THE EVOLUTION OF PLANT CLASSIFICATION
In the earliest European works on natural history—those of the
Aristotelian school—we meet with an attempt to classify the different
varieties of plants. It was inevitable that the writers of this school
should make such an attempt, since no mind trained in Greek philosophy
could be content to leave a science in the condition of a mere chaos
of isolated descriptions. At first the most obvious distinction, that
of size, was used as the chief criterion whereby to separate the
different groups of the vegetable kingdom. In the ‘History of Plants’
of Theophrastus, we find Trees, Shrubs, Bushes and Herbs treated
as definite classes, within which, cultivated and wild plants are
distinguished. Other distinctions of lower value are made between
evergreen and deciduous, fruiting and fruitless, and flowering and
flowerless plants.
Albertus Magnus, who kept alive in the Middle Ages the spirit of
Aristotelian botany, was more advanced than Theophrastus in his method
of classification. It is true that he divides the vegetable world into
Trees, Shrubs, Undershrubs, Bushes, Herbs and Fungi, but at the same
time he points out that this is an arbitrary scheme, since these groups
cannot always be distinguished from one another, and also because the
same plant may belong to different classes at different periods of its
life. A study of the writings of Albertus reveals the fact that he
had in mind, though he did not clearly state it, a much more highly
evolved system, which may be diagrammatically represented as follows.
The modern equivalents of his different groups are shown in square
brackets:—
[Illustration: Text-fig. 63. “Carui” [Ortus Sanitatis, Mainz, 1491].]
I. Leafless plants [Cryptogams in part].
II. Leafy plants [Phanerogams and certain Cryptogams].
1. Corticate plants [Monocotyledons].
2. Tunicate plants [Dicotyledons].
(_a_) Herbaceous.
(_b_) Woody.
The word _tunicate_ in the above table is used for the plants which
Albertus describes as growing “ex ligneis tunicis.” It seems clear
from this expression that he realised that there was an anatomical
distinction between Dicotyledons and Monocotyledons.
Considering how much Albertus had achieved, it is somewhat curious
that Cesalpino, who represented Aristotelian botany in the sixteenth,
as Albertus did in the thirteenth century, should have produced so
inadequate a system as his own contribution to the subject. We owe to
him one marked advance, the recognition, namely, of the importance of
the seed. On the whole, however, his classification savours too much
of having been thought out in the study, and it suffers by comparison
with other systems of about the same period, such as those of de l’Obel
and Bauhin, which were arrived at rather by instinct, acting upon
observation, than by a definite and self-conscious intellectual effort.
Cesalpino makes his main distinction, on the old Aristotelian plan,
between Trees and Shrubs on the one hand, and Undershrubs and Herbs
on the other. He divides the first of these groups into two, and the
second into thirteen classes, depending chiefly on seed and fruit
characters. Very few of these classes really represent natural groups,
and the chief of all distinctions among Flowering Plants, that between
Dicotyledons and Monocotyledons, which was foreshadowed by Albertus, is
almost lost to sight.
When we turn from the botanical philosophers to the herbalists proper,
we find an altogether different state of affairs. The Aristotelian
botanists were conscious, from the beginning, of the philosophic
necessity for some form of classification. The medical botanists, on
the other hand, were only interested in plants as individuals, and were
driven to classify them merely because some sort of arrangement was
necessary for convenience in dealing with a large number of kinds. The
first Materia Medica, that of Dioscorides, shows some attempt at order,
but the arrangement is seldom at all natural. Occasionally the author
groups together plants which are nearly related, as when he treats of a
number of Labiates, or of Umbellifers successively—but this is rare.
[Illustration: Text-fig. 64. “Buglossa” [Ortus Sanitatis, Mainz, 1491].]
Pliny was not, strictly speaking, a medical botanist, but at the
same time he may be mentioned in this connection, since his interest
in plants was essentially utilitarian. Like Theophrastus, he begins
his account of plants with the trees, but his reason for so doing is
profoundly different from that of the Greek writer, and illustrates
the divergence between what we may call the anthropocentric and the
scientific outlook upon the plant world. Theophrastus placed trees
at the head of the vegetable kingdom, because he considered their
organisation the highest, and most completely expressive of plant
nature; Pliny, on the other hand, began with trees because of their
great value and importance to man. As an example of his ideas of
arrangement, we may mention that he places the Myrtle and Laurel side
by side, because the Laurel takes a corresponding place in triumphs to
that accorded to the Myrtle in ovations!
