Porter — Organs of the Common Corticolous Ramalinue. 25 



hypha or mass of hyphae directly from the lichen to the centre, and the 

 possibility remains that the hyphae figured belong to some species of parasitic 

 fungus ; but, on the other hand, the section represents a state of affairs too 

 frequent to be the result of the chance co-operation of two sets of mycelia. 



Fig. 18 represents hypertrophy of the periderm and erosion of the wood. 

 Here the host is Alder, and the lichen probably B. pollinaria, as well-developed 

 plants of this species were frequent on this and neighbouring twigs. Not 

 only does the typical periderm tissue develop as a cushion round the base of 

 the lichen, but, besides the hyphal masses penetrating the external tissues, we 

 have tongues of tissue consisting of polygonal, pitted cells (t), apparently 

 eating their way into the otherwise normal wood (iv), as in fig. 19. 



Fig. 20 gives in detail the character of these ingrowths. 



V.— Discussion of Eesults. 



With regard to the structure of the cortex of these lichens, the figures 

 obtained hardly seem to justify the distinction between cortex and mechanical 

 tissue. For convenience the terms inner and outer cortex might be retained, 

 with the reservation that the one is a mere continuation, with more or less 

 change of direction, of the other. The swollen appearance of the apices 

 described by Speerschneider is probably due to his use of caustic potash, 

 since other less violent clearing agents, such as chloral hydrate, do not produce 

 this effect. The pseudoparenchyma described by Brandt has been alluded to, 

 and may certainly be more or less satisfactorily obtained by his method. If, 

 however, fig. 3 be examined closely, it will be noted that the cortex consists 

 of hyphae cut in all directions, the majority transverse. On treatment with 

 potash these last appear as isolated cell sections, whilst the interwoven 

 hyphae form more or less complete enclosures round them. On addition of 

 acetic acid this appearance becomes still more sharply defined, and on staining 

 with alcoholic iodine the hyphae become coloured so that the contents of the 

 cell sections show up clearly, and the interwoven hyphae apparently form 

 cell-walls around them. 



In the intraperidermal extensions of the cortical cells it is almost im- 

 possible to induce this parenchymatic appearance, and this may be attributed 

 to the fact that here the hyphae are more rarely interwoven, the main 

 direction being longitudinal. 



With regard to the action of potash as a clearing agent, it should be 

 remembered that this alkali is a solvent of usneic acid, which is apparently 

 the most common of lichen acids. In the case of B. thrausta it occurs as 

 minute crystals in the cortex, giving to that tissue its yellowish-green colour, 



R.I.A. PBOC, VOL. XXJCIV, SECT. B. [_^] 



