Wind._--Everything that we know about the wind suggests that it is a
potent agent of seed-dispersal, far excelling, for instance, that of
flowing water. “All the rivers flow into the sea,” that cemetery of
seeds, and their courses are at best mere spider-lines on a map. But the
wind, blowing where it listeth, is everywhere, always ready to snatch up
in its arms any seed of sufficient lightness, and to bear it away from
the parent; in fancy we can see tiny seeds borne by gales across
mountains and oceans. But we have to leave imagination out of account,
and examine prosaically the mechanical laws according to which such
transport is of necessity conducted. Any body liberated in still air
will fall vertically with a velocity which increases according to
well-known laws until the increasing resistance of the air to its
passage equals the effect due to gravity; it thenceforward continues to
fall at a uniform velocity, that velocity depending upon the nature of
the falling body. In all seeds which are sufficiently light to be at all
suitable for wind dispersal, the resistance of the air almost at once
counteracts acceleration due to gravity, so that the rate of fall may be
taken as uniform from the beginning. If the seed on liberation is
carried along by the wind, it will acquire almost immediately the
horizontal velocity of the air-current, but it will at the same time
move downward through the air with the same velocity as if the air was
still--just as a body dropped in a railway carriage will fall at the
same rate whether the train is moving or standing still. If we measure
the speed of fall of a seed in still air, then we can easily deduce the
distance to which it will be carried by a horizontal air-current of
given velocity if liberated at any given height above the ground. Thus,
if a seed liberated 100 feet from the ground falls that distance in half
a minute, and the wind is blowing at the rate of, say, 1,000 feet in
half a minute (or nearly 23 miles per hour, a good breeze), the seed
will be carried 1,000 feet before it reaches the ground. Its course will
be represented by the diagonal AD of the accompanying figure, where AB
represents the distance which the seed falls in the given time, and AC
the distance according to the same scale travelled by the wind in the
same period.
[Illustration: FIG. 12.]
But most seeds sufficiently light to be capable of extended flights are
liberated only a few feet from the ground; they are dependent on upward
eddies to raise them if they are to achieve more than a very short
migration. That such eddies, both upward and downward, occur on a windy
day we all know from experience; and it is they that make or mar the
fortune of most wind-borne seeds. Only some local or accidental excess
of upward over downward eddies will assist a seed on its journey; and as
every upward eddy must be compensated somewhere by a downward eddy, the
longer the journey is, the more such eddies tend to neutralize each
other. Over the sea--that most formidable barrier to plant
migration--eddies do not prevail as they do over rough ground, so that,
unless by a series of lucky eddies a seed is whirled up to a
considerable elevation before it leaves the shore, the chances of its
successful passage across a stretch of water are remote. Discussing the
possibility of seeds of Portuguese plants reaching the Azores, lying
800 miles to the westward, H. B. Guppy[4] shows, from observations on
the rate of fall of seeds made by several workers, that with a 50 miles
per hour horizontal wind the light-plumed seed of the Common Groundsel
(_Senecio vulgaris_), for instance, would require to be liberated at a
height of 9 miles above the ground if it is to reach the islands: or to
express it differently, if liberated at ground-level, the seed would
need to be raised 9 miles by upward eddies during its journey, even if
corresponding downward eddies were absent--which they certainly never
are. It is clear that if even light seeds are to achieve anything more
than short journeys, they must depend on exceptional disturbances of the
air, such as whirlwinds and tornadoes.
It is now time to examine the devices by which many seeds achieve a more
or less wide dispersal by means of the wind. Seeds possessing these
adaptations may be divided into three classes: (i.) Powder seeds, (ii.)
winged seeds, (iii.) plumed seeds.
