It has been just stated, that we have derived from botanical science a
philosophical language which enables us to describe the structure and
habits of any plant, with a luminous brevity and an unerring
perspicuity; but we are moreover indebted to botany for another service
no less important to the successful investigation of the Materia
Medica,—that of throwing into well defined groups, those plants which
possess obvious natural affinities, and which will be found at the same
time to present certain medicinal analogies; indeed, as a general rule,
we may admit the axiom, “_Quæ genere conveniunt, virtute
conveniunt_.”[85]
The _Umbelliferæ_ which grow on dry ground are aromatic, whilst the
aquatic species are among the most deadly poisons. The _Cruciform_
plants are aromatic and acrid in their nature, containing essential
oils, (hence the peculiar smell of cabbage-water, &c.) which are
obtainable by distillation; and Linnæus asserts that “among all the
_Leguminous_ or _Papilionaceous_ tribe there is no deleterious plant to
be found:” this however is not exactly true. Some of the individuals in
these natural orders, although very nearly related, do nevertheless
possess various, and even opposite qualities; in the leguminous tribe
above mentioned, which is as consistent as any one we possess, we have
the _Cytisus Laburnum_, the seeds of which are violently emetic, and
those of _Lathyrus Sativus_, which have been supposed at Florence to
soften the bones and cause death.
In the subdivision of a genus there is often a remarkable difference in
the properties of the species; there are, for instance, _Solanums_,
_Lettuces_, _Cucumbers_, and _Mushrooms_, both esculent and poisonous.
The _Digitalis_ or _Foxglove_, and the _Verbascum_, or common _Mullein_
of our fields, are included in the same Natural family, and yet the one
is as active, as the other is mild in its effects; the plants of the
natural family of _Contortæ_ abound with a highly acrid milky juice, but
Dr. Afzelius met with a shrub of this order at Sierra Leone, the milk of
whose fruit was so sweet, as well as copious, as to be used instead of
cream for tea; this is certainly what no one could have guessed from
analogy. The same individual will vary from culture or other
circumstances, as much as any two plants which have no botanic affinity;
the Chamomile, _Anthemis Nobilis_, with which we are well acquainted,
may have its whole disk changed by cultivation, to ligulate white
florets, destitute of medicinal properties. But, what is more
embarrassing, the different parts of the same plant have often very
different powers; a fact which is beautifully exemplified in the
_Podophyllum Peltatum_, or May Apple, the _leaves_ of which are
poisonous, the root powerfully cathartic, and the _fruit_ agreeably
esculent; so the leaves of the _Jatropa Manihot_ are employed as a
common esculent, while its root secretes a most virulent poison; but we
need not seek further for an example than the fruit of the Lemon, the
juice of which is _acid_, its seeds _bitter_, and its _rind aromatic_;
in some instances it happens that the energy of a plant is concentrated
in one particular part, and that all the rest is absolutely inert; thus,
the root of the _Convolvulus Scammonia_, is the only portion of that
plant which possesses any medicinal quality;[86] and the tree which
yields the drastic _Camboge_, presents at the same time an esculent
fruit, which is eaten by the natives with as much impunity as the
orange; yet, notwithstanding all these difficulties, botany is capable
of furnishing us with analogies which will lead to important conclusions
with respect to the medicinal properties of different vegetables.
The system of Linnæus, although in a great degree artificial,
corresponds in a surprising manner with the natural properties of
plants; thus a plant whose _calyx_ is a double valved _glume_, with
three _stamina_, two _pistils_, and one _naked seed_, bears seeds of a
farinaceous and nutritious quality; a flower with twelve, or more
_stamina_, all of which are inserted in the internal side of the
_calyx_, will furnish a wholesome fruit; whereas a plant whose flower
has five _stamina_, one _pistil_, one _petal_, and whose fruit is of the
_berry_ kind, may at once be pronounced as poisonous.
It is also in a great degree true that the sensible qualities of plants,
such as _colour_, _taste_, and _smell_, have an intimate relation to
their properties, and may often lead by analogy to an indication of
their powers; we have an example of this in the dark and gloomy aspect
of the _Luridæ_, which is indicative of their narcotic and very
dangerous qualities, as _Datura_, _Hyoscyamus_, _Atropa_, and
_Nicotiana_. _Colour_ is certainly in many cases a test of activity; the
deepest coloured flowers of the _Digitalis_, for example, are the most
active, and when the leaves of powerful plants lose their green hue, we
may conclude that a corresponding deterioration has taken place with
respect to their virtues; but Linnæus ascribed too much importance to
such an indication, and his aphorisms are unsupported by facts; for
instance, he says “Color pallidus _insipidum_, viridis _crudum_, luteus
_amarum_, ruber _acidum_, albus _dulce_, niger _ingratum_, indicat.”[87]
A peculiar heavy odour, which is well known, but is with difficulty
defined, is a sure indication of narcotic properties. Bitterness, when
not extreme, denotes a tonic quality, which will stimulate the stomach
and intestines, and promote the process of digestion. When the
bitterness is more intense and pungent,[88] as in _Aloes_, _Colocynth_,
_&c._ we may infer that such substances will produce a more active
effect upon the _primæ viæ_, and that catharsis will follow their
administration.
Botanical, like human physiognomy, may frequently afford an insight into
character, but it is very often a fallacious index. With regard to the
indications of _Smell_ and _Taste_, it may be observed that in the
examination of an unknown substance we instinctively apply to these
senses for information respecting its properties. It is certainly
reasonable to suppose, that those bodies which produce upon the organs
of taste a sensible, astringent, or pungent effect, may occasion an
impression, corresponding in degree upon the stomach or intestines,
which are but an extension of the same structure. But what numerous
exceptions are there to such a law? nay, some of the most poisonous
substances affect in a very slight degree the organs of taste,
especially those that belong to the mineral kingdom, as _Arsenious
Acid_, _Oxyd of Antimony_, _Calomel_, _&c._; yet some of these are,
perhaps, but apparent exceptions, depending upon the degree of
solubility which they possess, in consequence of which their energies
are not developed until they have traversed a considerable portion of
mucous surface. Nor ought it to be forgotten, that cultivation and
artificial habits may have blunted the natural susceptibility of our
organs, and in some instances changed and depraved their functions:
certain qualities for instance are so strongly connected with each other
by the chain of association, that by presenting only one to the mind,
the other links follow in succession.[89] It has been remarked, that
persons in social life, are more affected by vegetable odours, while the
Savage smells better the putrid and fœtid exhalations of animal
bodies:[90] thus the people of Kamskatcha, did not smell the perfume of
a vegetable Essence (_Aqua Melissæ_,) but they discovered by their
olfactory sense, a rotten fish, or a stranded whale at a considerable
distance.[91] There is no sense more under the dominion of imagination,
or more liable to be perverted by education, than those of taste and
smell; we are also liable to form unjust prejudices from the indications
of colour; for particular colours, from the influence of hidden
associations, are not unfrequently the exciting cause of agreeable or
unpleasant impressions. I have met with a person who regards green food,
if it be of an animal nature, with unconquerable aversion and disgust,
indeed an idea of unwholesomeness has not unfrequently been attached to
this colour, without the least foundation of truth; the bones of the Gar
fish, or Sea Needle, (_Esox Helone_,) have been deemed unwholesome from
the circumstance of their turning green on being boiled, although not a
single instance can be adduced in which that fish ever occasioned any
harm. I have met with persons who have been made violently sick from
eating the green part of the oyster;[92] an effect which can have no
other cause than that of unjust prejudice; these examples are sufficient
to shew, with what caution such indications respecting the medicinal
qualities of bodies are to be received.