This very marked condition was considered by Acharius[505] as of generic importance and the genus, Isidium, was established by him, with the diagnostic characters: “branchlets produced on the surface, or coralloid, simple and branched.” In the genus were included the more densely isidioid states of various crustaceous species such as Isidium corallinum and I. Westringii, both of which are varieties of Pertusariae. Fries[506], with his accustomed insight, recognized them as only growth forms. The genus was however still accepted in English Floras[507] as late as 1833, though we find it dropped by Taylor[508] in the Flora Hibernica a few years later.
The development of the isidial outgrowth has been described by Rosendahl[509] in several species of Parmelia. In one of them, P. papulosa, which has a cortical layer one cell thick, the isidium begins as a small swelling or wart on the upper surface of the thallus. At that stage the cells of the cortex have already lost their normal arrangement and show irregular division. They divide still further, as gonidia and hyphae push their way up. The full-grown isidia in this species are cylindrical or clavate, simple or branched. They are peculiar in that they bear laterally here and there minute rhizoids, a development not recorded in any other isidia. The inner tissue accords with that of the normal thallus and there is a clearly marked cortex, gonidial zone and pith. A somewhat analogous development takes place in the isidia of Parmelia proboscidea; in that lichen they are mostly prolonged into a dark-coloured cilium.
In Parmelia scortea the cortex is several cells thick, and the outermost rows are compressed and dead in the older parts of the thallus; but here also the first appearance of the isidium is in the form of a minute wart. The lower layers (4 to 6) of living cortical cells divide actively; the gonidia also share in the new growth, and the protuberance thus formed pushes off the outer dead cortex and emerges as an isidium ([Fig. 85]). They are always rather stouter in form than those of P. papulosa and may be simple or branched. The gonidia in this case do not form a definite zone, but are scattered through the pith of the isidium.
Here also should be included the coralloid branching isidia that adorn the upper surface and margins of the thallus of Umbilicaria pustulata. They begin as small tufts of somewhat cylindrical bodies, but they sometimes broaden out to almost leafy expansions with crisp edges. Most frequently they are situated on the bulging pustules where intercalary growth is active. Owing to their continued development on these areas, the tissue becomes slack, and the centre of the isidial tuft may fall out, leaving a hole in the thallus which becomes still more open by the tension of thalline expansion. New isidia sprout from the edges of the wound and the process may again be repeated. It has been asserted that these structures are only formed on injured parts of the thallus—something like gall-formations—but Bitter[510] has proved that the wound is first occasioned by the isidial growth weakening the thallus.
Fig. 85. Vertical section of isidia of Parmelia scortea Ach. A, early stage; B. later stage. × 60 (after Rosendahl).
B. Origin and Function of Isidia
Nilson[511] (later Kajanus[512]) insists that isidia and soredia are both products of excessive moisture. He argues that lichen species, in the course of their development, have become adapted to a certain degree of humidity, and, if the optimum is passed, the new conditions entail a change in the growth of the plant. The gonidia are stimulated to increased growth, and the mechanical pressure exerted by the multiplying cells either results in the emergence of isidial structures where the cortex is unbroken, or, if the cortex is weaker and easily bursts, in the formation of soralia.