MORPHOLOGY 



The development of the isidial outgrowth has been described by Rosen- 

 dahl 1 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 develop- 

 ment takes place in the isidia of Parmelia pro- 

 boscidea; in that lichen they are mostly pro- 

 longed into a dark-coloured cilium. 



In Parmelia scortea the cortex is several 

 cells thick, and the outermost rows are com- 

 pressed 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 some- 

 times 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 formec 1 on injured parts of the thallus something like gall- 

 formations but Bitter 8 has proved that the wound is first occasioned by 

 the isidial growth weakening the thallus. 



1 Rosendahl 1907. 2 Bitter 1899. 



B 



Fig. 85. Vertical section of isidia of 

 Parmelia scortea Ach. A, early 

 stage; B. later stage, x 60 (after 

 Rosendahl). 



