Why Certain Plants Are Acrid
Professor William B. Lazenby
EVER since my first lessons in botany, the characteristic
qualities and properties of plants have given me much thought. Why
certain plants produced aromatic oils and ethers, while others
growing under the same conditions produced special acids or
alkaloids, was a subject of endless speculation.
The pleasing aroma of the bark of various trees and shrubs, the
spicy qualities of the foliage and seeds of other plants; the
intense acridity; the bitterness; the narcotic, the poisonous
principle in woody and herbaceous species; all were intensely
This interest was biological rather than chemical. I cared less
for the ultimate composition of the oils, acids, alkalis, etc.,
than I did for their use or office in the plant economy, and their
effect upon those who might use them.
Perhaps no one plant interested me more from this point of view,
than the well-known Indian turnip (Arisoema triphyllum). As a boy I
was well acquainted with the signally acrid quality of this plant;
I was well aware of its effect when chewed, yet I was irresistibly
drawn to taste it again and again. It was ever a painful
experience, and I suffered the full penalty of my rashness. As an
awn from a bearded head of barley will win its disputed way up
one's sleeve, and gain a point in advance despite all effort to
stop or expel it, so did every resolution, every reflection,
counteract the very purpose it was summoned to oppose, and to my
sorrow I would taste the drastic, turnip-shaped corm wherever
It is a well-known fact that the liquid content of the cells of
plants contain numerous inorganic substances in solution. Among
these, not considering oxygen, hydrogen, nitrogen and carbon
dioxide, there are the salts of calcium, magnesium, potassium,
iron, sulphur and phosphorus. The above substances are found in the
cells of every living plant. Other substances like salts of sodium
and silica are also found, but these are not regarded as essential
to the life and growth of plants. They appear to be present because
the plant has not the power to reject them. Many of the substances
named above, are found deposited either in an amorphous or
crystalline form in the substance of the cell wall. In addition to
this, crystals of mineral matter, having various shapes and sizes,
are often found in the interior of cells. The most common of these
interior cell crystals are those composed of calcium oxalate and
calcium carbonate. Others composed of calcium phosphate, calcium
sulphate and silica are sometimes found. These crystals may occur
singly or in clusters of greater or less size. In shape they are
prismatic or needle-like.
It is not the object of this paper to treat of plant crystals in
general, but to consider the peculiar effect produced by certain
forms when found in some well-known plants.
The extreme acridity or intense pungency of the bulbs, stems,
leaves and fruit of various species of the Araceae or Arum family,
was recognized centuries ago. The cause of this characteristic
property or quality was, until a comparatively recent date, not
As far as I am aware the first scientific investigation of this
subject was made by the writer. At a meeting of the American
Association for the Advancement of Science held at Indianapolis in
1890, some studies and experiments were reported in a short paper
entitled "Notes upon the Crystals in certain species of the Arum
This paper expressed the belief that the acridity of the Indian
turnip and other plants belonging to the same family, was due to
the presence of needle-shaped crystals or raphides found in the
cells of these plants. This conclusion was not accepted by
Professor T. J. Burrill, of the University of Illinois, nor by
other eminent botanists who were present and took part in the
discussion that followed the reading of the paper.
The opposition was based mainly on the well-known fact that many
other plants like the grape, rhubarb, fuchsia, spiderwort, etc.,
are not at all, or but slightly acrid, although the raphides are as
abundant in them as in the Indian turnip and its allies.
Up to this time the United States Dispensatory and other works
on pharmacy, ascribed the following rather indefinite cause for the
acridity of the Indian turnip. It was said to be due to an acrid,
extremely volatile principle. This principle was insoluble in water
and alcohol, but soluble in ether. It was dissipated both by
heating and drying, and by this means the acridity is destroyed.
There was no opinion given as to the real nature of this so-called
More recently it has been intimated that the acridity may be due
to some ferment or enzyme, which has been derived in part from the
self-decomposition of protoplasm and in part by the process of
oxidation and reduction.
