unsymmetrical, one side being round and hairy, the other nearly flat
and smooth. The true apex of the fruit which bears the remains of the
flower, is also somewhat eccentric, and, when the seeds are ripe,
if it is touched even lightly, the fruit explodes and the seeds are
thrown to some distance.
Other cases of projected seeds are afforded by Impatiens, Hura, one
of the Euphorbiæ, Collomia, Oxalis, some species allied to acanthus,
and by Arceuthobium, a plant allied to the mistletoe, and parasitic
on juniper, which ejects its seeds to a distance of several feet,
throwing them thus from one tree to another.
Even those species which do not eject their seeds often have them
so placed with reference to the capsule that they only leave it if
swung or jerked by a high wind. In the case of trees, even seeds
with no special adaptation for dispersion must in this manner be
often carried to no little distance; and to a certain, though less,
extent, this must hold good even with herbaceous plants. It throws
light on the, at first sight, curious fact that in so many plants
with small, heavy seeds, the capsules open not at the bottom, as one
might perhaps have been disposed to expect, but at the top. A good
illustration is afforded by the well-known case of the common poppy,
in which the upper part of the capsule presents a series of little
doors, through which, when the plant is swung by the wind, the seeds
come out one by one. The little doors are protected from rain by
overhanging eaves, and are even said to shut of themselves in wet
weather. The genus Campanula is also interesting from this point of
view, because some species have the capsules pendent, some upright,
and those which are upright open at the top, while those which are
pendent do so at the base.
In other cases the dispersion is mainly the work of the seed itself.
In some of the lower plants, as, for instance, in many sea-weeds, and
in some allied fresh-water plants, such as Vaucheria, the spores[5]
are covered by vibratile cilia, and actually swim about in the water,
like infusoria, till they have found a suitable spot on which to
grow. Nay, so much do the spores of some sea-weeds resemble animals
that they are provided with a red “eye-spot,” as it has been called,
which, at any rate, seems so far to deserve the name that it appears
to be sensitive to light. This mode of progression is, however, only
suitable to water plants. In much more numerous cases, seeds are
carried by the wind.
In other instances, the plants themselves, or parts of them, are
rolled along the ground by the wind. An example of this is afforded,
for instance, by a kind of grass (Spinifex squarrosus), in which the
mass of inflorescence, forming a large, round head, is thus driven
for miles over the dry sands of Australia until it comes to a damp
place, when it expands and soon strikes root.
So, again, the Anastatica hierochuntica, or “Rose of Jericho,” a
small annual with rounded pods, which frequents sandy places in
Egypt, Syria, and Arabia, when dry, curls itself up into a ball or
round cushion, and is thus driven about by the wind until it finds a
damp place, when it uncurls, the pods open and sow the seeds.
These cases, however, in which seeds are rolled by the wind along the
ground, are comparatively rare. There are many more in which seeds
are wafted through the air.
Another mode, which is frequently adopted, is the development of long
hairs. Sometimes, as in Clematis, Anemone, and Dryas, these hairs
take the form of a long, feathery awn. In others the hairs form a
tuft or crown, which botanists term a pappus. Of this the dandelion
and John Go-to-bed-at-noon, so called from its habit of shutting its
flowers about midday, are well-known examples. Tufts of hairs, which
are themselves sometimes feathered, are developed in a great many
Composites, though some, as, for instance, the daisy and lapsana, are
without them; in some very interesting species, of which the common
Thrincia hirta of our lawns and meadows is one, there are two kinds
of fruits, one with a pappus and one without. The former are adapted
to seek “fresh woods and pastures new,” while the latter stay near
the parent plant and perpetuate the race at home.
In other cases seeds are wafted by water. Of this the cocoanut is one
of the most striking examples. The seeds retain their vitality for a
considerable time, and the loose texture of the husk protects them
and makes them float. Every one knows that the cocoanut is one of
the first plants to make its appearance on coral islands, and it is,
I believe, the only palm which is common to both hemispheres.
In a very large number of cases the diffusion of seeds is effected
by animals. To this class belong the fruits and berries. In them an
outer fleshy portion becomes pulpy, and generally sweet, inclosing
the seeds. It is remarkable that such fruits, in order, doubtless,
to attract animals, are, like flowers, brightly colored--as, for
instance, the cherry, currant, apple, peach, plum, strawberry,
raspberry, and many others. This color, moreover, is not present in
the unripe fruit, but is rapidly developed at maturity. In such cases
the actual seed is generally protected by a dense, sometimes almost
stony, covering, so that it escapes digestion, while its germination
is, perhaps, hastened by the heat of the animal’s body. It may be
said that the skin of apple and pear pips is comparatively soft; but
then they are imbedded in a stringy core, which is seldom eaten.
