62 



to the naked eye. They are, however, almost exactly like 

 those found on the mouldy bread, and they run both 

 between and w^ithin the dead and dying cells. They can 

 also be traced into and between adjoining living cells, 

 and when the seedling falls prostrate the remainder of 

 the plant is soon invaded, for it is kept moist by contact 

 with the damp soil. As the tissues further decay the 

 filaments of the fungus spread out over the soil reaching 

 across to other seedlings which then succumb in the same 

 sort of way. The fungus attacks the cells of the seed- 

 lings by first extracting w^ater, then boring through the 

 cell walls and finally killing the living protoplasm and 

 feeding upon the cell contents. Since the fungus growls 

 very rapidly a seedling may be reduced to a putrid mass 

 in a few hours. 



The life story of Pythium is typical of the most 

 thorough and destructive of plant parasites. The fila- 

 ments at first grow in the air spaces between the cells, but 

 later they enter and kill the living protoplasm. The 

 spread of the fungus at first is due to the growth over the 

 soil from its earlier victims to healthy seedlings which 

 are attacked in turn; but soon a more rapid means of 

 spreading comes into play. When a seedling has become 

 thoroughly infected by the filaments of the fungus the 

 ends of many of the branches of the latter begin to swell 

 out into globular bodies very like the spore-cases of Mucor, 

 only much smaller. These sw^ollen bodies are full of 

 protoplasm and serve a somewhat similar purpose to those 

 in Mucor. They are really special spore-cases, but in 

 further development they dift'er considerably from those 

 of Mucor. In that fungus the spore-cases w^ere borne on 

 filaments standing erect from the surface, the spores being 

 shed into the air. In Pythium, however, the spore-cases 

 are submerged in the film of moisture around the decay- 

 ing seedling, and the different behaviour probably depends 

 on this. When the spore case is mature a short tube grows 

 from the side and swells to form a globular body with a 

 very thin wall. Into this the whole of the protoplasm 

 passes from the spore-case ; it then rapidly divides into 

 9 or lo small rather oval masses of protoplasm, which 

 begin to writhe and wriggle wathin the thin vesicle. This 

 soon bursts, liberating the minute writhing spores which 

 swim about in the slight film of water on the soil or sur- 

 face of the seedling. When examined under high mag- 



