NEW DISEASE OF THE SILVER FIRS IN SCOTLAND II7 



approached. In the discoloured portion the walls of the cortical 

 cells swell and ultimately collapse ; in many cases the radial 

 walls are first disintegrated, and the tangential walls come into 

 contact. Owing to the shrinkage of the tissues, longitudinal 

 fissures appear in the interior of the stem. It appears that 

 hyphae do not pass into the stem while it is still living, but that 

 soon after discoloration of the tissues has taken place, the 

 hyphae enter from the leaf bases. 



In the case of branches on which the majority of the leaves 

 are dead, there must necessarily be a shortage of food material, 

 and death, no doubt, is partially accounted for by this circum- 

 stance. The lack of food, however, does not seem to be a 

 sufficient explanation of the rapid collapse of the tissues and 

 the consequent curvature of the stem. The appearance suggests 

 that some toxic substance is excreted by the hyphae, which can 

 bring about the death of cells before they are actually reached 

 by the fungus. This impression is strengthened by the fact that, 

 in the case of isolated diseased leaves, a zone of discoloured 

 tissue exists in the stem around the base of each. 



The infection by Rehmiellopsis is frequently accompanied by 

 infection with Chennes, especially on A. pectinata. A leading 

 shoot of this species infected with both parasites is shown in 

 PI. IV. Fig. 2 ; in this the fructifications of the fungus can be seen 

 on the leaves {a)^ and masses of eggs of the Chermes are borne 

 on the shoot just below the apex {b). In such a case it is 

 difficult to estimate the relative amount of damage brought 

 about by each of the organisms. Each organism can indepen- 

 dently bring about the death of the shoots. In the case of the 

 insect this can be easily seen in districts where Rehmiellopsis 

 is absent. Specimens of Abies nobilis, in which Chermes is 

 absent or only sparingly present, have been received severely 

 infected with the fungus, and in this case death of the shoots 

 is obviously entirely due to the latter. Where both organisms 

 are present death of the shoot takes place very rapidly. This 

 was demonstrated very forcibly by the examination of an area 

 in the west of Scotland, where, in about 80 acres of the common 

 silver fir, hardly a tree now remains alive; in this case the 

 trees were attacked when from 8 to 12 years old. 



A disease of the silver firs, caused by Cytospora Pinastriy 

 has been observed in several localities in this country, and may 

 possibly be mistaken for attack by Rehmiellopsis. In the case 



VOL. XXXVIII. PART II. I 



