ADAPTATION OF PLANTS TO THEIR ENVIRONMENT.
HOW PLANTS MANAGE TO LIVE.
The following lecture was recently delivered by Dr. A. Morrison before the
West Australian Natural History Society :—
As the land composing the State of Western Australia has been for many
ages undisturbed, by upheavals of a geological nature, the soil and
climate must have varied very little during all that time, so that at the
present day the conditions of existence for organic beings may be taken to
be similar to these prevailing many thousand years ago. Through all this
time successive generations of plants have been accommodating themselves
to the conditions surrounding them, with the alternative that, if they
could not so adapt themselves, they would suffer extinction. Individual
plants that have for any reason been able to withstand particular trying
conditions better than their companions have in consequence of this
survived, and their descendants, through their possession of those
qualities, have continued on their course. A certain amount of
modification of structure will accompany, if it be not the cause of, this
power of resistance, and when further modifications of the same kind
arise, or others equally favourable, or other characteristics are
introduced by crossing with different forms, the plant may thus, by a
process of natural selection, be so much altered from its original form as
to constitute a distinct species. Long-continued drought, for example,
would impose such a test of the endurance of plants ; and as a matter of
fact, drought, periodic or constant, is the most prominent and
characteristic feature of the environment of plants in Western Australia.
In only a few other regions on the earth’s surface is aridity of climate
so pronounced as in some parts of Australia, and yet the driest parts of
its area are furnished with plants able to live through it and propagate
their kind. We are thus led to infer that our native plants must be
particularly well adapted to droughty conditions, while the duration of
this dry environment, and of the resulting structural modifications, seems
to suggest the idea of fixity of the forms now existing, seeing that a
long-continued succession of similar seasonal cycles, with little
disturbance of soil conditions, must in the course of time have brought
each form of plant to a state of perfection in its adaptation to this
unvarying environment.
ACQUIRED CHARACTERS.
According to Weismann and others, acquired characters cannot be inherited,
the germ plasm being continued unaltered in structure from parent to
offspring, and therefore incapable of producing new characters ; so that
no matter how highly any particular characteristic may be developed during
the lifetime of an individual organism that characteristic cannot possibly
be inherited by the offspring, at birth that is to say. Hence they deny
the correctness of part of the theory of development propounded by Darwin,
who believed that the inheritance of acquired characters was one of the
means through which new species arose, besides the crossing of our species
or variety with another. All depends, however, on the sense in which the
characters called “acquired” have arisen. If acquired characters were
heritable in the sense that the progeny at birth would be provided with
them, then an animal or a plant at various stages of its course through
life ought to produce descendants showing characteristics corresponding to
those acquired by the parents at particular stages. Although the traits
evolved in an organ or tissue by the action of particular stimuli brought
to bear on them in their nevironment [sic] may not appear in the progeny
at their birth, there is at least no reason to suppose that the
descendants should be less capable of developing the same characteristics
in a similar way. On the contrary, each successive generation subjected to
the same conditions—to the same stimuli or the same absence of stimuli—may
carry the development of a characteristic further and further to an
indefinite degree, and the capacity for such specialisation may become
greater and greater the longer the favouring conditions keep the same. We
can scarcely impose a limit to the degree to which an organ may develop
under special stimuli, or to the successive forms an organ may pass
through in a long series of generations, all under the action of the vital
forces inherent in the germ cells. If an entirely new organ or a structure
of a different type were supposed to be included in the expression
“acquired character,” it would be quite reasonable to doubt the
possibility of such being produced in the individual or inherited the
descendants. But if the specialisation has taken place simply from the
action of the stimuli encountered in the environment on the qualities
inherent in the germ cells of the organism, it seems quite reasonable to
expect that not only the capacity but the structure evolved on typical
lines may in time become fixed and heritable and when by the action of
tile external forces (such as drought) educing those new traits those
organisms lacking them are cut off on account of unfitness, the characters
of those surviving may be very unlike the originals.
Apart from the direct inheritance of acquired characteristics, however, a
highly developed trait may have an important influence on the capacity or
chances of an individual plant or animal so provided, for effective
crossing with another variety.
