time, and consequently self-pollination is quite impossible. With these
it often happens that the anthers and stigma alternately occupy the same
position, so that the same part of the body of an insect which becomes
dusted with pollen in one flower rubs against the stigma of another.
Other flowers, such as the Forget-me-not, in which both stamens and
stigma are ripe together, project their stigmas above the stamens at
first, in order that an insect from another flower might touch the
stigma before it reaches the stamens, and thus cross-pollinate them; and
their stamens are afterwards raised by the lengthening of the corolla
until they touch the stigma. Thus the flowers attempt to secure
cross-pollination; but, failing this, pollinate themselves.
In the Common Arum or Cuckoo Pint, described on p. 106, we have
an example of a flower of peculiar construction, surrounded by a very
large bract in which insects are imprisoned and fed until the anthers
are mature, and then set free in order that they might carry the pollen
to another flower of which the stigmas are ripe.
Sometimes the flowers of the same species assume two or three different
forms as far as the lengths of the stamens and pistils are concerned,
the anthers of one being of just the same height as the stigma of
another, so that the pollen from the former will dust that portion of
the body of the insect which rubs against the latter Examples are to be
found among the Primulas, and in the Purple Loosestrife, both of which
are described in their place.
In some flowers the stamens are irritable, rising in such a manner as to
strike the insects that visit them; and in these cases the anthers
almost invariably deposit pollen on that portion of the insect's body
which is most likely to come in contact with the stigma of the next
flower visited. Again, in Sages, the anthers are so arranged that they
are made to swing, as on a see-saw, to exactly the same end.
These few examples will suffice to show that the structure and
conformation of flowers are subservient to the one great purpose of
securing the most suitable means of the distribution of pollen, and the
student who recognises and studies the various forms of flowers in this
connexion will find his work in the field doubly interesting.
III
CLIMBING PLANTS
Many plants have stems which grow to a considerable length, and which
are at the same time too weak to support the plants in the erect
position. A considerable number of these show no tendency to assume an
upward direction, but simply trail along the surface of the ground,
often producing root fibres at their nodes to give them a firmer hold on
the soil and to absorb additional supplies of water and mineral food.
Some, however, grow in the midst of the shrubs and tall herbage of
thickets and hedgerows, or in some other position in which it becomes
necessary to strive for a due proportion of light, and such plants would
stand but a small chance in the struggle for existence if they did not
develop some means of securing a favourable position among their
competitors.
These latter are collectively spoken of as climbing plants; but it is
interesting to note that in their seedling stage they are all erect, and
it is only after they reach a certain height that they commence to
assume some definite habit by which they obtain the necessary support,
or to develop special organs by which they can cling to objects near
them.
Some climbers produce no special organs for the purpose of fastening
themselves to surrounding objects, but trust entirely to the wandering
and more or less zig-zag nature of their feeble stems, and thus reach
the open light merely by a process of interweaving, as in the case of
the Hedge Bedstraw (_Galium mollugo_). Others adopt this same method of
interweaving, but at the same time develop some kind of appendages to
give them additional support. Thus, the Rough Water Bedstraw (_G.
uliginosum_), which sometimes reaches a height of four or five feet, has
recurved bristles all along its slender stem, and these serve as so many
little hooks, holding the plant securely on to the neighbouring rank
herbage of the marsh or swamp in which it grows, while the rigid leaves
further assist by catching in the angles of surrounding stems.
Another good example is to be seen in the common Goose-grass or Cleavers
(_G. aparine_) of our hedgerows, which also reaches a height of four or
five feet, and clings very effectually by means of the hooked bristles
of its stems and leaves.
The Marsh Speedwell (_Veronica scutellata_), though it grows to a height
of only one foot, is too weak to stand erect without support, and it has
quite a novel method of securing the aid of the plants among which it
grows. Its two topmost leaves at first stand erect over the terminal
bud, so that they are easily pushed through the spaces in the
surrounding herbage as the stem lengthens. They then diverge, and even
turn slightly downwards, thus forming two supporting arms, the holding
power of which is further increased by the down-turned teeth of their
margins. This process is repeated by the new pairs of leaves formed at
the growing summit of the stem, with the result that the plant easily
retains the erect position.
