A great variety of opinions have been entertained respecting the plant called esobh, translated 'hyssop' in the Authorized Version both of the Old and the New Testament; but it is difficult to fix with certainty on the plant intended. The first notice of it occurs in , where a bunch of hyssop is directed to be dipped in blood and struck on the lintels and the two side-posts of the doors of the houses in which the Israelites resided. It is next mentioned in;; , in the ceremony for declaring lepers to be cleansed; and again, in; , in preparing the water of separation. To these passages the apostle alludes in : 'For when Moses had spoken every precept to all the people, according to the law, he took the blood of calves, and of goats, with water, and scarlet wool, and hyssop, and sprinkled both the book and all the people.' From these texts we find that the plant must have been leafy, and large enough to serve for the purposes of sprinkling, and that it must have been found in Lower Egypt, as well as in the country towards Mount Sinai, and onwards to Palestine. From the following passages we get some information respecting the habits and the supposed properties of the plant. Thus, in , it is said, 'Solomon spoke of trees, from the cedar-tree that is in Lebanon, even unto the hyssop that springeth out of the wall;' and in the penitential psalm of David (), 'Purge me with hyssop, and shall be clean: wash me, and I shall be whiter than snow.' In this passage it is, no doubt, considered by some commentators that hyssop is used in a figurative sense; but still it is possible that the plant may have possessed some general cleansing properties, and thus come to be employed in preference to other plants in the ceremonies of purification. It ought, at all events, to be found growing upon walls, and in Palestine. In the account of the crucifixion of our Savior, the Apostle John says (), 'Now there was set a vessel full of vinegar, and they filled a sponge with vinegar, and put it upon hyssop, and put it to his mouth.' In the parallel passages of Matthew () and Mark (), it is stated that the sponge filled with vinegar was put upon a reed or stick. To reconcile these statements, some commentators have supposed that both the sponge and the hyssop were tied to a stick, and that one apostle mentions only the hyssop, because he considered it as the most important; while, for the same reason, the other two mention only the stick; but the simplest mode of explaining the apparent discrepancy is to consider the hyssop and the stick to be the same thing—in other words, that the sponge was affixed to a stick of hyssop.
A great variety of plants have been adduced by different authors as that alluded to in the above passages. Some contend for several plants belonging to the class of ferns, such as maiden-hair, wall-rue, pearlwort, and hair-moss: others for a species of wormwood, that it might be more distasteful to our Savior. The majority, however, have selected different kinds of fragrant plants belonging to the natural family of Labiatae, several of which are found in dry and barren situations in Palestine, and also in some parts of the Desert. Of these may be mentioned the rosemary, species of lavender, of mint, of marjoram, of thyme, of savory, of thymbra, and others of the same tribe, resembling each other much in characters as well as in properties: but it does not appear that any of them grow on walls, or are possessed of cleansing properties; and, with the exception of the rosemary, they are not capable of yielding a stick, nor are they found in all the required situations. Dr. Royle, who has recently investigated the subject, is of opinion that as the caper plant has an Arabic name, asuf, similar to the Hebrew esob or esof; as it is found in Lower Egypt, in the deserts of Sinai, and in New Jerusalem; as it grows upon rocks and walls, was always supposed to be possessed of cleansing qualities, is large enough to yield a stick; and as its different parts used to be preserved in vinegar, as its buds now are, he is warranted, from the union of all these properties in this plant, corresponding so closely to those of the original esof, in considering it as proved that the caper plant is the hyssop of Scripture.