oct.4. I9IS Sugar-Beet Seedlings and Phoma Betae 57 



rial by the unusual appearance of the nuclei. Dumb-bell forms, budding, 

 and indirect division were observed occasionally, but never in any large 

 number (PI. I, fig. 4, 5, 6). 



The most interesting phenomena in many respects, as well as the most 

 puzzling, are those associated with recovery and healing. Sugar beets 

 attacked by the fungus frequently send out new side roots from a point 

 above the invasion and succeed in preventing the destruction of this new 

 growth. Cases were common in which the region invaded and dis- 

 integrated had been confined to the outer tissue. The central vascular 

 region and the surrounding layers of cells resisted the attack and eventu- 

 ally succeeded in sloughing off the killed tissue. The fungus was fre- 

 quently found developing its pycnidia on the killed portions of such 

 recovering seedlings, while the host tissue, only a few cells below, appeared 

 perfectly normal (PI. II, fig. i). 



The most striking thing brought out by a study of the sections, however, 

 is the presence of the fungus apparently established in a condition of 

 reduced relative virulence in the interior tissue of beets which have recov- 

 ered from the attack and which are assured of making a good growth 

 (PI. II, fig. 2), In such cases even the invaded cells are not killed, 

 and the adjacent ones appear perfectly normal in every respect. So far 

 as has been observed, the cells thus invaded are adjacent to vascular 

 tissue, but the organism has never been seen in the conducting elements. 

 The infection is confined to a vertical chain of cells, and in no case was 

 more than a single unbranched hypha observed. 



The physiological relation here presented is an exceedingly interesting 

 one and its investigation is of the highest scientific and practical import- 

 ance. 



It is difficult to explain just how an organism capable of producing 

 such complete collapse in cells of seedlings should suddenly find its action 

 checked and confined to a saprophytic existence on an area of discarded 

 surface tissue, but the means by which it establishes itself within the 

 highly nutritive living cells of the interior and is at the same time com- 

 pelled to remain in a quiescent condition is still more problematical. 

 The condition presents a relatively highly developed type of parasitism 

 in which the organism voluntarily or by compulsion permits the comple- 

 tion of the normal life history of the host while securing for itself the 

 assurance of perpetuation through infection of the seed. The balance, 

 however, is not a perfect one, since, if the host encounters sufficiently 

 adverse conditions during either of the growing seasons or in storage, 

 the activity of the parasite is renewed and the sugar beet is destroyed, 

 thus preventing seed production and the perpetuation of the parasite 

 through the seedling channel. 