STRUCTURAL MODIFICATION.
The structural modifications by means of which plants are enabled to live
and flourish in dry climates are very varied and interesting. In some
cases the plant as a whole, or particular organs, are highly specialised
to cope successfully with drought, while the great majority of our species
show some indication in their structure of the necessity that exists for
special provision against this feature of the climate. In considering the
question of how this is effected it is necessary to study the relation of
the plant to water, the element of most vital importance to it. Plants
live and grow by the absorption of water from the ground by means of their
roots, which serve also to keep them in a fixed position. This water finds
its way through the stem and branches to the leaves, from the surface of
which it is exhaled into the air. This water is not pure when it enters
the plant, but contains dissolved in it in small quantities a number of
different mineral substances necessary in the formation of its tissues.
Sometimes, however, these ingredients in the water are prejudicial to the
growth of the plant, and may cause its death ; in other cases they may be
tolerated or even favourable to growth, while in others again they may be
the cause of peculiarities of structure sufficiently pronounced to have
caused botanists to describe as distinct species plants so modified by the
absorption of water charged with a particular mineral substance derived
from the soil in which they grew. These substances in the soil may be
injurious to the majority of other plants, so that toleration of them by a
particular variety may be the means of its advancement and spread, through
being relieved from the competition of other plants in that soil.
The great bulk of the woody tissues of plants, however, is not derived
from the water supplied through the roots, but from the atmosphere, which
contains carbonic acid gas, the source of the element carbon, the chief
constituent of wood. The extraction of carbon is effected by the vital
action of the leaves in their performance of the function of respiration.
In a large tree bearing many thousands of leaves it can be understood that
a very great quantity of water is required for its growth, but if the soil
does not contain much or soon becomes exhausted the growth will be
proportionately restricted. In soils that are very dry or liable to become
so at particular seasons plants must be modified in some such way as will
enable them to do with a scanty supply of water. One obvious way in which
moisture may be secured even during the dry season, is by an extension of
the root system to the deeper strata of the soil, which are less subject
to the disiccating [sic] influence of the sun and atmosphere than the
superficial layers. As we have a very long dry summer we should expect to
find the great majority of our native perennial plants to have roots long
enough to reach deep into the ground. Examples of this may be found by
anyone who will take the trouble to dig deep enough. As all example, let
me mention calythrix [?]lavesceus, a small shrub less than a foot high,
that makes the sandy scrub gay with its bright yellow flowers during the
hottest months of summer, when the majority of flowers have faded. You
will find that the clusters of branches appearing above ground spring from
a root-like underground stem running horizontally at 3 to 6 in. below the
surface, and that the true roots for absorbing moisture are attached to
this and extend downwards to a lower stratum of the soil. In like manner
the bush casuarina (C. distyla) of the sandy scrub sends out underground
shoots which at intervals come to the surface as young plants, thus
helping in its propagation independently of its seeds. The same thing
occurs more strikingly still in the Christmas tree (nuytsia floribunda).
When this tree becomes broken down, as easily happens oil account of the
brittleness of its wood, or succumbs to a bush fire, you frequently find
that within a radius of a few yards from the old tree a large number of
young bushes have made their appearance, sometimes in a circle, and these
are found attached to underground shoots given off by the old stock at
various depths below the surface of the ground.
ROOT SYSTEM.
