63 



nification each of the colourless swimming spores is seen 

 to be furnished with two excessively fine hair-like threads 

 of protoplasm which, by lashing the water incessantly, 

 bring about the movement of the spore. The active move- 

 ments continue for twenty minutes or half an hour, then 

 the spore comes to rest, rounds off and withdraws the 

 whips of protoplasm. If favourably situated on a seed- 

 ling it sends out a fine lilament which bores its wa}' 

 through the outer wall of one of the cells and grows into 

 the interior. It has been proved that the tip of the fila- 

 ment is able to do this, because it secretes a substance 

 which enables it to digest its wa}' through cell w^alls 

 pretty much as gastric juice renders our food materials 

 soluble. Once within the cells of the victim the fungus 

 branches and grows rapidly from cell to cell, spreading 

 destruction as it goes and deriving nourishment from the 

 product of this destruction. In a diseased seed-bed these 

 pr^ocesses are going on so continuously that the fungus 

 soon produces many thousands of the motile Sj3ores which 

 are able to attack new seedlings, thus accelerating the 

 progress of the disease. In addition, as we have already 

 seen, even the threads of the fungus grow across the soil 

 between the seedlings and directly produce new infection. 



It is well known that "Damping off will recur with 

 even greater virulence in seed-beds which show^ed the 

 disease the previous season. This is explained by the 

 existence of yet another chapter in the life history of 

 Pythium, by which resting spores are formed that can 

 pass the winter m the soil. At a late stage in the decay 

 of diseased seedlings many threads of the fungus give 

 rise to the resting spores. These possess the power of 

 lying dormant over a long period, and in this resemble 

 the seeds of higher plants, though they only consist of a 

 single thick-walled cell. AVhen, however, the temperature 

 is favourable and the conditions are sufficiently moist, the 

 thick coat bursts and a fungal filament grows out which 

 soon attacks any seedling that may be near, producing 

 spore-cases and motile spores as before. The resting 

 spores are produced in myriads m a diseased seed-bed ; 

 in fact, on one occasion, I estimated the presence of upwards 

 of half a million in a single diseased seedling observed 

 under the microscope. As the seedlings rot these spores 

 all find their way into the soil where they spend the 

 winter. 



