THE GEOGRAPHICAL DISTRIBUTION OF PLANTS
'No speculation is idle or fruitless that is not opposed to truth
or to probability, and which, whilst it co-ordinates a body of well
established facts, does so without violence to nature, and with a due
regard to the possible results of future discoveries.'
Sir Joseph Hooker.
In the vegetation of the British Isles the leading _rôle_ is played
by that large group to which the term Flowering Plants is frequently
applied. This group, including the two sub-divisions Dicotyledons
and Monocotyledons, is known by the name Angiosperms, a designation
denoting the important fact that the seeds are developed in an ovary
or protective seed-case (ἁγγειον, a vessel or box). The fact that
these highly elaborated products of development made their appearance,
so far as we know, at a comparatively late stage in the history of
the plant-world, attests their efficiency as a class and demonstrates
the rapidity with which they have overspread the surface of the earth
as successful competitors in the struggle for existence. As Darwin
wrote in a letter to Sir Joseph Hooker in 1881, 'Nothing is more
extraordinary in the history of the vegetable kingdom, as it seems to
me, than the _apparently_ very sudden or abrupt development of the
higher plants(11).' In another letter (1879) to the same friend we
read, 'The rapid development as far as we can judge of all the higher
plants within recent geological times is an abominable mystery(12).'
Making allowance for the probability, or indeed certainty, that the
imperfection of the geological record tends to exaggerate the apparent
suddenness of the appearance of this vigorous class, and allowing for
the fact that our knowledge of the records of the rocks in which the
highest plants first occur is very incomplete, we cannot escape from
the conclusion that this recently evolved group spread with amazing
rapidity. Various reasons may be suggested in explanation of the
dominant position which the Angiosperms hold in the floras of the
world. As an instance of their rapid increase during the Cretaceous
epoch[1], the period which has furnished the earliest satisfactory
records of Flowering Plants, the following statement by an American
writer may be quoted:--'The rapidity with which it [_i.e._ the group of
Flowering Plants] advanced, conquering or supplanting all rivals, may
be better understood when we remember that it forms 85% of the flora
of the Dakota group '; that is a series of sedimentary rocks in Dakota
referred by geologists to the middle of the Cretaceous period(13). In
the Wealden rocks of England, which are rich in the remains of Lower
Cretaceous plants, no undoubted Flowering Plant has so far been found.
[1] For the position of the Cretaceous and other systems in the
geological series, see the table on page 42.
The more efficient protection of the ovules, the germs which, after
fertilisation, become the seeds, the extraordinary variety in the
floral mechanisms for assisting cross-pollination, the arrangements
for nursing the embryo, and the structural features of the wood in
relation both to rapid transport of water and to the storage of food,
are factors which have probably contributed to the success of the
Angiosperms. The degree of weight to assign to each contributing cause
cannot as yet be satisfactorily determined, but the general question
raised by the recent origin of these latest products of evolution
offers a promising field for work. While admitting our inability
at present to do more than suggest possibilities, we may encourage
research by speculation.
The members of the Vegetable Kingdom placed next to the Flowering
Plants are the Gymnosperms or naked-seeded (γυμνὁς, naked) plants,
including (i) the Conifers, _e.g._ pines, firs, larches, the yew,
etc., (ii) a small group of plants known as the Cycads, whose
existing members, now almost confined to a few tropical regions, are
the descendants of a vigorous race represented by many species in the
floras of the Mesozoic epoch. A third sub-division of the Gymnosperms,
the Ginkgoales, is represented by a single survivor, which is described
in a later chapter as one of the most remarkable links with the past in
the plant kingdom.
The Gymnosperms are geologically very much older than the Angiosperms.
Members of this class played a prominent part in the vegetation of
the Coal age and it is certain that they existed in the still older
Devonian period. The only other group to which reference is made in
later chapters is that of the Ferns, one of the sub-divisions of a
large class known as the Vascular Cryptogams or Pteridophyta. These
plants, like the Gymnosperms, are represented in the oldest floras of
which recognisable remains have been preserved. The main groups of the
vegetable kingdom, founded on existing plants, are distinguished by
well-defined differences; they are comparable with separate twigs of a
tree springing from larger branches and these again uniting below in
a common trunk. The vegetation of to-day represents only the terminal
portions of the upper branches. As we descend the geological series,
records of extinct types are found which enable us either to trace
the separate branches to a common origin or to recognise a convergence
towards a common stock. Were a botanist to find himself in a forest of
the Coal age he would experience great difficulty in assigning some
of the plants to their systematic position: characters now regarded
as distinguishing features of distinct groups would be met with in
combination in a single individual. It is by the discovery of such
generalised types, which serve as finger-posts pointing the way to
lines of evolution, that the student of pre-existing plants has been
able to throw light on the relative antiquity of existing forms, and
to trace towards a common ancestry plants which now show but little
indication of consanguinity.
