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TEXTBOOK OF BOTANY 



382. Damping off. — Much loss is occasioned in green- 

 houses, seed beds, and often in fields by fungi which attack 

 young plants at the surface of the soil. An infected plant 

 at first appears slightly water-soaked; soon it wilts, and 

 then, as the shoot falls to the soil, the fungus spreads to aU 

 its parts and in a short time kills the plant. The fungus 



which most commonly 

 causes this trouble lives 

 as a saprophyte in the 

 upper layers of rich, 

 moist soils. The plant 

 body of the fungus is 

 made up of delicate 

 threads much like those 

 of the bread mold; it 

 reproduces by means of 

 spores, each of which, 

 when it becomes free, di- 

 vides into several small 

 cells that can swim 

 about in water by means 

 of delicate vibrating 

 threads. Gametes are 

 also produced which 

 unite to form thick- 

 walled zygotes (Fig. 

 212) ; it is probably by 

 means of these zygotes 

 that the fungus lives through the winter. Similar troubles 

 are also caused by several quite different fungi, among them 

 some of the black molds and some of the imperfect forms. 

 Damping off can be partially prevented by sterilizing the 

 soil either by heating it or by the use of formaldehyde ; it 

 can be checked, too, by careful cultivation so that the fungi 

 are not allowed to grow on the refuse that may be present. 



Fig. 212. — The zygote of one of the 

 " damping off " fungi ; a, wall of the 

 oogone, an organ in which the egg was 

 formed ; b, outer wall, i , middle, and d, in- 

 ner walls of the zygote ; e, fat body ; /, a 

 tube through which the antherozoid passed 

 to unite with the egg ; g, antherid ; h, zygote 

 nucleus; i, a vacuole in the cytoplasm of 

 the zygote ; _;, the branch of the fungus on 

 which the antherid was borne. 



