132 HEART-ROT CAUSED BY OTHER FUNGI 



mx 



of a state of tension can be ol^served in the cell walls them- 

 selves. The general form of the trachcides is unaffected, 

 but cracks appear in the walls, always rising from right to 

 left, as shown in fig. 54, and usually in tiers, one above 

 another. I'hese cracks appear first ift the summer wood, 

 and in all stages are more conspicuous in the summer wood 

 than the spring wood. They do not rupture the complete 



wall, but the thickening on each 

 side of the middle lamella cracks 

 independently, and whereas on 

 one side the cracks run from right 

 to left, on the other side they 

 appear (as seen through the wall) 

 to run from left to right, whilst 

 the middle lamella itself remains 

 intact. Naturally the tension 

 breaks away weak places in the 

 walls, and bordered pits andhjqDhal 

 bore-holes are commonly centres 

 for cracking, and, where both sides 

 of the wall come within focus, the 

 cracks appear as a cross as shown 

 in the figure. 



Later on, larger cleavages occur 

 after attack by Polijporns in the wood in the transverse, 

 Schwdnitzii, showing cracks y ^ ^ f 'incrpnti-il ninnps 



in the walls: ^./<.,hyi.lian)<)ro- '^*^^^' ^^^ tangential pianes, 

 hole; />.p., bordered pit; h.c, and it is these cleavages which 

 horizontal crack; w.r.,nicdul- j^^^^. jjecome filled with the cheesv 



lai-v ray. 



mycelium. Also if a section ot 



a ti'uiik witli a rotten core is allowed to dry, further 



in-egular cracks occur in all directions in the wood, so that 



in such wood the micr<)scoi)i(' details are apt to become 



obscured. 



(iencral remarks on Polyporus Schweiiiitzii. We know 



very little indeed about the mode of infection of this fungus. 



Fn fact the text-books give all the infoi'mation which is 



available on the subject when they say that the fungus 



first attacks the i-oots of a tree and grows \\\) from the roots 



Fig. 54.— Tangential longi- 

 tudinal section of larcli wood 