Confining our attention to the dominant group of plants in the British
flora, namely the Flowering Plants, we may profitably consider the
question, though we cannot satisfactorily answer it,--which members of
this group are entitled to be regarded as the most ancient inhabitants?
The past history of our native plants, and their geographical range,
not only in the British Isles but on the Continent of Europe, are
subjects well worthy of the attention of field-botanists whose
interests are apt to be confined within too narrow bounds. There are
numerous problems relating to the composition of the present vegetation
of Britain which might be discussed in reference to the relative
antiquity of plants; but in a single chapter it is impossible to do
more than call attention to certain considerations which are frequently
overlooked by students of British species.
It is customary to speak of the British flora as consisting for the
most part of species introduced into this country by natural means,
while some plants owe their introduction to human agency or are
'escapes' from cultivation. It is by no means an easy task in some
instances to decide whether a species is native or introduced, but
in some cases, a few of which are mentioned, there is no doubt as to
the alien nature of the plants. The term 'native' needs a word of
explanation. It is not intended to convey the idea that a plant so
designated came into existence on British soil and spread thence to
other regions; but by native species we mean such as have reached this
country by migration from other lands, or it may be in some instances
have actually originated in this part of Europe. One of the best known
aliens in Britain is the American water-weed, _Elodea canadensis_
(or _Anacharis alsinastrum_), which was discovered about sixty years
ago in a canal near Market Harborough in Leicestershire(14). In all
probability this North American species was introduced into England
with timber. Once established, it spread through the waterways with
alarming rapidity and became a serious pest. Elodea affords an
admirable instance of the serious interference with the balance of
Nature by the introduction of a new competitor into an environment
conducive to vigorous development. Another foreign water-plant,
_Naias graminea_, for the importation of which Egyptian cotton
may be responsible, has been recorded from the Reddish canal near
Manchester(15). This African and Asiatic species occurs in Europe
only as a colonist; it is said to have been introduced into Italy
with East Indian rice. A more recent case of alien immigration due to
unintentional human agency is that of _Potamogeton pennsylvanicus_,
a pond-weed of Canada, the United States, Jamaica, and elsewhere.
Specimens of this species were first noticed in 1907 in a canal near
Halifax close to the effluent from a cotton mill. Since its discovery
the plant has slightly extended its range. It is suggested by Mr
Bennett, who first identified the alien, that its fruits were brought
to this country in goods from the United States(16).
Of the introduction of these and other foreign plants we have
satisfactory records; but there are many others which may owe their
presence to man's agency, though we have no information as to their
arrival.
It has long been recognised that several members of the British flora
are related to Scandinavian species. The Scandinavian flora, as Sir
Joseph Hooker says in his well-known paper on the 'Outlines of the
Distribution of Arctic plants,' 'not only girdles the globe in the
Arctic Circle, and dominates over all others in the North Temperate
Zone of the Old World, but intrudes conspicuously into every other
temperate flora, whether in the northern or southern hemisphere, or on
the Alps of tropical countries'(17). The view generally held is that
during the Glacial period this Arctic flora was driven South, and aided
by land-bridges, which were afterwards submerged, many of the northern
migrants found a more congenial home in Britain. It is however by no
means improbable that this conclusion may have to be considerably
modified. Mr and Mrs Reid, as the result of their careful analysis
of the Pre-Glacial Flora of Britain, express the opinion that 'the
pre-glacial plants suggest climatic conditions almost identical with
those now existing, though slightly warmer' (27, 2). It is noteworthy
that the list of plants given in their paper does not include any
typical Arctic species. The occurrence on the mountains of Scotland
and elsewhere of such plants as _Silene acaulis_, _Dryas octopetala_,
_Saxifraga oppositifolia_ and other Saxifrages, _Rubus chamaemorus_
(the Cloudberry), and the dwarf Birch illustrate the Arctic-Alpine
element in our flora.
