132 HEART-ROT CAUSED BY OTHER FUNGI 



of a state of tension can be observed in the cell walls them- 

 selves. The general form of the tracheides 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. These cracks appear first in 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 tn the 

 walls, and bordered pits andhyphal 

 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 



FIG. 54. Tangential longi- 

 tudinal section of larch wood 



after attack by Polyporus in the wood in the transverse, 



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

 ^^ beCOme filled with the che6S y 



mycelium. Also if a section of 

 a trunk with a rotten core is allowed to dry, further 

 irregular cracks occur in all directions in the wood, so that 

 in such wood the microscopic details are apt to become 

 obscured. 



General remarks on Polyporus Schweinitzii. We know 

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

 In fact the text-books give all the information which is 

 available on the subject when they say that the fungus 

 first attacks the roots of a tree and grows up from the roots 



