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 microscope.
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 the mouth.
In the other case the insoluble mucilage which surrounded the crystals prevented all free movement and they produced no irritation.
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.