Here the question appeared to rest. At all events I was unable
to glean any further knowledge from the sources at my command.
Some time later the subject was taken up in a more comprehensive
manner and the following report is the first detailed description
of an investigation that has occupied more or less of my leisure
for some years.
A dozen or more species of plants have been used for examination
and study. Among these were:
Indian turnip (Arisoema triphyllum).
Green dragon (Arisoema dracontium).
Skunk cabbage (Spathyema).
Wandering Jew (Tradescantia).
Most of the plants selected were known to have crystals in
certain parts. Some of them were known to be intensely acrid. In
these the acridity was in every instance proportional to the number
The following order of study was pursued and the results of each
step noted. Only the more salient points of the methods employed
and the conclusions reached are presented.
1. The Character of the Taste Itself.—It was readily noted that
the sensation produced by chewing the various acrid plants was
quite different. For example, the Indian turnip and its close
allies do not give the immediate taste or effect that follows a
similar testing of the onion or horse-radish. When the acridity of
the former is perceived the sensation is more prickling than
The effect produced is more like the pricking of numerous
needles. It is felt not only upon the tongue and palate, but
wherever the part tasted comes into contact with the lips, roof of
mouth or any delicate membrane. It is not perceived where this
contact does not occur.
The acridity of the onion and horse-radish is perceived at once
and often affects other parts than those with which it comes into
2. The Acrid Principle Is Not Always Volatile.—This is shown by
the fact that large quantities of the mashed or finely grated corms
of the Indian turnip and allied species, produced no irritation of
the eyes or nose even when these organs were brought into close
contact with the freshly pulverized material. This certainly is in
marked contrast with the effect produced by freshly grated
horse-radish, peeled onions, crushed mustard seed when the same
test is applied.
It seems fair to assume that in the latter case some principle
that is volatile at ordinary air temperatures is present. The
assumption that such principle is present in the former has no
In order to test this matter further a considerable quantity of
the juice of the Indian turnip was subjected to careful
distillation, with the result that no volatile principle or
substance of any kind was found.
Various extractive processes were tried by using hot and cold
water; alcohol, chloroform, benzene, etc. These failed in every
instance to remove any substance that had a taste or effect
anything like that found in the fresh Indian turnip.
3. The Acrid Principle Is Not Soluble in Ether.—Inasmuch as
various works on pharmacy made the claim that the active or acrid
principle of the plants in question was soluble in ether, this was
the next subject for investigation. The juice was expressed from a
considerable quantity of the mashed Indian turnip. This juice was
clear and by test was found to possess the same acrid property as
the unmashed corms.
Some of the juice and an equal quantity of ether were placed
into a cylinder and well shaken. After waiting until the ether had
separated a few drops of the liquid were put into the mouth. For a
little time no result was perceived, but as soon as the effect of
the ether had passed away the same painful acridity was manifest as
was experienced before the treatment with the ether. A natural
conclusion from this test was that the acridity might come from
some principle soluble in ether.
Observing that the ether was quite turbid and wishing to learn
the cause, a drop or two was allowed to evaporate on a glass slide.
Examining the residue with a microscope it was found to consist of
innumerable raphides or needle-like crystals. Some of the ether was
then run through a filter. The filtrate was clear. An examination
showed it to be entirely free from raphides, and it had lost every
trace of its acridity. The untreated acrid juice of the Indian
turnip, calla, and other plants of the same family was then
filtered and in every instance the filtered juice was bland and had
lost every trace of its acridity. These tests and others that need
not be mentioned, proved conclusively that the acridity of various
species of the Arum family was not due to a volatile principle, but
was due to the needle-shaped crystals found so abundantly in these
Several questions yet remained to be answered. (1) If these
needle-like crystals or raphides are the cause of the acridity of
the plants just mentioned, why do they not produce the same effect
in the fuchsia, tradescantia and other plants where they are known
to be just as abundant? (2) Why does the Indian turnip lose its
acridity on being heated? (3) Why does the dried Indian turnip lose
It was first thought that the raphides found in plants having no
acridity, might be of different chemical composition than those
which produce this effect.
