WOOD SOIL CONTACT CULTURE TECHNIQUE 113 



not decrease and at all stages of decay corresponds to about 35% of the 

 original dry weight of the block. Since the amount of water does not de- 

 crease as the amount of wood substance decreases, there is an increase in the 

 percentage of water expressed in terms of dry weight as shown by curves 

 labeled 3 in Fig. 1. Such an increase in the amount of water relative to the 

 remaining wood substance would tend to limit decay if the optimum moisture 

 content for initiating decay is considered to be at or near fiber saturation. 



If the absolute amount of water in the blocks does not change during the 

 progress of decay, then the final amount of moisture divided by the initial 

 weight of the blocks should give a percentage figure fairly close to the initial 

 fiber saturation of approximately 30%. The curves labeled 2 in Fig. 1 show- 

 ing the moisture content based on the initial weight indicate that this is the 

 case. For example, Fig. 1 shows that the average for all organisms is 4% 

 greater than 30% after one month, 7% greater after two, 6.3 after three, and 

 only 2.1 greater after four months. Between the third and fourth months 

 the soil showed signs of drying out and examination of all of the moisture 

 curves, Fig. 1, indicates a loss of water through the bottle caps, which 

 accounts partially for the discrepancy. The 5-10% increase in water over 

 the original fiber saturation may be due to slight condensation on the 

 blocks or to the respiratory activity of the fungi in breaking down carbo- 

 hydrates into CO2 and water. 



The average amount of decay (curve 1) and the final moisture content 

 referred to the initial weight (curve 2) for all the organisms obtained by 

 the wood-water (A) and wood-soil (B) assays are compared in Fig. 2. The 

 water content of the blocks in the water test varied from 55-165% based 

 on the oven-dry weight of wood and the decay was much less in amount 

 and uniformity than that obtained by the wood-soil technique with the 

 same organisms. From the results for individual blocks, the limiting water 

 content at which no decay took place was determined as 78% for Porta 

 incrassata, 84% for Coniophora cercbzlla, and 66% for Polyporiis vapor- 

 arius. In a few instances, despite the full cell saturated conditions, decay did 

 take place. Examination of these blocks indicated that most of the decay 

 was confined to the surfaces of the blocks. This indicates that lack of 

 oxygen was the limiting factor. 



When wood was supported on glass rods over agar (kolle flash technique) 

 full cell saturation of the blocks often occurs due to capillarity of the glass, 

 condensation of water, accidental contact between wood and agar, and 

 conduction by the fungus filaments. That the water content of the wood 

 in the kolle flask technique is also too high is indicated by the "optimum" 

 moisture content of 150% and the rclati\ely small weight losses due to 

 decay, less than 10%, cited in the experiments of Benton and Ehrlich.^' 

 The amount of decay was again shown to be affected by the water content 