By powder seeds are meant seeds of very small dimensions. Reduction in
size, if carried far enough, greatly facilitates dispersal by wind. This
is because the resistance offered by the air is relatively greater for a
smaller body than for a larger one, so that rate of fall decreases as
the size of the falling body diminishes--we all know how even a heavy
material, if reduced to powder, will fall more slowly than when forming
a single mass. Most of the spores of the “Flowerless Plants”--Ferns,
Mosses, Fungi, etc.--are exceedingly minute, and have as a result a very
slow rate of fall, and a consequent power of long-distance dispersal by
wind. For instance, the microscopic spore of the puff-ball _Lycoperdon_
falls so slowly that, if we take again Guppy’s Azores example, it could
traverse the 800 miles in a 50 miles an hour gale if it commenced its
flight only 86 feet above the ground. Such spores are, in fact, so
buoyant that they form a normal constituent of the air--as we know, for
instance, by the rapidity with which they will discover and germinate
upon a piece of cheese, forming bluemould--and with little doubt they
are capable of reaching under favourable circumstances the most distant
of oceanic islands. But in the Flowering Plants with which we are mainly
concerned reduction in size is not carried far enough to confer any
great amount of buoyancy. The minute seeds of the Poppies (_Papaver_),
for instance, fall about 10 feet in a second. Applying again Guppy’s
Azorean case, we find that though these would cover the distance in
sixteen hours, they would fall in that time about 100 miles, unless
raised during the journey to that extent by the excess of upward eddies
as compared with downward ones--a quite impracticable proposition. In
the Orchids alone do we find among the powder-seeded Flowering Plants a
really effective buoyancy; this is due to the fact that great reduction
in size is accompanied by very loosely disposed tissue enclosing the
seed in a kind of net, and by the resistance to the air thus offered,
greatly reducing the rate of fall. The seed of the Marsh Helleborine
(_Epipactis longifolia_) falls only about 1/15 as fast as that of the
Poppies, and would thus, under the same conditions, be carried fifteen
times as far.
To pass on. Some seeds, many of them of considerable size as compared
with those which we have just considered, have coverings which are
furnished with a membranous wing (Fig. 13, _d_), sometimes extending all
round the seed, as in the Elm (_Ulmus_), more often placed at one side,
as in the Sycamore (_Acer_). The effect of such wings is to reduce the
rate of fall, imparting to the seed an irregular zigzag motion, as in
the former case, or a spinning motion as in the latter. A Sycamore seed
with the wing removed will fall four or five times as fast as with the
wing present. But while a well-developed wing forms a more efficient
dispersal device than mere reduction in size as found in Seed Plants,
the rate of fall of wing seeds as a whole shows that these appendages do
not fit them for anything but short voyages.
We may then pass on to consider the plumed seeds, which possess by far
the most efficient as well as the most beautiful devices for aiding
dispersal found among wind-borne seeds. These plumed seeds belong to
many different groups of plants, and the tufts of delicate hairs which
give them their buoyancy arise in different ways. Among the _Compositæ_,
the Order which furnishes the most familiar of our plumed seeds, the
plume is formed by modification of the upper part of the calyx, which in
so many common plants is small, green, and leaf-like; the lower part of
the calyx in the _Compositæ_ is tough, persistent, and close-fitting,
forming an additional protection for the seed. The plume springs either
from the top of the seed, as in the Thistle, or is borne on a slender
stalk, as in the Dandelion. It consists of a ring or radiating mass of
hairs of beautiful delicacy, often bearing short
[Illustration: FIG. 13.--WING-SEEDS AND PLUME-SEEDS.
_a_, Mountain Willowherb (_Epilobium montanum_), 2/1; _b_,
Dandelion (_Taraxacum officinale_), 2/1; _c_, Mountain Avens
(_Dryas octopetala_), 1/1; _d_, Scotch Fir (_Pinus sylvestris_),
2/1; _e_, Reed-mace (_Typha latifolia_), 2/1.]