These colored fruits form a considerable part of the food of monkeys
in the tropical regions of the earth, and we can, I think, hardly
doubt that these animals are guided by the colors, just as we are, in
selecting the ripe fruit.
In these instances of colored fruits, the fleshy edible part more or
less surrounds the true seeds; in others the actual seeds themselves
become edible. In the former the edible part serves as a temptation
to animals; in the latter it is stored up for the use of the plant
itself. When, therefore, the seeds themselves are edible they are
generally protected by more or less hard or bitter envelopes, for
instance, the horse chestnut, beech, Spanish chestnut, walnut, etc.
That these seeds are used as food by squirrels and other animals is,
however, by no means necessarily an evil to the plant, for the result
is that they are often carried some distance and then dropped, or
stored up and forgotten, so that in this way they get carried away
from the parent tree.
In another class of instances, animals, unconsciously or unwillingly,
serve in the dispersion of seeds. These cases may be divided into two
classes, those in which the fruits are provided with hooks and those
in which they are sticky. The hooks, moreover, are so arranged as to
promote the removal of the fruits. In all these species the hooks,
though beautifully formed, are small; but in some species they become
truly formidable. Two of the most remarkable are Martynia proboscidea
and Harpagophyton procumbens. Martynia is a plant of Louisiana, and
if its fruits once get hold of an animal it is most difficult to
remove them. Harpagophytum is a South African genus. The fruits are
most formidable, and are said sometimes to kill lions. They roll
about over the dry plains, and if they attach themselves to the skin,
the wretched animal tries to tear them out, and sometimes getting
them into his mouth perishes miserably.
The cases in which the diffusion of fruits and seeds is effected by
their being sticky are less numerous, and we have no well-marked
instance among our native plants. The common plumbago of South
Europe is a case which many of you no doubt have observed. Other
genera with the same mode of dispersion are Pittosporum, Pisonia,
Boerhavia, Siegesbeckia, Grindelia, Drymaria, etc. There are
comparatively few cases in which the same plant uses more than one
of these modes of promoting the dispersion of its seeds, still there
are some such instances. Thus in the common burdock the seeds have
a pappus, while the whole flower-head is provided with hooks which
readily attach themselves to any passing animal. Asterothrix, as
Hildebrand has pointed out, has three provisions for dispersion: it
has a hollow appendage, a pappus, and a rough surface.
The next point is that seeds should find a spot suitable for their
growth. In most cases, the seed lies on the ground, into which it
then pushes its little rootlet. In plants, however, which live
on trees, the case is not so simple, and we meet some curious
contrivances. Thus, the mistletoe, as we all know, is parasitic
on trees. The fruits are eaten by birds, and the droppings often,
therefore, fall on the boughs; but if the seed was like that of most
other plants it would soon fall to the ground, and consequently
perish. Almost alone among those of English plants it is extremely
sticky, and thus adheres to the bark.
I have already alluded to an allied genus, Arceuthobium, parasitic on
junipers, which throws its seeds to a distance of several feet. These
also are very viscid, or, to speak more correctly, are imbedded in a
very viscid mucilage, so that if they come in contact with the bark
of a neighboring tree they stick to it.
Among terrestrial species there are not a few cases in which plants
are not contented simply to leave their seeds on the surface of the
soil, but actually sow them in the ground.
I have already alluded to the Cardamines, the pods of which open
elastically and throw their seeds some distance. A Brazilian species,
C. chenopodifolia, besides the usual long pods, produces also short,
pointed ones, which it buries in the ground.
Arachis hypogæa is the ground-nut of the West Indies. The flower is
yellow and resembles that of a pea, but has an elongated calyx, at
the base of which, close to the stem, is the ovary. After the flower
has faded, the young pod, which is oval, pointed, and very minute,
is carried forward by the growth of the stalk, which becomes several
inches long and curves downward so as generally to force the pod into
the ground. If it fails in this, the pod does not develop, but soon
perishes; on the other hand, as soon as it is underground the pod
begins to grow and develops two large seeds.
A remarkable instance is afforded by a beautiful south European
grass, Stipa pennata, the structure of which has been described by
Vaucher, and more recently, as well as more completely, by Frank
Darwin. The actual seed is small, with a sharp point, and stiff,
short hairs pointing backward. The upper end of the seed is produced
into a fine twisted cork-screw-like rod, which is followed by a
plain cylindrical portion, attached at an angle to the corkscrew,
and ending in a long and beautiful feather, the whole being more
than a foot in length. The long feather, no doubt, facilitates the
dispersion of the seeds by wind; eventually, however, they sink to
the ground, which they tend to reach, the seed being the heaviest
portion, point downward. So the seed remains as long as it is dry,
but if a shower comes on, or when the dew falls, the spiral unwinds,
and if, as is most probable, the surrounding herbage or any other
obstacle prevents the feathers from rising, the seed itself is forced
down and so driven by degrees into the ground.
