PYTHIUM 241 



IV. PERONOSPORE^E 

 PYTHIUM DE BARYANUM 



I. Sow seeds of the common garden Cress (I^pidium sativuni) 

 thickly in a flower-pot : cover it over with a glass plate, and 

 keep it well watered, so that the young seedlings grow up in an 

 atmosphere saturated with water. After a few days the head 

 of some of the seedlings may be seen to have bent over, owing 

 to insufficient support of the stem : examination will show that 

 the curvature is a sharp one, so that it is not due to general 

 weakness : further that the stem is thin and flabby at the point 

 of curvature : while fungal filaments may be observed in close 

 contact with the stem at that point, and it is this Fungus 

 (Pythium de Baryanum) which is the cause of the disease 

 termed by gardeners " damping off": it is of common occurrence 

 in propagating pits which are kept too warm and moist. 



If the Cress cultures be kept damp for some days longer, a 

 thick felt of hyphae will be formed, which will bind the seedlings 

 together : and finally the disorganization, which usually begins 

 near the base of the hypocotyledonary stem, will spread through- 

 out the seedlings, causing complete rotting. 



II. Mount part of a stem of one of the collapsed seedlings in 

 water, and examine under a low power : observe 



1. That the tissues show an abnormal appearance at the 

 point of curvature, their colour is yellowish, and the individual 

 cells show signs of having lost their turgidity. 



2. That numerous colourless branched hyphse extend along 

 the surface of the seedling, being most numerous at the point 

 of curvature, and less frequent further up. 



R 



