6i 



bracket fungi and the honey agaric, for instance, live as 

 parasites on trees. The vegetative filaments of these 

 fungi ramify in the tissues of the tree, gaining entrance 

 often through wounds, and only the reproductive bodies 

 are produced outside. But very many diseases of plants 

 are caused by fungi which, like the saprophytic mould 

 Mucor^ are so small that they can only be studied with 

 the aid of the microscope. Such a minute parasitic fungus 

 called Pythimn de Baryanuin often causes the "'Damping- 

 off " of seedlings. 



The 'VDamping~off disease very frequently appears 

 when the seeds of many plants are sown too thickly and 

 grown under conditions which are too warm and moist. 

 Young seedlings begin to die off in patches and soon 

 present a very characteristic rotten appearance. The disease 

 is very commonly met with in gardens and greenhouses, 

 occurring in seed-beds of all kinds a few days after the 

 germination of the seeds. It is most abundant in very 

 wet weather and when the beds are kept too shaded and 

 badly ventilated; crowding of seedlings also favours the 

 progress of the disease. At first a few^ individual seed- 

 lings are attacked at or near the surface of the ground, 

 the tissues in this region having a water-soaked appear- 

 ance. Soon the cells collapse, and being unable to 

 support the weight of the cotyledons the seedlings 

 fall prostrate. Those immediately around are similarly 

 affected, the disease spreading through the seed-bed m 

 ever widening circles until practically the whole may be 

 destroyed. When the plants fall over they become pale 

 and rotten, and soon the whole bed is reduced to a moist 

 mass of decaying seedlings. This mass is seen, on closer 

 examination, to be covered with the very delicate threads 

 of a fungus somewhat like the mould on bread. Often 

 the filaments can be seen to have spread from the first 

 diseased seedlings to the outer parts of the circle. These 

 filaments most often belong to the fungus Pythium, which 

 is so called because of its ability to produce rotting. The 

 fungus continues to grow in the dying seedlings, and the 

 filaments may form a dense felt over the whole seed-bed. 



If a seedling is examined under the microscope shortly 

 after it is attacked, the collapse of the tissues just above 

 the ground is seen to be due to the destruction of the 

 cells at that spot. This destruction is caused by the fila- 

 ments of the fungus w^hich at this stage are seldom visible 



