30 Proceedings of the Royal Irish Academy. 



tree the more slowly do the outer cortical layers die, and so much the more 

 suitable are the conditions for the growth of lichens." He compares the 

 absence of lichens on Beeches grown under favourable conditions, i.e., on 

 calcareous soil, with their abundance on the slow-growing Beeches of sandy 

 soils. 



Liudau in his section on lichens, in his edition of Sorauer's Plant 

 Diseases, 1 states that it is impossible for the hyphae to penetrate within the 

 periderm tissues except possibly by means of cracks or fissures already 

 present. He had not observed any cases of the presence of hyphae in living 

 tissues in his researches on a list of species which includes those of liamalina. 

 Ee appears, however, to have used as material chieliy the larger species of 

 tree for his researches on fruticulose lichens. Further evidence must, however 

 be sought before his conclusion can be disproved, founded as it is on many 

 investigations and much knowledge of lichen-growth. 



There is, however, this further consideration. He differentiates between 

 the harmful actions of lichens on fruit and on forest trees. In the former 

 case he points out that the lichen holds water, and so keeps the bark moist, 

 hastening its decomposition, and at the same time offering itself as a shelter 

 to noxious insects. In the latter he considers that wealth of lichen-growth 

 indicates unfavourable conditions, dampness of situation, exclusion of light, 

 poor soil ; and that these are the causes of the death of such trees rather than 

 the lichens themselves. On the other hand, it' conditions are so bad that the 

 rate of annual apical growth is reduced to a minimum, the lichen may cover 

 the tips of the branches and cause suffocation. The remedy is here obviously 

 to allow the entrance of light and wind. In the case of forests on a large 

 scale these statements hold, hut for the small scattered woodlands of the 

 British Isles it is more frequently found that the maximum lichen-growth is 

 on the side of the wood, and of the trees in the wood, which is exposed to the 

 prevailing wind, i.e., the moisture-laden south-west. The light factor is here 

 not of such great importance as it would be in a wood of great extent- 

 Judging, indeed, from the luxuriant growth of lichens on the exposed sides of 

 .-united Oak, Birch, Hawthorn, and Sloe around our coasts and on the highest 

 wooded portions of our mountains, it would seem that moisture and slow 

 growth of the host were the important factors. The presence of salt in the 

 moisture is apparently not harmful to the lichens, though wind-blown sand is 

 undoubtedly so, the action probably being a purely physical one. 



- rauer, P. Handbuch der rilanzenkrankheitcn ii, 1908, p. 484. 