A chemical examination proved beyond question that the raphides
were of the same composition. The needle-shaped crystals in all the
plants selected for study were composed of calcium oxalate. The
crystals, found in grape, rhubarb, fuchsia and tradescantia were
identical in form, fineness and chemical composition with those
found in the plants of the Arum family. How then account for the
painfully striking effect in one case and the non-effect in the
other? This was the perplexing question.
In expressing some juice from the stems and leaves of the
fuchsia and tradescantia it was found to be quite unlike that of
the Indian turnip and calla. The juice of the latter was clear and
limpid; that of the former quite thick and mucilaginous. There was
no difference as to the abundance of crystals revealed by the
After diluting the ropy, mucilaginous juice with water, and
shaking it thoroughly with an equal volume of ether, there was no
turbidity seen in the supernatent ether. Allowing a few drops of
the ether to evaporate scarcely any crystals could be found.
Practically none of them had been removed from the insoluble
mucilaginous covering. Here and there an isolated specimen was all
that could be seen. So closely were these small crystals enveloped
with the mucilaginous matter that it was almost impossible to
separate or dissect them from it.
It was now easy to explain why certain plants whose cells were
crowded with raphides were bland to the taste, while other plants
with the same crystals were extremely acrid.
In one case the crystals were neither covered nor embedded in an
insoluble mucilage, but were free to move. Thus when the plant was
chewed or tasted the sharp points of these needle-like crystals
came into contact with the tongue, lips and membranous surface of
In the other case the insoluble mucilage which surrounded the
crystals prevented all free movement and they produced no
Why do these intensely acrid, aroid plants lose their acridity
on being heated? It is well known that the corms of the Indian
turnip and its allies contain a large amount of starch. In
subjecting this starch to heat it becomes paste-like in character.
This starch paste acts in the same manner as the insoluble
mucilage. It prevents the free movement of the crystals and in this
way all irritant action is precluded. In heating the Indian turnip
and other corms, it was found that the heat applied must be
sufficient to change the character of the starch or the so-called
acridity was not destroyed.
One other question remains to be answered. It has long been
noted that the old or thoroughly dried corms of the Indian turnip
are not acrid like those that are fresh. The explanation is simple.
As the plant dries or loses its moisture, the walls of the cells
collapse and the crystals are closely encased in the hard, rigid
matter that surrounds them. This prevents free movement and the
crystals can not exert any irritant action.
It is generally believed by biologists that the milky juice,
aromatic compounds, alkaloids, etc., found in plants have no direct
use in the economy of the plant. They are not connected with the
nutritive processes. They are excretions or waste products that the
plant has little or no power to throw off. There can be little
doubt, however, that these excretory substances often serve as a
means of protection. Entomologists have frequently stated that the
milky juice and resins found in the stems of various plants act as
a protection against stem boring insects. In like manner the bulbs,
stems and leaves of plants that are crowded with crystals have a
greater immunity from injurious biting insects than plants that are
free from crystals. It is quite generally believed that the
formation of crystals is a means of eliminating injurious
substances from the living part of the plant. These substances may
be regarded as remotely analogous to those organic products made by
man in the chemical laboratory.
Some progress has been made in this direction, but so far the
main results are certain degradation-products such as aniline dyes
derived from coal tar; salicylic acid; essences of fruits; etc.
Still these and many other discoveries of the same nature do not
prove that the laboratory of man can compete with the laboratory of
the living plant cell.
Man has the power to break down and simplify complex substances
and by so doing produce useful products that will serve his
purposes. We may combine and re-combine but so far we only replace
more complex by simpler combinations.
The plant alone through its individual cells, and by its living
protoplasm has fundamentally creative power. It can build up and
restore better than it can eliminate waste products.
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