Turning to the herbals themselves, we find that the earliest show no
trace of a natural grouping, the plants being, as a rule, arranged
alphabetically. This is the case, for instance, in the Latin and German
Herbarius, the Ortus Sanitatis and their derivatives, and even in the
herbals of Brunfels and of Fuchs in the sixteenth century. In Bock’s
herbal, on the other hand, the plants are grouped as herbs, shrubs and
trees, according to the classical scheme. The author evidently made
some effort, within these classes, to arrange them according to their
relationships. In the preface to the third edition he writes—“I have
placed together, yet kept distinct, all plants which are related and
connected, or otherwise resemble one another and are compared, and have
given up the former old rule or arrangement according to the A.B.C.
which is seen in the old herbals. For the arrangement of plants by the
A.B.C. occasions much disparity and error.”
Although the larger classificatory divisions, as now understood,
were not recognised by these early workers, they had at least a dim
understanding of the distinction between genera and species. This
dates back to Theophrastus, who showed, by grouping together different
species of oaks, figs, etc., that he had some conception of a genus.
We owe to Konrad Gesner the first formulation of the idea that genera
should be denoted by substantive names. He was probably the earliest
botanist who clearly expounded the distinction between a genus and a
species. In one of his letters he writes—“And we may hold this for
certain, that there are scarcely any plants that constitute a genus
which may not be divided into two or more species. The ancients
describe one species of Gentian; I know of ten or more.”
Very little of Gesner’s botanical work was ever published, and it
was left to Fabio Colonna to put before the botanical world the true
nature of genera. He held most enlightened views on the subject, and,
in 1616, clearly stated in his ‘Ekphrasis’ that genera should not be
based on similarities of leaf form, since the affinities of plants are
indicated not by the leaf, but by the characters of the flower, the
receptacle, and, especially, the seed[29]. He brought forward instances
to show that previous authors had sometimes placed a plant in the
wrong genus, because they only attended to the leaves and ignored the
structure of the flower.
[Illustration: Text-fig. 65. “Nenufar” = Waterlily [Arnaldus de Villa
Nova, Tractatus de virtutibus herbarum, 1499].]
In the writings of Gaspard Bauhin, at the end of the sixteenth and the
beginning of the seventeenth century, the binary system of nomenclature
is used with a high degree of consistency, each species bearing a
generic and specific name, though sometimes a third, or even a fourth,
descriptive word is added. These extra words are not, however, really
essential. In the preface to the ‘Phytopinax’ (1596) Bauhin states
that, for the sake of clearness, he has applied one name to each plant
and added also some easily recognisable character[30].
The binomial method was foreshadowed at a very early date, for in a
fifteenth-century manuscript of the old herbal ‘Circa instans,’ to
which we have referred on p. 24, this system prevails to a remarkable
extent.
When we turn to those general schemes of classification which were
evolved by the herbalists of the sixteenth century, we are at once
struck by the great difference existing between the principles on which
these schemes are based, and those at which we have arrived at the
present day. To classify plants according to their uses and medicinal
properties is obviously the first suggestion that arises, when the
universe is regarded from a simple, anthropocentric standpoint. In
the Grete Herball of 1526 we get a ludicrously clear example of
this method, applied to the special case of the Fungi. “Fungi ben
mussherons.... There be two maners of them, one maner is deedly and
sleeth [slayeth] them that eateth of them and be called tode stoles,
and the other dooth not.” This account of the Fungi occurs also in
the earlier manuscript herbal, ‘Circa instans,’ mentioned in the last
paragraph.
This theory of classification has been shown in more recent times to
contain the germ of something more nearly approaching a natural system
than one would imagine at first sight. Both Linnæus and de Jussieu
have pointed out that related plants have similar properties, and, in
1804, A. P. de Candolle, in his ‘Essai sur les propriétés médicales des
Plantes, comparées avec leurs formes extérieures et leur classification
naturelle,’ carried the argument much further. He showed that in no
less than twenty-one families of flowering plants, the same medicinal
properties were found throughout all the members of the order. This is
very remarkable, when we remember that the state of knowledge at that
time was such that de Candolle was obliged to dismiss a large number
of orders with the words “properties unknown.” Quite recently the
subject of the differentiation of groups of plants according to their
chemistry has again come to the fore, and, in the future, chemical
characters will probably be numbered among the recognised criteria for
use in elaborating schemes of classification.