The opinion is held by many Swiss botanists that their Alpine species
have in large measure been derived from non-glaciated parts of the
Pyrenees, that is from a region which was presumably able to retain its
flora at a time when more northern lands were exposed to extreme Arctic
conditions. My friend Dr Moss believes that some of the so-called
Scandinavian plants came to Britain from Central Europe after the
retreat of the ice; if this view is correct it means that some at least
of our Arctic-Alpine plants reached these islands by a southern rather
than by a northern route.
Interesting examples of far-travelled northern plants recently
described by Professor Engler of Berlin afford additional illustrations
of the general principles enunciated many years ago by Sir Joseph
Hooker. A species of flowering Rush, _Luzula spicata_ var. _simensis_,
occurs at an altitude of 3600 metres in Abyssinia and on Kilimanjaro.
_Luzula spicata_ is found in the whole of the Arctic and Subarctic belt
in Scotland, Auvergne, the Jura mountains, and elsewhere. The species
probably began its career in the northern hemisphere where it grew
abundantly on the higher ground in the Arctic Circle: it eventually
travelled along the North American Andes and appeared in Mexico under a
guise sufficiently distinct to warrant the use of another name, _Luzula
racemosa_. In an eastern direction it reached the Himalayas and is
represented in Abyssinia by a closely allied form. From Abyssinia to
Kilimanjaro _Luzula spicata_ 'had to travel a long distance; but it is
not impossible that it either still exists or has existed previously on
a few of the high mountains between Abyssinia and Kenia, from which,
having advanced to the Kilimanjaro, it again produced new forms....
At any rate, it is impossible to do without distribution of seeds of
alpine plants by air-currents or by birds from one mountain to the
other in explaining the history of distribution'(18).
The majority of British plants are identical with species in Central
and Northern Europe: of these, some are among the most widely spread
English species, _e.g._ the Daisy and Primrose, while others, such as
the Oxlip (_Primula elatior_), are confined to the Eastern counties,
and others, such as the Cheddar Pink (_Dianthus caesius_), are
restricted to Western counties.
Before considering a small section of the British flora which is the
most interesting from the point of view of origin, a short digression
may be allowed in order to call attention to the importance of a
branch of science which Darwin spoke of as 'that grand subject, that
almost keystone of the laws of creation, geographical distribution,'
and in 1847 referred to as 'that noble subject of which we as yet
but dimly see the full bearing.' It was largely as the results
of his study of distribution in the Galapagos Islands that Darwin
determined to 'collect blindly every sort of fact which bears anyway
on what are species.' The acceptance of the view 'that each species
was first produced within a single region'(19), raises the subject of
geographical distribution to a far higher plane than it occupied in
pre-Darwinian days. Although most people are familiar with some of
the commoner means by which plants are able to colonise new ground
through the adaptation of their fruits and seeds to various methods of
transport, the conception of a plant as a stationary organism tends
to prevent due allowance being made for the comparative facility with
which, in the course of successive generations, a species is able to
wander from place to place. The individual animal is endowed with
powers of locomotion enabling it to seek new feeding-grounds and to
avoid enemies; but with the exception of some of the simplest forms a
plant cannot move--'le matin la laisse où la trouve le soir.'
The rate of travel may or may not be rapid, but in a comparatively
short time, if the conditions are favourable, a tree may spread over
a wide area. Mr Ridley, Director of the Botanic Gardens, Singapore,
writes as follows in reference to the rate of travel of one of the
common Malayan trees (_Shorea leprosula_), which bears winged fruits
particularly well adapted to wind-transport: 'If we assume that a tree
flowers and fruits at 30 years of age and the fruits are disseminated
to a distance of 100 yards, that the furthest fruits always germinate
and so continue in one direction, it will be seen that under such most
favourable circumstances the species can only spread 800 yards in 100
years, and would take 58,666 years to migrate 100 miles'(20).
