CHAPTER XXVII. 



PHYCOMYCETES. (a) OOMYCETES. 



Two of the commonest and most destructive of fungal parasites will 

 serve to illustrate the Oo-mycetes. They both belong to those non- 

 septate Fungi which habitually produce distinct male and female 

 organs, comparable to those seen in the higher Siphonaceous Algae, 

 such as Vaucheria. Like them also they include in their life-history 

 a stage where zoospores are motile in water. This, together with 

 their close dependence upon moisture during vegetation, justifies 

 for them the name Phy corny cetes, or Alga-like Fungi. 



THE " DAMPING-OFF FUNGUS" (Pythium debaryanuni). 



When Mustard and Cress are sown thickly, and kept too warm 

 and damp, the seedlings are liable to the disease of " damping-off," 

 the plants quickly rotting with an unpleasant 

 smell. Many other seedlings, and especially 

 Cucumbers and Melons, are subject to it ; in 

 fact, the disease is one of the commonest diffi- 

 culties of the gardener, and ruins the efforts of 

 many amateurs. It makes its appearance at 

 definite spots in the seed-beds, and if not checked 

 it spreads thence in ever-increasing circles. The 

 first sign is the collapse of a seedling, owing to 

 the shrinking of its cortex, usually at some 

 point above the soil-level, the stem being no ^| 



longer able to support the weight above (Fig. 4 b 



348). If the diseased plant be examined micro- FIG. 348. 



,..,,. .,, , ,. , , . . . ,, . A young Cress-seedling 



SCOplCally its tiSSUeS Will be lOUnd tO be riddled attacked by Pythium at d, 

 ,1 i j'j.1. u u ii. 11 just above the ground-line c. 



through and through by rather coarse colourless 6= r oot. o=cotyiedons and 

 threads, enclosed by a cell-wall and filled with g|S viSd:"* 80 (After 