[Illustration: Text-fig. 66. “Nenuphar” = _Nymphæa alba_ L., White
Waterlily [Brunfels, Herbarum vivæ eicones, Vol. I. 1530]. _Reduced._]
[Illustration: Text-fig. 67. “Gele Plompen” = _Nuphar luteum_ Sm.,
Yellow Waterlily [de l’Obel, Kruydtbœck, 1581].]
In the history of botanical classification, the first advance from the
purely utilitarian standpoint was marked by the recognition of the
fact that the structure and mode of life of the plants themselves are
of importance. In the work of writers such as Dodoens and d’Aléchamps,
to take two typical examples, we find the issues curiously confused
by the working of three different principles side by side; that is
to say, by the simultaneous insistence (i) on the habitat, (ii) on
the “virtues,” and (iii) on the structure, as affording clues to the
systematic position of the plant in question. The herbalist thus erects
his scheme on a basis consisting of a confused medley of ecological,
medical, and morphological principles. An enumeration of the eighteen
headings, under which d’Aléchamps, in 1586, described the vegetable
kingdom, so far as it was then known, will show the perplexities which
surrounded the first gropings after a natural system. His headings may
be translated as follows:—
I. Of trees which grow wild in woods.
II. Of fruits growing wild in thickets and shrubberies.
III. Of trees which are cultivated in pleasure gardens and orchards.
IV. Of cereals and pulse, and the plants which grow in the field
with them.
V. Of garden herbs and pot herbs.
VI. Of umbelliferous plants.
VII. Of plants with beautiful flowers.
VIII. Of fragrant plants.
IX. Of plants growing in marshes.
X. Of plants growing in rough, rocky, sandy and sunny places.
XI. Of plants growing in shady, wet, marshy and fertile places.
XII. Of plants growing by the sea, and in the sea itself.
XIII. Of climbing plants.
XIV. Of thistles and all spiny and prickly plants.
XV. Of plants with bulbs, and succulent and knotty roots.
XVI. Of cathartic plants.
XVII. Of poisonous plants.
XVIII. Of foreign plants.
Among these eighteen groups, the only ones which have any pretension to
being natural are VI (Umbellifers) and XIV (Thistles), and these merely
approximate roughly to related groups of genera. Among the Umbellifers
we meet with _Achillea_ and other genera which do not really belong to
the order, whilst, with the Thistles, there are grouped other spiny
plants, such as _Astragalus tragacantha_, which, in a natural system,
would occupy a place remote from the Composites.
[Illustration: Text-fig. 68. “Ninfea” = Waterlily [Durante, Herbario
Nuovo, 1585].]
In spite of the fact that improved systems of classification, to which
we shall shortly refer, were put forward in the sixteenth and early
seventeenth centuries, we find that, as late as 1640, John Parkinson in
his well known herbal, divided all the plants then known into seventeen
classes or tribes—the sequence in which these classes were placed
having, in most cases, no meaning at all. A few of his tribes are
natural, but many are valueless as an expression of affinities. As an
example we may mention his third class, “Venemous, Sleepy, and Hurtfull
Plants, and their Counterpoysons,” and his seventeenth, “Strange and
Outlandish Plants.” In Parkinson’s classification, we see Botany
reverting once more to the position of a mere hand-maid to Medicine.
In the first book of Dodoens’ ‘Pemptades’ (1583) the principles
of botany are discussed. The old Aristotelian classification into
Trees, Shrubs, Undershrubs and Herbs is accepted, but with some
reservations. The author points out that an individual plant may, owing
to cultivation, or from some other cause, pass from one class into
another. He instances _Ricinus_, which is an herbaceous annual with us,
but a tree in other countries[31].
The general scheme of classification, which Dodoens propounded,
has much in common with that of d’Aléchamps, which we have already
outlined. Within the larger groups, he shows a stronger perception of
natural grouping than appears in his arrangement of the larger classes
themselves. He often grouped together genera which we now regard as
members of the same natural order, and species which we now look upon
as belonging to a single genus. For instance he brought together genera
belonging respectively to the Geraniaceæ, Hypericaceæ, Plantaginaceæ,
Cruciferæ, Compositæ, etc. In some cases, however, he was only
partially successful, as in the Umbelliferæ, among which he described
_Nigella_ (Love-in-a-Mist) and a couple of Saxifrages. This example
shows how little stress was laid on the flowers and fruit at this time,
from the point of view of classification. The general habit, and the
shape of the leaves were the features that received most attention.