branches; these hairs are tightly packed together when the fruit is
young or during damp weather, but on a dry day when it is ripe they
spread out, and the seed, breaking away from its attachment, is floated
off by the wind. In many species the plume or _pappus_ is only lightly
attached to the seed, so that if on a voyage an obstacle is encountered
the seed drops off, while the now useless parachute drifts away. But
though the plume seeds of the _Compositæ_ are the largest and most
beautiful among our common plants, they are not the most efficient for
dispersal. The fluffy seeds of the Willowherbs (_Epilobium_) and of the
Willows (_Salix_), for instance, fall at a slower rate than those of
almost any _Compositæ_, while by far the most buoyant seed in the
British flora is that of the Reed-mace (_Typha_). In this case the seed
itself is minute, and is situated on a very slender stalk, from near the
base of which springs a tuft of delicate hairs. This seed takes
thirty-four seconds to fall twelve feet. Using once more the Azorean
example, it could cross the 800 miles of sea if it had an initial
elevation of 3-1/3 miles, or was raised to that amount during the
sixteen hours occupied by its passage.
Summing up, then, we find that the plume seeds are the most efficient of
all seeds for extended flights by the agency of the wind. If the
efficiency of the seeds of the Reed-mace, the most buoyant among British
plants, be taken as 100, the efficiency of the Willowherbs is between 60
and 70, of Willows 45 to 70, the best of the Thistles 35 to 40,
Dandelion 25. Even the best of the winged seeds are much less efficient,
Elm and Scotch Fir being about 20, Sycamore and Ash 9 or 10. Of powder
seeds, the efficiency of several Orchids tested ranges from 35 to 65,
and Broomrapes (_Orobanche_) from 20 to 25. Most of the powder seeds are
far below these, the efficiency of seeds of _Papaver dubium_, for
example, being only 4·5 on the same scale. This last figure is
representative of the many small-seeded plants in the British
flora such as are found among the _Crucifercæ_, _Caryophyllaceæ_,
_Scrophulariaceæ_, etc. The relative efficiency of such comparatively
large seeds as those of many of our Leguminous plants would be about 1
on the same scale.
4. _Dispersal by Animals._--The coverings of many seeds are provided
with hooks or barbs, and others with stiff hairs, which render them
liable to become entangled in the hair or fur of passing animals.
Examples will occur at once to the reader, as this character occurs in
the case of many familiar plants, such as Burdock (_Arctium_),
Enchanter’s Nightshade (_Circæa_), Avens (_Geum_), and so on. Without
doubt these hooked fruits often secure a wide local dispersal by the aid
of cattle, sheep, rabbits, and so on: the state of one’s trousers or
stockings after walking the autumn woods is often very suggestive in
this regard. Again, herbivorous quadrupeds eat seeds in quantities, many
of which are capable of germination after passing through the animal’s
body. But while the dispersal obtained by such means may often aid in
spreading a species over a tract of land, it does not generally aid in
the crossing of barriers, such as mountains or sea, on account of the
limitations to the movements of such animals. To arrive at a true
estimate of the importance of the animal kingdom in regard to plant
migration, we have to study the movements, habits, and food of birds, to
whose wanderings neither mountains nor seas set a barrier. Seeds are
carried about by birds in two ways--by becoming attached to their
feathers or feet, or by being eaten and subsequently ejected. The first
case belongs to the class of phenomena which we have just been
considering, save that the smooth plumage of birds, and their frequent
preening of their feathers, tends to keep their coats free from
extraneous material. But at least in wet weather minute seeds must often
cling to feathers and to feet, and mud which may contain seeds may
easily be present on a bird’s toes during flight. More important is the
question of _endozoic_ dispersal--where seeds are transported in the
alimentary canal of birds. Some families, like the Finches and Tits,
which eat great numbers of seeds, are inimical instead of helpful to
dispersal, because the seeds which they devour are crushed and
afterwards digested. But in many cases the seeds are swallowed whole,
and are usually in no way injured by their passage through a bird’s
body. Frequently, indeed, the seeds have not to run the gauntlet of the
digestive juices of the alimentary canal, being disgorged from the
stomach along with other hard material prior to digestion. Birds which
live on berries or other juicy fruits are the most important in