[Illustration: PRICKLES OF THE WILD ROSE.]
The Wild Roses and Brambles growing in the hedgerows support themselves
among the other shrubby growths by the interlacing of their stems, but
are also greatly aided by the abundance of prickles with which these
stems are armed. The prickles, even if erect, would afford considerable
assistance in this respect; but it may be observed that they are
generally directed downwards, and often very distinctly curved in this
direction, and so serve to suspend the weak stems at numerous points.
We often find the Bramble growing in abundance on heaths and downs, in
situations where suitable props do not exist. In this case the younger
shrubs simply trail along the ground, or form low arches as the weight
of the stems and their appendages cause the apex to bend to the ground.
Yet if we turn to the older shrubs of several years' growth we find that
they have succeeded in reaching a height of some feet. The first stems
of these shrubs formed low arches as we have just described, and then
they gave rise to branches which were first erect, but were afterwards
bent downwards in the same manner, forming arches rising higher than
their predecessors. This continued, year after year, till at last a long
series of stems, forming arch above arch, reached the present height,
the older stems, at the bottom, now dead, serving to support the whole
mass above.
[Illustration: IVY, SHOWING THE ROOTLETS OR SUCKERS.]
Some climbing stems produce little roots by means of which they can
cling firmly to available supports. Such are very common among tropical
plants, but our Ivy affords a splendid example. The roots so formed may
appear in clusters at special points of the stem, or in long lines
running longitudinally on it, and they are produced on trailers as well
as on climbers. In fact, we can draw no fine distinction between the
former and the latter in this respect, and even the Ivy will sometimes
trail along the ground after the manner of the Periwinkle, which roots
itself at several points as it proceeds.
The rootlets of the Ivy and other climbers of the same habit always
avoid the light; and if they are not originally formed on the side of
the stem facing the supporting surface, they soon turn towards the
latter, and give rise to little clinging suckers that firmly adhere. If
they come in contact with a bare rock, or with a surface from which no
nutriment can be derived, they serve the one purpose of clinging only;
but if they reach even a small amount of nutritive soil, they produce
absorbent fibres that are capable of extracting food.
The ivy usually clings to the bark of trees or to old walls, the
crevices of which often contain some small amount of transported soil,
or more or less organic soil formed by the growth and decay of low forms
of vegetable life; and thus the tree is enabled to obtain a little food
from the objects that give it the necessary mechanical support.
The well-known Virginian Creeper (_Ampelopsis_) produces rootlets by
means of which it can cling to very smooth surfaces. Its light-avoiding
'tendrils' always turn to the wall or other supporting body; and, on
coming in contact with it, give off little branches which diverge like
the toes of the tree-frog, and produce little adhesive discs which hold
on firmly by the aid of a sticky secretion.
Perhaps the most interesting of all climbing plants are those which
twine their stems around the props afforded by the neighbouring growths.
As before stated, the stems of these plants are erect when very young;
but after they have reached a certain height the top of the stem bends
to one side, and then, as the growth proceeds, it turns slowly round and
round, describing a circle in the horizontal plane, thus seeking some
support round which it can twine.
The rate at which the top of the stem revolves varies in different
plants, and also in the same plant according to the temperature and
other conditions affecting the growth. In some species the upper portion
describes a complete circle in less than two hours during warm weather,
while in others a single revolution may occupy one or two days.
It will be seen, from the nature of these movements, that the revolving
stem is far more likely to come in contact with erect, rather than with
horizontal supports, and observations made on twining stems will show
that they seldom fix themselves round supports which are placed
horizontally or only on a slight incline. In fact, some of these stems
seem quite unable to twist themselves spirally except round an axis
that is either erect or forms a very large angle with the horizontal
plane.