Resemblances and differences between the forms of the leaves alone
must naturally appear to the botanist of the present day to be a very
inadequate basis for a general system of classification. Nevertheless
Mathias de l’Obel worked out a scheme on these lines which had great
merit, and was a considerable advance on previous efforts. He put
forward his system in his ‘Stirpium adversaria’ (1570-71) and used it
also in his later work. It was thus published much earlier than the
very primitive schemes of d’Aléchamps and Dodoens to which we have
just referred. The best point of his system is that, by reason of
their characteristic differences of leaf structure, he distinguishes
the classes now known to us as Monocotyledons and Dicotyledons. He
introduces a useful feature in the shape of a synoptic table of
species which precedes each more or less natural group of plants. The
superiority of his classification to the other arrangements in the
field at the time was immediately realised. We have evidence of this in
the fact that, after his ‘Kruydtbœck’ was published, Plantin brought
out an album of the wood-engravings used in the book, which, although
they had also appeared as illustrations to the works of Dodoens and de
l’Écluse, were now arranged as in the scheme put forward by de l’Obel,
“according to their genus and mutual relationship[32].”
There seems little doubt that de l’Obel made a more conscious effort
than any of his predecessors to arrive at a natural classification, and
that he realised that such a classification would reveal a unity in
all living beings. In the preface to his ‘Stirpium adversaria nova’ of
1570 he writes—“For thus in an order, than which nothing more beautiful
exists in the heavens or in the mind of a wise man, things which are
far and widely different become, as it were, one thing.”
De l’Obel’s scheme is not expressed in the clear manner to which we
have become accustomed in more modern systems, because, in common
with other botanists of his time, he did not, as a rule, give names
to the groups which we now call _orders_, or draw any sharp line of
distinction between them.
De l’Obel’s arrangement, in spite of its good features, had serious
drawbacks. The anomalous Monocotyledons, such as _Arum_, _Tamus_,
_Aloe_ and _Ruscus_, are scattered among the Dicotyledons, while
_Drosera_ (the Sundew) appears among the Ferns, and so on. Similarities
of leaf form, which are now regarded merely as instances of
“homoplastic convergence,” are responsible for many curious groupings.
For instance in the ‘Kruydtbœck’ we find the Twayblade (_Listera_)
the May Lily (_Maianthemum_) and the Plantain (_Plantago_) described
in succession, while, in another part of the book, various Clovers
(_Trifolium_), Wood Sorrel (_Oxalis_) and _Anemone hepatica_ are
grouped together. It is also not surprising that the Marsh Marigold
(_Caltha_), the Waterlilies (_Nymphæa_ and _Nuphar_), _Limnanthemum_
and Frogbit (_Hydrocharis_) should follow one another, or that de
l’Obel should have brought together the Broomrape (_Orobanche_), the
Toothwort (_Lathræa_), the Bird’s-nest Orchid (_Neottia_) and a number
of Fungi. In these latter instances the author has really arrived
at genuine biological (though not morphological) groups. He has
recognised, on the one hand, the marked uniformity of the type of leaf
characteristic of “swimming” water-plants, and, on the other hand, he
has observed the leaflessness and absence of green colour, which are
negative features common to so many saprophytes and parasites.
[Illustration: Text-fig. 69. “Tussilago” = _Tussilago farfara_ L.,
Coltsfoot [Fuchs, De historia stirpium, 1542]. _Reduced._]
The perception of natural affinities among plants which, in the
sixteenth and seventeenth centuries, was gradually, in a dim,
instinctive fashion, arising in men’s minds, is perhaps best expressed
in the work of Gaspard Bauhin, especially in his ‘Pinax theatri
botanici’ (1623). This work is divided into twelve books, each book
being further sub-divided into sections, comprehending a variable
number of genera. Neither the books nor the sections have, as a rule,
any general heading, but there are certain exceptions. For instance,
Book II is called ‘de Bulbosis,’ and a section of Book IV, including
eighteen genera, is headed ‘Umbelliferæ.’ Some of the sections
represent truly natural groups. Book III, Section VI, for example,
consists of ten genera of Compositæ, while Book III, Section II
includes six Crucifers. Other sections contain plants of more than one
family, but yet show a distinct feeling for relationship. For instance,
Book V, Section I includes _Solanum_, _Mandragora_, _Hyoscyamus_,
_Nicotiana_, _Papaver_, _Hypecoum_ and _Argemone_—that is to say four
genera from the Solanaceæ followed by three from the Papaveraceæ. The
common character which brings them together here is, no doubt, their
narcotic property, but, although no definite line was drawn between the
plants belonging to these two widely sundered families, the order in
which they are described shows that their distinctness was recognised.