There is, however, one type of distribution--what is called
discontinuous distribution--to which special attention should be
directed on account of its intimate association with questions
relating to the past history of living organisms. Many examples might
be quoted from both the animal and plant kingdoms in support of the
view that discontinuous distribution is a criterion of antiquity. When
identical or very nearly identical plants occur in regions separated
from one another by areas in which the particular species is unknown,
the inference is either that the surviving individuals are remnants
of a large number formerly distributed over a wider continuous area,
or that in the course of evolution similar conditions in widely
separated areas led to the production of identical types. The former
view is much the more probable: it is consistent with the conclusions
arrived at on other grounds as to the connexion between discontinuous
distribution and ancient lineage. The explanations of the widespread
occurrence among different races of similar objects or legends afford
an analogous case. As Dr Andrew Lang points out in _Custom and Myth_,
it is held by some students that the use of the bull roarer--to cite a
specific instance--by different peoples denotes descent from a common
stock, though he considers the more probable explanation to be that
similar minds, working with simple means towards similar ends, might
evolve the bull roarer and its mystic uses anywhere.
The Cedars of Lebanon afford an interesting example of discontinuous
distribution. They illustrate how a species, which may be assumed to
have originated in one region, in the course of its wanderings may
undergo slight changes until, at a later stage when the plants have
disappeared from parts of the once-continuous area, the remaining
outlying groups of individuals are spoken of under different specific
names. The cedars of Lebanon, known as _Cedrus libani_, occur as
isolated groups on the Lebanon hills as outliers of the larger
forests of the Taunus 250 miles distant. The African cedar, _Cedrus
atlantica_, is separated from the Lebanon cedar by a distance of 1400
miles. Approximately the same distance divides the Lebanon cedar from
the deodar, _Cedrus deodara_, which extends from Afghanistan along
the Himalayas almost to the confines of Nepal. Sir Joseph Hooker
regards the three cedars as varieties of one species which once
formed a continuous forest: he attributes the present discontinuous
distribution, in part at least, to the effects of a warmer succeeding
a colder climate. The less favourable conditions drove the vegetation
of the lowlands to seek more congenial habitats at higher altitudes.
In this connexion it is interesting to find that in Algeria the cedar
is confined to the higher ground where the snow lies long in the
spring(21).
The Tulip Tree of North America and Central China affords one of
many examples of existing flowering plants which illustrate the
close connexion between present distribution and past history. The
genus Liriodendron, often cultivated in the south of England, is
now represented by two species, the best known of which--the Tulip
Tree, _Liriodendron tulipera_--extends from Vermont to Florida and
westwards to Lake Michigan and Arkansas. The leaves bear a superficial
resemblance to those of the Sycamore, but are as a rule easily
distinguished by the truncated form of the apex; the specific name
was suggested by the tulip-like form of the flowers. Fossil leaves
of Liriodendron are not uncommon in the Cretaceous rocks of Disco
Island in latitude 70° N., where they occur with other flowering
plants which bear striking testimony to the mildness of the Cretaceous
climate in high northern latitudes. One of the associated flowering
plants is a species of Artocarpus, described by Dr Nathorst as
_Artocarpus Dicksoni_ which bears a close resemblance to _A. incisa_
the bread-fruit tree of the southern tropics of the Old World. Without
attempting to deal fully with the past history of Liriodendron, it may
be confidently stated that the records of the rocks are consistent with
the idea of antiquity suggested by the present distribution of the two
surviving species.
Islands such as Great Britain and Ireland, situated a short distance
from a continent, contain many plants which are widely spread in
different parts of the world, together with a very small number
peculiar to the British Isles though closely allied to species on the
neighbouring continent or to plants farther afield. The occasional
recognition of species previously believed to be confined to Britain
tends to reduce the short list of our endemic types.
An enquiry into the origin of an island flora involves a consideration
of the data in regard to changes in level and relative distribution of
land and water in the course of geological evolution. It is generally
agreed that at no distant date, in a geological sense. Great Britain
and Ireland were united to the continent. There is, however, another
fact to reckon with, namely the prevalence of Arctic conditions in
northern Europe when a thick sheet of ice spread over the greater
part of the British Isles. There can be no doubt that the severity of
the climate during the Glacial period was such as to destroy a large
proportion of the vegetation. The question is, were all the flowering
plants destroyed or were some of the hardier species able to survive,
either on the higher peaks which kept their heads above the level of
the ice or on the southern fringe of England beyond the ice-covered
region? It is impossible to give a definite answer: the probability
is that nearly all the pre-Glacial species were destroyed, but it is
not impossible that some Alpine-Arctic plants escaped extinction,
while others retreated to more southern and less Arctic areas by means
of a land-connexion with France or crossed the intervening sea by
ocean-currents, by animal agency, or by wind.