Should the twining stem succeed in reaching a favourable prop, it
immediately commences to bend itself round and round, forming a more or
less compact spiral; and it is probable that the slight pressure, caused
by the contact, acts as a stimulus which incites the peculiar mode of
growth.
The direction which the spiral takes is not always the same. In the Hop,
Honeysuckle, and the Climbing Buckwheat or Black Bindweed, the direction
is always the same as that of the hands of a clock; while in the
Bindweeds the spiral is invariably contra-clockwise. Further, it is not
possible to compel any species to turn in a direction opposite to that
which it naturally follows. Its stem may be forcibly twined in the wrong
direction any number of times, but the free end will always follow its
natural course as soon as it is left undisturbed.
[Illustration: STEM OF THE BINDWEED, TWINING TO THE LEFT.]
Should the stem of a young twining plant fail to reach a suitable
support, it bends over, not being sufficiently rigid to support itself,
and at last the apex reaches the ground. Then, starting afresh from this
second position of rest, it begins to ascend, and its upper end again
commences to revolve as before. The chances are that it will, from this
second position, find something round which it can twine; but failing
this its summit may again and again bend to the ground, thus renewing
its attempts from various positions more or less distant from one
another, and in each effort so made the revolving upper end of the stem
gradually lengthens, and describes a larger and larger circle in search
for a favourable prop.
A twining stem sometimes has the advantage of additional support
afforded by the stiff nature of the base of the stem, which is often
rendered still more rigid by a twist or torsion resembling that of the
strands of a rope. Such advantage is often still further increased by
the presence of longitudinal ridges of the stem, frequently bearing rows
of hooked prickles or hairs that hold on to any object touched. Again,
the base of the stem, even though it reaches nothing round which it can
twine, sometimes takes the form of a spiral, thus forming a good
foundation for the upper portion as it seeks out a convenient prop. Yet
another contrivance to secure the same end may be observed in the
Greater Bindweed and some other plants. The stems, failing to secure a
favourable hold, twine round one another, thus producing a kind of rigid
cable for the support of the upper extremities as they revolve in order
to find stems round which to form their spirals.
Should all the methods and contrivances of the twining plant fail it in
its attempts to secure an uppermost place among the surrounding herbage
or shrubs, it is compelled to trail along the ground. But such a
position is most disadvantageous and unnatural to it, and usually
results in a stunted and sickly plant that may produce no flowers.
Most of the twining plants of our country are of short duration. Many,
like the Climbing Buckwheat, are annuals; while others, as the Hop and
the Bindweeds, though they have perennial roots, produce fresh stems
each season. The Honeysuckle and the Bittersweet, however, have
perennial, woody stems which increase in thickness year by year, though
the latter does not twine very much, and seems to take an intermediate
place between the typical twiners and the plants which support
themselves by merely interlacing their stems with the neighbouring
plants or shrubs.
[Illustration: STEM OF THE HOP, TWINING TO THE RIGHT.]
Some twining stems are unable to form their spirals round thick
supports, and after making some attempt to do so grow off at a tangent
to seek some less bulky prop. It has been observed, for instance, that
the Hop cannot grasp a pole that is more than four inches in diameter.
In many cases, too, the spirals of the twining stem increase in diameter
after they are first formed, and can thus adapt themselves to the
increasing size of a living stem round which they have grown. The
spirals of the Honeysuckle, however, do not increase in this way; and
consequently, when they surround the trunk or branch of a young tree,
the latter is constricted, often to such an extent that it is strangled
and becomes stunted in its growth.
Another class of climbing plants cling to their surroundings by means of
tendrils, which are modifications of leaves or shoots that grow spirally
like the stems we have been considering.
Whatever be the origin of a tendril, it generally grows straight until
it has reached some favourable support, and in order to obtain such
support it performs circular movements similar to those of the tips of
twining stems. Like these stems, too, the tendril is always sensitive,
and forms a close spiral round the object it touches.