seed-dispersal. As Barrows says: “The seed-eaters are not the
seed-planters; on the contrary, the insectivorous birds more often sow
seeds than the true seed-eaters.” “Seeds which _simply contain_
nourishment are eaten and destroyed, while seeds which _are contained in
nourishment_ are eaten and survive.”[5] It is for this reason that, if
we look under a tree on which Blackbirds or Thrushes perch, we shall
often find young plants of Bramble (_Rubus_), Ivy (_Hedera_), Holly
(_Ilex_), or Yew (_Taxus_). There can be no doubt that birds eat and
subsequently eject vast numbers of seeds still capable of germination;
many observations and calculations might be quoted. But when we come to
apply the facts to the problem of long-distance dispersal, or the
passage across serious barriers, we find that important limiting factors
must be taken into account. The digestion of birds is remarkably rapid,
food being ejected from a half to three hours after being eaten, so that
a bird eating seeds and at once flying off in a straight line at, say,
50 miles per hour could not convey seeds more than 150 miles. Secondly,
many observations show that on migration birds generally travel with
empty stomachs and clean plumage and feet. It is clear, therefore, that,
as in the case of wind dispersal, we must look to exceptional
circumstances, not normal conditions, to provide opportunities for long
journeys on the part of seeds. But for the transfer of seeds from France
to England, for instance, or from England to Ireland, it is clear that
birds furnish a far more efficient medium than wind or water. In one
important particular, dispersal by animals has a great advantage over
dispersal by wind--that it is practically independent of the weight of
the seeds. Thus, the heaviest of British seeds, the acorn, is carried
about by Rooks, just as the hazelnut is scattered by Squirrels, or a
head of Burdock fruits by a passing sheep.
Having thus arrived at some idea of the high efficiency for dispersal of
many kinds of seeds, it is with some little surprise that we observe--as
we may on any country walk--that the plants which arise from these are
in general no more abundant or more widely distributed than others which
possess seeds devoid of any apparent advantages in this respect--seeds
which cannot fly nor float, nor cling to a passing creature, and which
are not eaten to any extent by birds so far as observation goes. The
truth is, we have to remember, as emphasized in a previous chapter, that
the world is already densely populated by plants, all of which survive
by reason of their being specially fitted for their several habitats.
They have fought in the great struggle for existence, and have
established their right to the places which they occupy; they will not
readily give way to any newcomer whose seeds happen to be imported into
their strongholds. Of course exceptions can be quoted, where plants
accidentally or intentionally introduced by man into new areas have not
only maintained a foothold, but have spread remarkably. Note the case of
the Sweetbrier (_Rosa eglanteria_) in New Zealand, of the Mexican
_Bryophyllum calycinum_ in many Tropical countries, of the American
Monkey-flower (_Mimulus Langsdorfii_) in our own islands; but these are
admittedly exceptional. It is nearer the truth to say that the troubles
of an immigrant only begin where dispersal ends; and that the chance of
seeds carrying out a successful migration is much greater than the
chances of their giving rise to a new colony when that migration is
successfully accomplished. Every head of the Reed-mace liberates about a
quarter of a million seeds of marvellous lightness; yet the Reed-mace
does not increase in the country, nor is it a particularly abundant
plant even in its chosen habitats. The Foxgloves (_Digitalis purpurea_)
in a wood shed, each plant, say a hundred thousand seeds; yet on an
average only one of these attains maturity, otherwise the species would
become more abundant in the area. This enormous destruction of seed is
largely due to competition. The reception which a plant receives in its
new home is the thing that matters, and that may usually be summed up in
the phrase “House full.”
Nevertheless, the present flora of Great Britain is in the long run the
result of migration from surrounding areas; so that ease of dispersal
has undoubtedly played its part in the building up of our vegetation.
Conditions under which rapid dispersal has obviously an advantage occur
when by some exceptional circumstances the natural vegetation is
destroyed within an area, as by a flood or landslide. Such conditions
are produced artificially each season over much of our own country by
the operations of agriculture. Their results will be considered in a
subsequent chapter.