LEAVES
--R. Lloyd Praeger
The stems of plants are the framework on which the leaves and
flowers are spread out to catch the light and air, and we find
definite relations existing between the form, position, and strength
of stems, and the shape, weight, and function of the organs which
the stems support. The branches of an apple or pear tree have to
be sufficiently strong not only to withstand the stress of winter
gales, and the burden, of the wealth of blossom and foliage of early
summer, but also the weight of the abundant fruit of autumn. It is
interesting to note that among our cultivated fruits strength of
stem has not kept pace with the increase in weight of fruit due
to artificial selection, so that in gardens our artificial fruits
must needs, in a season of abundance, be supported by artificial
stems--by props and crutches--lest, like the legs of the prize turkey
in the _Christmas Carol_, the branches might snap like sticks of
sealing-wax. In evergreen trees, the weight of snow is a serious
contingency that must not be neglected. Nor must the chance of
accident owing to wandering animals be left out of account. The young
ash saplings, a few feet in height, are as pliable as willow-wands,
and spring back into their places as we force our way through them;
but the knobby twigs of an old ash tree, which swing clear in the air
high overhead, are brittle, and snap across if we attempt to bend
them; the elasticity of the whole bough is sufficient to bring them
safely through the heaviest storm.
Between the form of a twig and that of the leaves which it bears we
can generally at once perceive a relation. The little leaves of the
birch are borne on twigs slender as a piece of twine. The oak and
elm, with larger leaves, require a stouter twig for their support.
The sycamore and ash have twigs which are stouter still. The large
leaves of the horse chestnut are borne on very thick twigs, in which
the principle of the hollow column is introduced.
The arrangement of the leaves on the stem, or _phyllotaxis_, is a
question of the first importance. The leaves must be so grouped that
all may receive as much light as possible. So far as can be arranged,
there should be no overlapping, nor should any of the available space
be wasted. On the stem of the ash, or sycamore, or teazel, the large
leaves are arranged in alternate pairs, the direction of the axis
of each pair being at right angles to that of the next. Thus two
spaces or _internodes_ separate any pair of leaves from the nearest
pair which, being placed in the same position, might overshadow it.
This is a very simple case, which we shall find to be the rule when
we examine plants in which the leaves are borne in opposite pairs.
When leaves are borne in whorls of three a similar rule will be found
to hold good. The position of the leaves of any whorl is such that
they are vertically below or above the _spaces_ between the leaves
of the next whorl. It will be seen at once that the amount of light
received by each leaf is materially increased by this arrangement.
If in a theatre we can look between the heads of two people in the
row immediately in front of us, the head of a person in the next row
beyond, even though directly before us, does not much interfere with
our view of the stage. In most cases, however, the arrangement of the
leaves on the stem is much more complicated than this. The leaves
usually emerge singly. If we join by a line the point of emergence
of a leaf with that of the next leaf above it on a stem, and that
again with the next, a spiral will be the result, along which at
equal intervals we reach the _nodes_, or points where leaves are
borne. And the distance between these nodes will be always found to
bear some definite relation to the total length of the spiral line
in making one complete revolution round the stem. If the distance
from node to node is one-half of this whole distance, it signifies
that the leaves are borne alternately on opposite sides of the
stem, each leaf being vertically below the second one higher up the
stem--a very common arrangement. Or the leaves may be borne three
to each spiral revolution, so that the position of each leaf shifts
one-third way round the stem as compared with the preceding leaf.
If we look along such a stem, the leaves will appear to be borne in
three vertical rows, with an equal angle between each. Examining some
other plant, we may find that we have to go as far as the fifth leaf
before we find one vertically above the one from which we started,
and if we measure the horizontal distance from any leaf to the next
above or below it, it will be found to equal two-fifths of the total
circumference, so that we have to go five times two-fifths way round
the stem, or two complete revolutions, before completing the cycle.
This is called a two-fifths phyllotaxis. In many other cases, the
arrangement is immensely more complicated, and need not be entered on
here. What is important for us to note at present is that by means of
this orderly mathematical arrangement, the leaves are so distributed
that each fulfils its functions to the best advantage.