Some tendrils will grow spirally without ever touching a support, but
these often become stunted and wither, while those which reach and
embrace a stem or other structure are apparently incited to a luxuriant
growth by the stimulating effect of the pressure produced.
When the tip of a tendril is successful in gripping a stem firmly, the
portion behind it often takes part in the spiral movement, thus becoming
shorter, and pulling the support towards its own plant in such a manner
as to bring it within the reach of additional tendrils.
Of course the tendrilled plants have a much better chance of securing a
suitable support than the twiners, for the latter have to depend on the
searching and clinging powers of but one structure, while the tendrils
are usually very numerous on the same plant, and throw themselves out in
all directions in search of the required aid. The production of tendrils
as a means of support is also much more economical than the method of
clinging by a twining stem, for the former are usually very slender,
while the latter must necessarily be sufficiently thick to convey the
nutritive requirements of the whole plant; and thus the process of
clinging by tendrils is more in accordance with the usual economy of
Nature.
We have observed that twining stems can, as a rule, twine round only
those supports which are erect or nearly so. This is not the case with
tendrils, which are better adapted for twisting round horizontal stems
and leafstalks. Often, too, they pass from one branch or leaf to
another, and so secure the plant to which they belong by fastenings both
above and below. Further, while the clasping part of a tendril often
becomes hard and rigid, the portion between this and the plant may
remain green and flexible. This latter portion also frequently forms a
new spiral in the opposite direction, thus rendering the connexion
between the plant and its support so supple and elastic that no damage
is likely to accrue from the motions caused by the wind.
The tendrils which form long spirals are generally modified stems or
leaves, or they may be elongated leaflets of a compound leaf. Those
which are modified stems may be distinguished by their growth from the
axils of the leaves, denoting that they had their origin in axillary
buds after the manner of branches generally; and also, sometimes, by the
fact that they bear imperfect leaves in the form of little scales. The
tendrils of the Common or White Bryony (p. 96) are of this
nature; while those of the Grape Vine are either modified floral stems
or altered flower-stalks.
In some cases the entire leaf may be changed into a tendril, in which
instance its true nature is revealed by the presence of a bud in its
axil, as in many ordinary foliage leaves. More frequently, however, the
'leaf-tendril' is an altered leaflet of a compound leaf, such as we see
in the Peas and Vetches; and it is interesting to note in such cases
that the loss entailed by the conversion of leaflets into tendrils is
often compensated for by the formation of leaf-like stipules which are
capable of performing the function of leaves. In fact, we often find
that the size of the stipules is proportional to the number of tendrils
produced; and that when the leaflets are considerably reduced in number
by their conversion into tendrils, not only are the stipules large and
leafy, but the stem itself may be extended laterally into broad
wing-like expansions which do the work of foliage leaves.
Interesting illustrations of this are to be found in the Yellow Vetch--a
rather rare plant sometimes seen in sandy fields--in which all the
leaves are converted entirely into tendrils, and their function
performed by very large leafy stipules; also in the Narrow-leaved
Everlasting Pea of bushy places, in which the leaflets of the compound
leaves are all converted into tendrils with the exception of two, the
work of which is aided by the stipules and by the 'wings' of the stem
and petioles. In the Rough-podded Vetch, too, the stems and petioles are
winged to serve the same end; and other British members of this genus
have either large stipules or winged stems, or both, to compensate for
the loss of leaflets that have been modified into tendrils.
In other climbers the blade of the leaf is not reduced in size, even
though the leaf serves the purpose of a tendril, the function of
clinging being assigned exclusively to the petiole or leaf-stalk. This
may be observed in the Wild Clematis and the Bryony, in both of which
the petiole forms a ring round any branch or stem with which it comes in
contact. These petioles are apparently equally sensitive on all sides,
and are therefore ready to cling to any available support, whether above
or below. In the Clematis the leaves are at first at right angles to the
stem of the plant, but they afterwards turn downwards, and thus
transform themselves into so many anchors which give additional aid in
supporting the climber among the other hedgerow plants and shrubs.
