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 Vancheria. 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 Phycomycetes, or Alga-like Fungi. 



The " Damping-off Fungus " (Pythium debaryanum). 



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

 and damp, the seedlings are hable 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. 

 348). If the diseased plant be examined micro- 

 scopically its tissues will be found to be riddled 

 through and through by rather coarse colourless 

 threads, enclosed by a cell-wall and filled with 



413 



/I 



l< b 



Fig. 348. 

 A young Cress-seedling 

 attacked by Pythium at d, 

 just above the grouiid-Une c. 

 b^Toot. fl=cotyledons and 

 plumule. (Nat. size.) {After 

 Marshall Ward.) 



