22 Proceedings of the Royal Irish Academy. 



When reagents were used with hand sections the method of Lindau was 

 adopted, i.e. treatment with alcohol to drive out ah - , transference to chloral 

 hydrate, and finally, after preliminary examination, treatment with chlor- 

 zinc-iod or iodine solution ; this method results in a limited and uniform 

 swelling of the section. 



For microtome sections the thallus, with a squared block of the stem or 

 twig to which it was attached, was fixed in alcohol, and then transferred 

 slowly from pure alcohol to pure xylol (25 per cent, alcohol, 50 per cent., 

 100 per cent. ; 5 per cent, xylol, 10, 25, 50, 70, 100 per cent.). 



Penetration of the tissues by paraffin wax dissolved in xylol is a slow 

 process which cannot safely be hastened iii any way, since the tissue of the 

 periderm tends to burst, making complete sections impossible, and creating 

 artificial lacunae. 



I i differentiating stains chlor-zinc-iodine was used; it gave better effects 

 when used after potash. Fuchsin gave good results when varying thicknesses 

 of cork cell wall.- were t" 1"- examined, or to show up the path of a hypha 

 between the walls. In cases where hyphae apparently passed through acork 

 cell wall the illusion is best dispelled by uBe of Hoffmann's Blue. 



[V. — DSSI IUPTION. 

 1. i;.„ ral anatomy of the thallus. 



I ig. i represents a longitudinal section of the thallus of R. forinacea, 

 illustrating the closely packed longitudinal hyphae of the general cortical 



:•• (■ i : they turn outwards to the surface at their apices, but hardly at 

 right angles to the axis; the general effect may, however, be roughly stated 

 as " more or less at right angles." Further, the assemblage of these apices is, in 

 the specimen depicted, scarcely sufficiently developed in comparison with the 

 rest of the cortex to justify w . regarded as a tissue sua, generis. In 



uncus obtained from more exposed localities a broader expanse of apices 

 does undoubtedly occur, but is correlated with a greater width of longi- 

 tudinally arranged tissue; a similar correlation is observable in the lighted 

 ami unlighted surfaces of the thallus of R.fraxin<". This contrast between 

 the cortices of the two surfaces is not so marked in thalli of R. farinacm, since 

 in that species there is greater tendency to cylindrical development of the 

 laciniae. 



In the figure we have an example of the emergence of the gonidial (g) and 

 medullary (m) tissues at the surface, but whether the phenomenon is con- 

 nected with the development of soredial structure or an " atempor " is 

 irrelevant to the present investigation. 



