THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 223 



the dead remains, and by means of external hyphae and zoospores attacks 

 other seedUngs near by. The disease spreads rapidly, and in a comparatively 

 short time large numbers of seedling plants are killed. Pythhim deharyamim 

 also lives as a saprophyte in the soil, remaining there indefinitely before 

 once again becoming parasitic on some higher plant. It should be noted 

 that many other species of Pythium also cause disease among higher plants. 



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Fig. 213. — Pythium debaryonum on seedlings. A, Normal seedlings. 

 B, Seedlings showing " Damping-off." 



Pythium consists of long branched coenocytic hyphae, transverse septa 

 being absent, except in association with the sex organs. The contents of the 

 hyphae consist of a vacuolated mass of protoplasm in which nuclei are 

 embedded. The infecting hypha may find its way into the host through a 

 stoma, or may gain an entrance by dissolving the cell wall, probably by enzyme 

 action. Once inside it grows rapidly, branching and forming an extensive 

 mycelium. Although for the most part the hyphae remain in the inter- 

 cellular spaces of the stem, they have the power of penetrating into the cells. 

 Usually the mycelium is restricted, at any rate in the early stages of the 

 disease, to the cortical region of the hypocotyl just above soil level ; the cells 

 of the host die and collapse at this point and the seedling falls over. Later 

 the hyphae make their way into the vascular bundles, where they absorb 

 food substances which are being carried to different parts of the plant. It 

 is the appropriation of this food and the formation of waste products by the 

 Fungus which lead to the death of the host and so cause the Fungus to 

 become saprophytic. Long before the death of the seedling, however, 

 reproductive organs are developed. Reproduction may be either by asexual 

 or sexual methods. 



Asexual Reproduction 



The asexual reproductive organs are formed almost exclusively on aerial 

 hyphae. Such hyphae are produced from a mycelium which grows out from 

 inside the host, in the presence of a damp atmosphere, which favours growth 