Although we possess but imperfect information as to the extent and
duration of land-bridges between Britain and the continent, there are
no special difficulties in the way of accounting for the presence
of Scandinavian, Germanic, and other elements in the British flora.
There are, however, other and more difficult problems to consider in
reference to a small group of flowering plants which are met with in
the west and south of Ireland, also, to a less extent, in Cornwall and
in a few other localities in the south-west of England. In Connemara
in the west of Ireland, where hard frosts are unknown and winter snows
are rare, there are three kinds of Heath, St Dabeoc's Heath (_Daboecia
polifolia_), the Mediterranean Heath (_Erica mediterranea_) and
_Erica Mackaii_ which are not found elsewhere in the British Isles or
in the whole of northern Europe, but reappear in the Pyrenees. The
London Pride (_Saxifraga umbrosa_), another Pyrenean plant, grows
on the south and west coast of Ireland from Waterford to Donegal.
_Arbutus Unedo_, the Strawberry tree, which flourishes in the Killarney
district of County Kerry and occurs in neighbouring localities, has
a wide distribution in the Mediterranean region. Devonshire and
Cornwall possess two other Heaths, _Erica ciliaris_, which extends into
Dorsetshire and occurs in north Brittany, and _Erica vagans_, both
Pyrenean species, while a Mediterranean plant. _Gladiolus illyricus_,
grows in the New Forest.
In 1846 Edward Forbes dealt with the problems presented by the
distribution of British plants in an essay which has exercised a
far-reaching influence. When Forbes published his work, comparatively
little was known as to the possibilities of transport of seeds and
fruits across barriers of water(22). His conviction that the known
means of dispersal were insufficient to account for the presence of
Mediterranean or Lusitanian plants in Ireland led him to turn to
geology for a solution of the problem. He was thus led to put forward
the view that in the course of the Tertiary period when, as we know
from palaeontological evidence, the climate of north and west Europe
was much warmer than it is now, and long-before the beginning of the
climatic changes which culminated in the Glacial period, there was a
land-connexion between the west of Ireland and the south-west of the
continent. Mr Praeger, whose work on the Irish flora is well known to
systematic botanists, agrees with the conclusions of Forbes, and sees
in the Portuguese and Mediterranean plants 'relics of a vegetation
which once spread along a bygone European coast-line which stretched
unbroken from Ireland to Spain'(23). If this explanation is correct it
entitles Arbutus, St Dabeoc's heath and other members of this southern
group to be regarded as a very old section of our flora. There is,
however, another side to the question: granting that a certain number
of Irish plants were able to withstand the rigours of an Ice age, it
is hardly likely that the strawberry tree and other southern types,
which it is admitted flourish in the south-west of Ireland because of
the mildness of the climate, were of the number of those which endured
an extreme Arctic phase. Moreover, if these Mediterranean species
are survivals from the Tertiary period, if they have been isolated
since pre-Glacial days as an outlier of a southern flora, we might
fairly expect that during the long interval between their arrival and
the present day new forms would have been produced closely related
to, though not identical with, the parent types. This, however,
has not been proved to be the case. Darwin in speaking of Forbes'
Essay in a letter to de Candolle in 1863 says that he differs from
most of his contemporaries 'in thinking that the vast continental
extensions of Forbes, Heer, and others are not only advanced without
sufficient evidence, but are opposed to much weighty evidence'(12).
The alternative view is to regard Arbutus and its compatriots as
post-Glacial arrivals and not as survivals from a widely spread
Tertiary flora.
A recently published account of the New Flora of the volcanic island
of Krakatau furnishes an instructive and remarkable demonstration of
the facility with which a completely sterilised island, separated
by several miles of ocean from neighbouring lands, may be restocked
with vegetation(24). In 1883 the island of Krakatau, then densely
covered with a luxuriant tropical vegetation, was partially destroyed
by a series of exceptionally violent volcanic explosions. After this
catastrophe only a third of the island was left: the surface was deeply
covered by pumice and volcanic ash and no vestige of life remained. In
1906 a party of botanists who spent a few hours on Krakatau collected
137 species of plants: the vegetation was in places so dense that it
was with the greatest difficulty they penetrated beyond the coastal
belt, and some of the trees had reached a height of 50 feet. The seeds
and fruits of this new flora have been carried by ocean-currents,
by wind, and by the agency of birds from other islands in the Malay
Archipelago. The nearest islands, except the small island of Sebesi,
about 12 miles distant, are Java and Sumatra, separated from Krakatau
by a stretch of water about 25 miles in breadth. It is reasonable to
wonder whether, had Forbes known of this and similar modern instances
of the capabilities of plants as travellers, he would have adopted the
view he did. In this connexion it may be added that in recent years the
glaciation of Ireland has been shown to be more extensive than it was
believed to be when Forbes wrote his essay.