These plants illustrate one arrangement by which the drought of a long dry
summer, and other untoward conditions may he overcome ; but there are many
other ways in which the root system is modified in a special way to retain
life in the plant, and even at shallow depths in the soil. Such are seen
in bulbs, tubers, and various forms of swollen roots or underground stems,
in all of which water is stored up during the wet seasons of the year,
that the germ of the plant may be kept alive through the droughty period,
to start into active growth again when the next rains supply moisture
sufficient for the growth of a new plant which forms roots and provides a
new bulb or succulent organ in which water is again stored up. It is not
only in the underground portions of plants that this storage of water is
effected ; we see it also in the stems and branches of plants such as
those of the cactacrae and some euphorbiaceae, giving them quite a
peculiar character. This type of plant is seen in the most arid regions,
where only a scanty rainfall over a short period of the year must be made
the most of for growth and storage, before the hot, dry air takes it all
back from the ground. Active transpiration is not possible in these dry
surroundings, and as leaves are not required to carry off surplus water
they are suppressed altogether or reduced to scales or prickles, the stems
themselves taking their place and performing their functions to the extent
required In these plants, and in succulent plants generally, water is
stored up in cellular tissue set apart for the purpose, to be drawn upon
when necessary. There are many native plants around us with succulent
leaves in which water is stored against drought, such as the saltbushes,
mesembryanthemums (or pig-face), members of the portulaca family,
including the ice-plant in which water-containing cells on the surface of
the leaves glisten like particles of ice. Some of the lobelias —poisonous
plants not uncommon with us—frequently have their juices so carefully
stored and protected from evaporation in stem and flower that we may find
plants still standing upright in the ground, flowering and ripening their
seed, although the root and lower part of the stem with the leaves on it
are dead and brittle. Others, of the Portulaca family, are so retentive of
life that they may be used for room decoration without water for weeks or
months, while under heavy pressure within sheets of drying paper they
continue their growth for long periods.
THE LEAF.
As the leaf is the respiratory organ of the plant besides being concerned
in transpiration, its formation and intimate structure are important
objects of study. The leaves are in direct continuity with the roots
through the medium of the vascular fibres, which pass from the roots
through stem and branches, and from these through the stalk of the leaf,
breaking up on entering its blade into an expanded network visible to the
naked eye, so that every minute portion of its area is supplied with
water. The surface of the leaf is studded over with minute pores, which
allow the water to escape into the air as vapour, and at the same time let
in the air for respiratory purposes. The small cavities to which the pores
or stomata give access are lined with delicate living cells which are the
active agents in the function of respiration or breathing. The carbonic
acid gas entering with the air becomes dissolved in the water that has
descended from the roots, and is thus presented to the active living cells
which decompose it into the oxygen which escapes into the atmosphere,
while the carbon is appropriated for the formation of the woody tissue of
the plant. The breathing pores or stomata are so affected by the state of
the atmosphere that when it is very dry the two cells guarding them so
alter their form and position as to close the opening, while with a moist
atmosphere and a full current of sap flowing they become more turgid and
bulge outwards so as to form an opening that allows communication with the
air again. It can be well understood that in a dry climate a large
distilling apparatus of the kind indicated would not generally be suitable
; for lack of moisture it could not be kept in action. The extensive scale
on which broad-leaved trees in a moist atmosphere transfer water raised
from the ground to the air has to be reduced, so that the quantity exhaled
shall be proportioned to the amount present in the soil. Accordingly the
broad leaves, with innumerable breathing pores, must have their surface
diminished in extent, and the cellular tissue and stomata reduced in
proportion. In conformity with this requirement we find that in a very
large number of our local plants the foliage is of a very stiff, spiny,
and harsh character, the soft cellular tissues being reduced to a minimum,
the leaves being narrow, with prominent ribs, and ending in sharp points.
As we have a copious rainfall in the winter months, a luxuriant growth of
some broad-leaved plants is encouraged, but the leaves of such, not being
deciduous, are protected by many interesting devices from the influence of
the dry air and hot sunshine of summer, which directly tend to induce
transpiration and rob the plants of their moisture. The leaves of the
eucalypti, acacias, and others are set in a vertical position, so that
their surfaces will be parallel to the sun’s rays at the hottest time of
the day, and so receive less heat. In others the leaves are covered with
hairs, especially on the under surface, and sometimes they, or the whole
plant, are enveloped in a covering of hairs so dense and so matted
together as to resemble thick flannel or blanket. Such a covering not only
protects the substance of the leaf from excessive heat or cold or other
injurious influences, but it may retain for a considerable time such
moisture as may come to it from rain or dew, to the benefit of the plant.
