OZONIUM ROOT EOT 21 



present, depending on the diameter of the strand. The hyphse stain 

 red, but owing to the fact that the cell contents are not as dense 

 as in the central hypha, they do not stand out so sharply. 



When branching of the strands occurs the central hypha only is 

 found to branch, whereas the outer band of smaller hyphse do not 

 branch, but run along continuously. Just what the function of this 

 large central hypha is was not determined. It appears to be con- 

 tinuous and probably serves as a food reserve for the mycelium 

 which invades the root tissues of the plant. 



The diameter of the strands varies greatly. Those found on the 

 mesquite root (pi. 7, A) are at least twice the diameter of the 

 strands found on cotton and alfalfa roots (pi. 7, B, and pi. 8, A) 

 during the summer. Likewise they are much more compact and 

 consist of more layers of smaller hyphse. Strands observed on cot- 

 ton roots which wintered over were found to be more like those 

 studied on the mesquite roots both in size and compactness. The 

 strands found during the summer months are of a looser nature and 

 not so well organized. Using various types of polarized light, no 

 differences could be detected between the nature of the component 

 parts of the strands or between the strands found on the various 

 hosts. 



The acicular hyphse (pi. 7, A and B) stain very lightly or not 

 at all. They arise from more or less triangular cells in the outer 

 band of hyphse. 



MODES OF ENTRANCE 



Three avenues of entrance into the root tissues of cotton plants 

 have been observed — through lenticels (pi. 9, B), through wounds 

 of various kinds, and by the direct penetration of the hyphse be- 

 tween the cork cells. (PL 8, A, B, and C.) Infection of the roots 

 through the lenticels is of common occurrence. Direct penetra- 

 tion of the hyphse between the cork cells is rather uncommon. 



On alfalfa roots, in addition to the three portals mentioned above, 

 a fourth one has been noted, which ranks next to the lenticels 

 in the frequency with which it has been observed. Entrance in this 

 instance is through the small fibrous rootlets attached to the main 

 taproot. (PL 10, A and B.) Penetration usually takes place in the 

 angle at the junction of a rootlet and the taproot, generally through 

 a break in the corky layer. Direct penetration of the alfalfa roots 

 appears to be effected more easily and is more common than with 

 cotton. This is due to the fact that the layer of cork cells on cotton 

 roots is thicker and more compact than the cork cells on the alfalfa 

 roots. 



MECHANICS OF ENTRANCE 



The hyphse involved in the actual penetration of the roots arise 

 from the central hypha of the strands. The central hypha branches 

 profusely on the side of the strand in contact with the roots. Soon 

 an accumulation of mycelium (pi. 9, A) is formed, composed of 

 rather large, irregular-shaped, thick-walled, many-nucleated hyphse, 

 at first with only an occasional cross wall. The protoplasm is 

 extremely dense and granular and stains a deep red with Pianese's 

 stain. The contents of these hyphse appear similar in all respects 