Some of Bauhin’s other groups, however, which, like that just
discussed, are distinguished by their properties, or, in other words,
by their chemical features, have no pretension to naturalness from a
morphological standpoint. This is the case with the group described in
Book XI, Section III under the name of “Aromata,” which consists of a
heterogeneous assemblage of genera belonging to different orders, which
are only connected by the fact that they all yield spices useful to man.
[Illustration: Text-fig. 70. “Plantago major” = Plantain [Fuchs, De
historia stirpium, 1542]. _Reduced._]
[Illustration: Text-fig. 71. “Althæa Thuringica” = _Lavatera
thuringica_ L. [Camerarius, Hortus medicus, 1588].]
There is no doubt that, on the whole, Bauhin was markedly successful
in recognising affinities within small cycles, but he broke down on
the broader question of the relationships between the groups of genera
so constituted. This is, however, hardly surprising when we remember
how much difference of opinion exists among systematic botanists, even
to-day, upon the subject of the relations of the orders to one another.
Like de l’Obel, Bauhin seems to have believed in the general principle
of a progression from the simpler to the more highly developed forms.
His application of this principle led him to begin with the Grasses
and to conclude with the Trees. The question as to which groups among
the Flowering Plants [Angiosperms] are to be considered as relatively
primitive, is still, at the present day, an open one, but it would be
generally conceded that Bauhin’s arrangement cannot be accepted. There
is little doubt, from the standpoint of modern botany, that the Grasses
are a highly specialised group, while the “tree habit” has been adopted
independently by many plants belonging to entirely different cycles
of affinity, and thus, except in rare cases, it cannot be used as a
criterion of relationship.
On the subject of the relations of the Cryptogams (flowerless plants)
to the Phanerogams (flowering plants), Bauhin had evidently no clear
ideas, but such could hardly be hoped for in the state of knowledge of
that time. We find, for instance, the Ferns, Mosses, Corals(!), Fungi,
Algæ, the Sundew, etc., sandwiched between some Leguminosæ, and a
section consisting chiefly of Thistles.
The classification put forward by the Bohemian botanist, Zaluziansky,
in 1592, although in its general features no better than that of
Dodoens, or of d’Aléchamps, and certainly less satisfactory than
that of de l’Obel or the later scheme of Bauhin, is an improvement
on all of these in one particular, namely, that he begins with the
Fungi and deals next with Mosses. After the Mosses he describes the
Grasses, and his classification concludes with the Trees. He was thus
evidently attempting to pass from the simpler to the more complex,
and his arrangement indicates that, unlike certain other botanists of
his time, he looked upon the Lower Cryptogams as comparatively simple
and primitive plants. He was not so clear-sighted, however, on the
subject of the Ferns, for he placed them with the Umbelliferæ and some
Compositæ, no doubt because he was influenced by the form of the leaf.
[Illustration: Text-fig. 72. “Pulsatilla” = _Anemone pulsatilla_ L.,
Pasque-flower [Camerarius, De plantis Epitome ... Matthioli, 1586].]
It is curious that Cesalpino, who, as we have pointed out, had arrived
at the very important principle that the seed and fruit characters
were of major value in classification, yet put forward a system which
was distinctly inferior to that of Gaspard Bauhin, although the
latter appears to have been guided by no such general principles.
Probably the reason for this is to be sought in the fact that no
system of classification can represent natural affinities, unless it
takes into account the nature of the plant as a whole. It is true
that, compared with the characters of the reproductive organs, the
leaf-form and habit, owing to their plasticity, have to be used with
great discretion as systematic criteria, but, nevertheless, no system
of classification can afford to ignore them entirely. Cesalpino based
his scheme too exclusively upon seed characters, to the neglect even
of the structure of the flower, and, curiously enough, although he
laid so much stress upon the nature of the seed, he did not grasp the
fundamental distinction between the embryos of the Monocotyledons and
the Dicotyledons, due to the possession of one, and two seed-leaves
respectively. The chief drawback of his scheme, however, was his
failure to realise that living organisms are too complex to fall into a
classification based on any one feature, important as that feature may
prove to be when used in conjunction with other characters.
Those herbalists, on the other hand, who attacked the problem of the
classification of plants without any preconceived, academic theory,
depended, one might almost say, on the glimmerings of common sense
for the recognition of affinities. This was no doubt a dim and fitful
illumination, but it was at least less partial than the narrow,
lime-light beam of a rigid theory.