During the growing season leaves may be somewhat soft and delicate, but
with a gradually increasing intensity of sunshine and dryness of the air,
the superficial layer of cells forming its substance may become thickened
into an impervious cuticle, preventing evaporation of the moisture below,
and modifying the effects of the light and heat of the sun. In other cases
fluids are excreted on the surface of the leaf, so that the epidermis is
protected and the stomata effectually sealed with wax, gum, resin, or
lime. In some leaves the stomata are so deeply sunk below the surface as
to be removed from the direct action of the light and heated air, or they
may be situated in the bottom of grooves which themselves become shrunk
and partly closed when the air is dry. In some grasses and sedges you may
observe that as they grow the leaves are quite flat, but shortly after the
stalk is plucked you find that the leaves have lost their flat character
and appear curled up lengthwise, the change being due to the stoppage of
the sap current when the stem was severed from the roots. One of the
plants now coming into flower about Perth is Grevillea oxystigma, a small
shrub with abundance of white blossom. You will notice that as long as the
rainy season lasts the leaves are mostly flat, but at a later date, in the
dry season, they appear more often as double-barrelled tubes each half of
the blade having curved backwards to the midrib as if to exclude the dry
air from the under surface, which is already protected by a covering of
minute hairs. This rolled condition appears to be more constant in the
drier districts of the interior, such as Kellerberrin or the Stirling
Ranges, while in these localities other species of G. show the peculiarity
in a still more pronounced degree. The rolling back of the margins is
often so tightly effected in some plants, that the leaf must be broken up
if you wish to see the hidden under-surface. From this condition of the
leaf it is only a step further to one that is quite solid with the under-
surface obliterated and a groove marking the position of the midrib ; and
from that again we easily arrive at the terete form, as it is called, like
a knitting-needle, quite smooth all round and presenting to the air the
least possible area of the surface, and ending in a sharp point most
frequently.
INJURIOUS EXTERNAL INFLUENCES.
It is interesting to note how the stems of trees and other plants are
protected from injurious external influences. In a visit to the North-West
I was struck with the fact that the few trees seen had their trunks and
branches provided either with a rough thick corky bark or had them smooth
and quite white as if painted with a cooling composition such as that
applied to the roofs of houses. In either case it evidently serves as a
protection from the excessive sunshine and heat of that region. The heat
of the sun and air is probably the cause of the development of the thick
protective cuticle, like that on the palm of the village blacksmith, and
the white stems reflect the light and heat of the sun, so that the
interior of the stems will not suffer injury. When the moisture in the
soil is not pure water, but contains particular substances dissolved in it
such as salt, many plants would be injured or killed if planted there,
while others, like saltbushes would grow and thrive. The saltbushes may
grow well in an ordinary soil, so that a distinctly brackish condition is
not indispensable to them : but they have a tolerance or affinity for
salt, and may extract more of it from an ordinary soil than other plants
do, and store it up in their tissues. They thrive better in a moderate
degree of brackishness, however, and their roots, when not in active
growth, might suffer no injury from a somewhat concentrated solution of
salt, as those of other plants would. The subject of the adaptation of
plants to their surroundings may be said to be co-extensive with the
vegetable kingdom, and we may go on speaking of examples of their ways and
means of doing so indefinitely ; but it would be advantageous to consider
some of the means by which the economically important family of grasses
maintain their position under trying conditions. In some grasses—for
example, Sesleria aenciefolia, a South European species—a remarkable
intricately-woven tunic is provided for the covering and protection of the
lower part of the stalks. The sheaths of the lower leaves—the part of the
leaf embracing the stem—are composed of longitudinal and crossing zigzag
fibres so woven together as to form an intricate network in the tissue of
that part of the leaf. When the upper part of the leaf withers and drops
away in dry weather this fibrous net remains as a protective clothing for
the steam [sic] of the grass during the trying period between summer and
the following spring. Instead of having a woven fibrous tunic the base of
the leaf sheath may be more strawlike in its nature, as in some of our
native grasses, but in drought either texture is calculated to protect the
plant, to absorb moisture when the chance occurs, and to retain it for a
longer or shorter time during intermediate dry periods.
