OZONIUM ROOT EOT 23 



be wondered at that under optimum conditions for the development 

 of the disease a plant is killed within one week after the strands are 

 first found on the roots. 



As has already been pointed out, the point of attack centers on 

 the taproot of both cotton and alfalfa plants within the first foot of 

 soil. Sections were made of both the taproots and lateral roots out 

 away from the diseased areas and near the ends of both lateral roots 

 and taproots of plants just wilting. These sections do not show the 

 presence of the fungous strands on the surface of the roots or of the 

 invading hyphae within the root. From the ends of the healthy 

 taproots and lateral roots sections have been taken at regular inter- 

 vals to the union of the taproots with the lateral roots of cotton. On 

 plants just wilting, sections made from lateral roots 30 to 50 centi- 

 meters away from their union with the taproot show no strands on the 

 surface, although the outer xylem vessels were found to be filled with 

 the fungous hyphae. (PL 11, B and C.) Entrance to these vessels 

 occurred at the main point of attack on the taproot. Similarly, 

 sections made of alfalfa beyond the diseased portions for 50 centi- 

 meters showed the presence of fungous hyphae within the outer 

 xylem vessels, while no strands were present on the roots. The 

 tissues of these sections were intact and healthy appearing in all 

 respects, so that it is clear that the Ozonium strands do not attack 

 the roots below the first foot of soil, but they center their attack on 

 the taproot above this point. 



Sections were made in the discolored regions of the stem above the 

 junction of the root and the stem, but no fungous mycelium was 

 found within the tissues, so that the discoloring of the tissues must 

 be due to causes other than the presence of an invading fungus. 



DISCUSSION 



The development of the strand hyphae when they come in contact 

 with the root tissues should be studied more at length, to determine 

 how the central hypha of the strands branches to build up the 

 mycelial accumulations preparatory to the formation of the wedge 

 that is pushed down through the cells of the lenticel or the cork. So 

 far as the writers are aware, this type of entrance has not as yet been 

 described. 



The most common portal for Ozonium hyphae entering the root 

 tissues of alfalfa and cotton is the lenticel. A study of lenticels, 

 their number, size, and vertical distribution on the roots, the age 

 of the plants when they first appear, and the effects of irrigation 

 on their development, should determine the importance and fre- 

 quency of lenticel infection and solve some of the points which are 

 not as yet clearly defined. Until this is done the complete develop- 

 ment of the disease will remain obscure. 



'Whether infection occurs through the lenticels or cork cells, the 

 method of entrance is the same and appears to be mechanical, namely, 

 the formation of a hypha! wedge which pushes its way down between 

 the cells until the cork cambium is reached, when there appears to 

 be a rapid differentiation of the hyphae. These hyphae advance in 

 all directions, and they may be found intercellular and intracellular. 

 The accumulation of the hyphae in the cortex cells is remarkable. In 