The shape of leaves offers an almost inexhaustible field for
observation and scientific speculation. Mr. Ruskin has said: “The
leaves of the herbage at our feet take all kinds of strange shapes,
as if to invite us to examine them. Star-shaped, heart-shaped,
spear-shaped, arrow-shaped, fretted, fringed, cleft, furrowed,
serrated, sinuated, in whorls, in tufts, in spires, in wreaths,
endlessly expressive, deceptive, fantastic, never the same from
footstalk to blossom, they seem perpetually to tempt our watchfulness
and take delight in outstripping our wonder.” The size of leaves
will naturally vary inversely as their number. A plant of a certain
size--say a tree--will require a certain total area of leaf for the
manufacture of the requisite amount of plant-food. If we cut the
branch of a horse chestnut and of a beech where each had exactly a
diameter of one inch, or two, or six inches, and counted and measured
the leaves on each, while the number of beech leaves would immensely
exceed the number of chestnut leaves the total leaf-area would be
about the same in each case. This area of green leaf, then, must be
spread out to the best advantage. In this connection, a beautiful
relation between the shape of leaves and their arrangement on the
stem may frequently be remarked. Lay a twig of beech on a sheet of
white paper, and note how small are the interstices between the
leaves through which the paper may be seen. The shape of the leaves,
and the intervals at which they are borne, are so related that an
almost continuous expanse of green is offered to the sunlight. A
more remarkable case may be seen in the lime, whose leaves are
quite inequilateral, being contracted on one side at the base and
expanded at the other, in order the more exactly to fill the space
which is available. The elm likewise furnishes a beautiful example
of close-fitting leaves. In most trees in which, like the beech,
hazel, and elm, the leaves lie in close-ranked rows in the same plane
as the twig which supports them, we find more or less oval leaves,
their breadth varying with the space between the leaves, _i. e._,
the length of the internode. In trees such as the horse chestnut or
sycamore, on the other hand, the leaves grow in opposite pairs, and
are typically arranged on upright twigs, the leaf-stems projecting at
a wide angle from the twig, with the surface of the leaf horizontal.
In this case space is not so curtailed; the leaf is larger, and more
or less circular in outline; and the great increase of length in the
internodes, as compared with the trees lately considered, prevents a
too great overshadowing of the lower leaves by those higher up the
shoot.
In plants which have a very short axis--which have in popular
language “no stem”--a difficulty arises as to how all the leaves
shall receive a due amount of light, since all arise from the same
point. This is met in several ways. The leaves are often placed at
different angles, the outer leaves, which are the lowest and oldest,
spreading horizontally near the ground, the newest rising almost
vertically in the centre, the intermediate being disposed at various
angles between these extremes. Another solution of the difficulty
is effected by a continued growth of the leaf-stalks, each leaf
steadily pushing itself outward so that the whole form a slowly
expanding circle, in which each leaf-blade successively occupies
a position commencing at the centre, ending at the circumference.
Such leaf-blades, it is almost needless to say, are widest at the
extremity, since that is the portion which receives most light; often
the blade is roundish, and placed at the end of a bare leaf-stalk,
which pushes it further and further from the centre, as other leaves
arise. Such arrangements are well seen in many of our biennial
plants. During their first season they form a close leaf-rosette of
this kind, which manufactures during the summer and winter a supply
of plant-food to be stored for the building up of the tall flowering
stem of the succeeding year. The stork’s-bills, crane’s-bills,
teazel, and other plants will occur to the reader as examples.
In the case of some plants, the normal position of the blade of
the leaf is not horizontal, but vertical. The black poplar and its
relation the aspen furnish well-known instances. If we examine the
stalk of an aspen leaf we notice that while the lower part of it
is circular in section, the part near the leaf is much flattened,
permitting free movement in the plane of the leaf-blade. This,
together with the position in which the leaves are borne on the
twigs, causes the leaves to hang vertically. One result is that the
light can stream almost unbroken through the branches even to the
ground below, the wealth of foliage producing but a faint tremulous
shadow as the leaves rustle in response to every breath of air. Well
does Scott, seeking for a simile, say in _Marmion_:
“Variable as the shade
By the light quivering aspen made.”
A peculiar point about these vertical leaves should be noted. On
the under side of leaves are situated a myriad of tiny openings
(_stomata_, mouths) through which the plant absorbs carbon dioxide
from the atmosphere, and having taken from it the carbon, liberates
the oxygen, the stomata being also used for the escape of the surplus
water of the plant. Now, the reason why these mouths are situated in
most plants on the under side of the leaves is no doubt because they
are thus protected from cold and rain and storm, and their work less
interfered with. In the aspen, with its vertical leaves, either side
of which is equally exposed to atmospheric vagaries, there is nothing
to choose between the two sides as regards the position of the
stomata, and as a matter of fact, these are equally distributed over
both sides of the leaf. A further modification of this kind we may
find in plants like the water-lily, the leaves of which float on the
surface of water. Following out our line of argument, we would expect
to find the stomata confined to the _upper_ side of such a leaf, so
that they may be in contact with the atmosphere, and this is exactly
what we do find. Plants whose leaves are all continually below the
surface of the water, such as the water lobelia and many pond-weeds,
must perforce be content with obtaining the carbon dioxide which they
require from the small quantity of that gas which is to be found
dissolved in the water.