PROTECTIVE COVERING.
In Australia and South Africa another form of protective covering has been
evolved in the native grasses of those parts of the world. On the bases of
the leaf sheaths, instead of woven fibrous or straw as described, we find
a great development of hairs of a woolly nature so copious as to
completely cover the base of the stem and give it a bulbous appearance.
These woolly hairs are sometimes woven into a feltlike fabric of
appreciable thickness, giving effective protection from injurious outside
influences, and at the same times preventing the escape of moisture, while
it absorbs water from the atmosphere and retains it for the replenishment
of that required in the vital processes carried on in the living tissues
of the plant. Eragrostis eriopoda, a North-West grass, gives a typical
example of this arrangement, which may he observed, though to a less
degree in other species of Eragrostis, Stipa, Panicum, Danthonia, etc. It
would be easy to demonstrate the power of this felted tunic to retain
moisture by wetting a clump of one of these grasses and a similar clump of
another grass unprovided with a hairy covering, and laying both in an open
place to dry, when it would be found that the felted covering would still
be moist long after the other had become perfectly dry.
In the sandy soil about Perth and in the South-West generally during the
spring months may be seen a tall purplish grass, bearing at the top of its
stalk a loose bunch of flowers somewhat resembling quaking-grass in shape.
This is Poa nodosa, which grows up during the wet season, and continues
for some time further into the dry period, flowering and ripening its
seeds when the sand in which it grows is almost constantly dry. If the
base of the stem is examined there will be seen one, two or three bulbous
swellings of the base of the stalk, just below the surface of the sandy
soil. These swellings are succulent, and contain a store of water in
anticipation of the plant’s reeds during the course of the long periods of
dry weather that are sure to follow. A similar formation is found in a
variety of well-known Timothy grass that grows in the drier districts
about the Mediterranean Sea. There is no specific difference between this
variety and the typical form (Phleum pratense) found in moister
localities, beyond the presence of the bulbous swelling of the underground
stem, and when this form is transplanted to cultivated ground and supplied
with sufficient moisture, the stems lose their bulbous character. Whether
Timothy grass grown in dry localities in Western Australia ever assumes
this bulbous form, perhaps some of our agriculturists may be able to say.
If it should be able to adapt itself to our climate it may be worth
encouraging for that reason alone ; but whether the bulbous form be
imported as such from its native steppes or modified from the ordinary
form by a process of acclimatisation, it would be scarcely reasonable to
expect it to possess the same succulency as the grass grown with abundant
moisture. It might prove of value in dry districts, but at the same time
our native grasses, long used to the climate, may be found, if properly
tested by experiment and analysis, to be quite as nutritious.
INDIGENOUS FODDER PLANTS.
Under a sort of fatuity we neglect the plants growing in our own soils and
search all over the world for new kinds of fodder plants, without giving a
thought to those growing in our own vicinity. There are many grasses and
saltbushes adapted to withstand the most severe droughts, and all that is
done is to take advantage of the bounty of nature and use up all the
herbage provided till it is in danger of extermination. The grasses are
known to be highly nutritious, but no steps are taken to preserve them or
to extend the area of their growth ; while the saltbushes, though they
have been analysed and proved, in foreign countries to have high feeding
qualities, receive scarcely any consideration, as if they were thought as
indestructible as the sand or as inevitable as drought. In connection with
this subject we have to distinguish between perennial plants and annuals.
The pastoralists are concerned with the former, but their scheme of
exploitation of the country grazed upon does not include provisions for
the future continuation of the fodder plants originally provided. On the
other hand the cultivator of the soil imports plants from foreign
countries, apparently without always considering whether the conditions of
existence here are fairly comparable with those of their home country.
Fodder plants from India, with its combination of heat and moisture may
grow here, but they could not be expected to do as well in our dry season
as when in their own home. Some plants form that country or from the basin
of the Nile might produce abundance of fodder or fruit, with the help of
irrigation ; but land is so plentiful and the population is so scanty in
Western Australia that irrigation does not recommend itself to cultivation.
