Porter — Organs of the Common Corticolous Ramalinae. 29 



four annual rings had for length of lobe from centre "25 inches, and at 



a distance of - 37 inches a small specimen of the same species occurred on a 

 branch showing three annual rings, the intervening surface being covered by 

 a thin layer of gonidia containing hyphal tissue and the periderm being 

 traversed horizontally by hyphae. On the other hand, this superficial layer 

 has been seen in connexion with XanihorUi, covering a leuticel without 

 sending downward growths into it. 



Injurious Effect of Lichen Growth on Trees. 



Though it is usually asserted that lichen growth does not to any great 

 extent injure trees from the point of view of the forester, there is no doubt 

 that it is regarded unfavourably by the arboriculturist, and in particular, 

 the fruit grower. Even the forester may admit that boughs broken off in a 

 storm are often covered by a luxuriant growth of lichens, but there is no 

 direct evidence that such growth renders boughs more liable to damage by 

 wind. 



The indirect evidence on which Lindau bases his assertion that the effect 

 of the hyphae is directly injurious may be summarized as follows : — 



1. They block the lenticels. 



2. Packing tissue of lenticels disappears. 



3. Cork layers are torn apart horizontally. 



4. Apart from lenticels, vertical cracks are forced open. 



5. Cell walls are frequently torn across, especially in Oaks. 



6. Atmospheric decomposition of the cell walls causes decrease in thick- 

 ness of cork cell walls from within outwards, thus rendering the cork less 

 impermeable. 



In fig. 12 we have evidence of more definitely harmful action, as here the 

 hyphae occur in periderm, bast, cambium, and even the youngest cells of the 

 wood. That, in this case, the hyphae are not those of a parasitic fungus is 

 proved by the fact that they did not occur in these positions except between 

 the young plant and its supposed parent. A parasite could hardly be limited 

 to this short distance. 



This is, as yet, the most striking case observed in which the hyphae were 

 found inside the periderm, and occurs in Primus spimosa, the Sloe, a shrub of 

 very leisurely growth. Hartig 1 remarks that " the slower the growth of a 



'Hartig, R. Diseases of Trees : Trans. Somerville and Marshal] Ward, London, 1804, 

 p. 55. 



