DISEASES OF FLOWERING PLANTS 201 



171. Microscopical nature of " damping-off " fungus. The 

 vegetative or feeding portion of the fungus spreads through 

 the tissue of the leaf or stem in slender, colorless, myeelial 

 threads which branch in a complex manner. The threads pass 

 between the cells and frequently penetrate the cell wall. This 

 they accomplish by means of a ferment which they are sup- 

 posed to excrete, and increasing rapidly in size, they absorb the 

 cell contents, until the tissue dies. They have never been found 

 to possess haustoria or true parasitic roots. 



172. Asexual reproduction. After the vegetative mycelium 

 has ramified through the cells of the host-plant, ascending 

 hyphal branches may produce chains of conidia. These conidia 

 are of various forms. They may germinate at once by thrust- 

 ing out a slender tube through the cell wall, which elongates 

 into a myeelial strand exactly like the mycelium from which it 

 was produced. Some conidia exhibit a resting period and may 

 be distinguished by thicker walls. Sporangia may also be 

 produced from the mycelium, and in form and size resemble the 

 conidia. Their method of germination differs, however. De 

 Bary describes it as follows : l " The gelatinously thickened wall 

 at its apex suddenly expands into a thin-walled spherical vesicle, 

 and into this at the same moment the whole of the protoplasm 

 of the cell, which is hitherto undivided, or has shown only tran- 

 sitory beginnings of division, streams rapidly, within a few mo- 

 ments of time at most ; there it breaks up at once into a number 

 of swarm-spores (zoospores), which issue from the delicate swell- 

 ing vesicle and finally germinate." 



The swarm-spores have two cilia and by means of these they 

 swim rapidly about in the drops of water surrounding the soil 

 particles, arid spread the pathogen with great rapidity. Even- 

 tually, they lose their motive power, and develop into the in- 

 fecting mycelium. 



Atkinson states 2 that in this species, the swarm-spore stage 

 (zoospore) is not so abundant as the oogonia, or egg stage. 



173. Sexual reproduction. A microscopical examination 

 of the tissue of seedlings which have been killed by this fungus 

 shows rounded or spherical bodies, from three to five times 

 thicker than the myeelial threads. They are the oogonia, or 

 eggs, of the sexual stage of reproduction. They may be at the 

 terminal of the myeelial strand, or at the end of short branches 

 of the mycelium, or they may occur as enlargements, with no 

 reference to the end of the strand. 



1 " Fungi Mycetozoa and Bacteria," p. 137. 



2 " Damping Off," Bulletin 34, Cornell Experiment Station. 