There would seem to be no insuperable objection to the conclusion
that the Mediterranean plants in Ireland and in the south of England
reached their present home after the retreat of the ice at the end
of the Glacial period, and after Ireland became an island. A full
consideration of the problem is beyond the scope of this book, but
I have briefly stated the case, not with the authority of an expert
but in order to draw attention to a particularly fascinating study in
plant-migration.
[Illustration: Fig. 2. _Eriocaulon septangulare_ With. West Connemara.
(Photograph by Mr W. R. Welch.) ]
In a volume by W. Canton entitled _A Child's book of Saints_ a story is
told in which the presence in Ireland of Mediterranean species receives
a more picturesque explanation. The Monk Bresal was sent to teach the
brethren in a Spanish monastery the music of Irish choirs. In later
years Bresal longed for a sight of his native land, though he loved his
home and 'every rock, tree, and flower' in his adopted country. After
returning to Ireland, his thoughts reverted to Spain; 'it appeared
to him as though he was once again in a granite nook among the rocks
beside the Priory'; he saw the ice-plant with its little stars of white
flowers sprinkled with red (the London Pride) and a small evergreen
tree from which he had often gathered the orange-scarlet berries
(Arbutus). The Prior of the Spanish monastery 'with heavenly vision
saw Bresal gazing at the evergreen tree and the ice-plant, and turning
to the trees blessed them and commended them to go and make real his
dream. As Bresal brushed away his tears he saw with amazement at his
feet the ice-plant and hard by the evergreen tree.'
The plant represented in Fig. 2 is another British species which tasks
the ingenuity of students of plant-geography. This is the Pipe Wort
(_Eriocaulon septangulare_), the sole representative in Europe of a
certain family of Monocotyledons: it flourishes in the west of Ireland
and in the western islands of Scotland but nowhere else in Europe; it
is native on the other side of the Atlantic in Canada and the northern
United States of America. Mr Praeger in describing the striking mixture
of species in the west of Ireland writes, 'The pool from which we
gather the American Pipe Wort is fringed with Pyrenean Heathers. The
cracks which are filled with the delicate green foam of the maiden hair
are set in Bearberry and Spring Gentian; _Habenaria intacta_, far from
its Mediterranean home, sends up its flower-spikes through carpets of
mountain Avens; and St Dabeoc's Heath and the dwarf Juniper straggle
together over the rocky knolls'(25).
The presence of Eriocaulon on the western edge of Europe may be
attributed to migration in pre-Glacial days from North America by way
of a land-connexion, of which Greenland and Iceland represent surviving
portions. The opinion held by Forbes, and advocated by some later
naturalists, that the southern companions of Eriocaulon in the west of
Ireland are survivors from a Tertiary flora which have lived through
the Ice Age, is consistently extended to the Pipe Wort. On the other
hand, before yielding to the temptation to regard these American and
Mediterranean species as links with the Tertiary period, we must be
convinced that the possibilities of post-Glacial introduction, even
without the aid of land-bridges, have been exhausted. The Pipe Wort is
a botanical puzzle which affords a good example of the accession of
interest to field-botany effected by a knowledge of the distribution
of the component members of the British flora. The problem of its past
history suggests an experimental enquiry into the adaptability of its
seeds to dispersal, and emphasises the importance of the co-operation
of botanists and geologists in a common endeavour to trace the origin
of British plants.
In addition to the Pipe Wort, mention may be made of three other
American flowering plants recognised in the Irish flora. _Sisyrinchium
angustifolium_ recorded from the west of Ireland is native in temperate
North America; the orchid, _Spiranthes romanzoffiana_, met with in the
south and north of Ireland, is widely spread in Canada and the northern
States, while _Sisyrinchium californicum_, a native of California
and Oregon, was discovered by Mr Marshall in marshy meadow-land near
Wexford(26). In the case of the more recently discovered American
immigrants, the possibility of human introduction must be borne in
mind, though there are no special reasons for doubting that some, as in
the case of Eriocaulon, reached the Irish coast by natural agencies.