IV
EARLY SPRING
The work of the botanist is light during the early spring, especially if
his attention is directed only to plants and trees in their flowering
stages; but, to one whose ambition is to study Nature in all her varied
phases, this season of the bursting of the bud, when all things are
awakening into new life, is full of interest, and demands no small
amount of time.
The first flowers observed in the spring are mainly those hardy weeds
which may be seen in bloom almost through the year, such as the
Shepherd's Purse, Chickweed, Groundsel, White Dead Nettle, Red Dead
Nettle, and Henbit Dead Nettle. These are soon followed by the Furze,
Strawberry-leaved Cinquefoil, Snowdrop, Hazel, Common Whitlow-grass, and
other flowers that are truly blossoms of the spring. All these will be
described in turn, according to their various habitats; the object of
the present short chapter being to note those signs of early spring
which demand the attention of the lover of Nature while flowers are as
yet few and inconspicuous.
A ramble over bleak downs and moors during the cold days of early spring
will probably reveal but little of interest in the way of vegetable
life, but in sheltered vales and woods, copses, and protected waysides,
there is much to be observed. Here it is that we find the hardy weeds
which have continued to bloom throughout the winter months; the earliest
of the spring flowers; the fresh green foliage of herbs and shrubs that,
in more exposed situations, have been completely denuded; the first
tender seedlings appearing above the ground long before the frosts are
over; and the expanding 'leaf-buds' showing their green while elsewhere
all life seems dormant.
This is the season when the young botanist requires his notebook more
than the collecting-book or vasculum; for his records of early flowers,
and of the times of the appearance of the leaf in our trees and shrubs,
will prove of great interest when compared with the corresponding events
and times of other years. Not only do our spring seasons vary
considerably from year to year in such a manner as to alter the general
times of appearance of leaf and flower, but the vicissitudes of our
climate even change the order in which these events occur.
The general study of the buds of trees should commence before they begin
to burst. We commonly speak of the buds as winter buds, but it should be
known that they were formed in the preceding summer or autumn, and have
remained dormant throughout the winter. There is usually a _terminal
bud_ at the tip of each twig, and _lateral buds_ at the sides. If we
examine a lateral bud we find immediately beneath it a more or less
distinct scar, denoting the position of a leaf that fell in the autumn,
thus showing that the bud in question was formed in the axil or angle of
the leaf. These observations should be verified by examining the trees
in autumn, while the leaves still exist.
It is not sufficient that we are able to recognise trees when in leaf;
they should be known equally or almost as well during the winter and
early spring while the branches are bare, and this is usually easily
accomplished by making ourselves acquainted with the general form of
each tree as viewed from a distance, and, on closer inspection, with the
nature of the bark and the character of the buds.
All our forest trees are of the exogenous type; that is, their stems
increase in thickness by the addition of new wood formed outside the
older wood and underneath the bark. Thus the bark, which is composed of
a layer or mass of dead, sapless cells, is gradually pushed outward as
the stem thickens. The result is that the bark is either more or less
fractured, as in the Elm and the Oak, or it flakes off and falls to the
ground, as is the case with the Plane and the Birch. A new layer of bark
is always formed during each summer, and this, in turn, either cracks or
peels away; but while, in the former instance, the accumulated bark
presents a very rugged appearance, and becomes very thick, in the latter
case it remains smooth, and is always thin.
Then again, how are we to account for the great variety in the general
forms of our different trees--the irregular, crooked nature of the Oak;
the slender, but denser branching of the airy Birch; and the tall,
pyramidal form of the Lombardy Poplar? All this is easily understood if
we carefully observe the positions of the buds as seen during the winter
months; and watch the development of these buds during early spring.
[Illustration: TREES IN WINTER OR EARLY SPRING
1. Hazel, with catkins. 2. Ash. 3. Oak. 4. Lime, with remains of the
last season's fruits.]