24 BULLETIN 1417, U. S. DEPARTMENT OF AGRICULTURE 



some instances mats of mycelium have been observed in the cortex 

 filling 10 to 12 rows of cortex cells, where the latter have collapsed. 



The fact that a cotton or an alfalfa plant can be killed in one 

 week shows that the fungus develops within the tissues of the roots 

 very rapidly. Further, the large mass of mycelium present on the 

 roots of the plants after they are wilted indicates that the fungus 

 is a voracious feeder. This nrycelium apparent!}^ serves as a food 

 reserve for the strand hyphse, which are found well in advance of the 

 wilting plants, as has already been pointed out. The fact that this 

 type of nrycelium serves as a reserve is further substantiated when 

 one notes the rapidity with which the fruiting or conidial mats are 

 formed (p. 13). 



It would be interesting to trace the connection between the my- 

 celium found on the wilting plants and the strand hyphse found well 

 in advance to these plants, to determine just how the strand hyphse 

 are formed and how the central hypha arises. At any rate it is 

 clearly evident that we are dealing with a polymorphic fungus, the 

 complete development of which can not be determined until the origin 

 and function of the different types of hyphse are studied at length. 



SUMMARY OF PART II 



Material representing various stages of root rot caused by Ozonium 

 omnivorwn on alfalfa and cotton roots, together with Ozonium 

 strands on mesquite roots, was collected at Sacaton, Ariz., and San 

 Antonio, Tex., during the summer of 1924 in connection with an 

 investigation of root rot. These materials were fixed in three com- 

 mon fixatives, embedded in paraffin, stained with Pianese's differ- 

 ential stain, and a study made of the pathological anatomy of the 

 disease. 



The strand hyphse are composed of a large central hypha around 

 which are bands of smaller hyphae. The acicular hyphse so conspicu- 

 ous on the strands arise from cells of triangular shape in the outer 

 band of smaller hyphse. 



The central hypha of the strands branches profusely to form a 

 dense weft of large, irregular-shaped, thin-walled, many-nucleated 

 hyphse, at first showing only an occasional cross wall at points on 

 the roots. 



Entrance to cotton roots is effected by the hyphse of these wefts 

 through the lenticels, through wounds of various lands, and by 

 direct penetration of the fungus between the cork cells. In addition, 

 a fourth portal was observed in alfalfa roots — the entrance through 

 the small fibrous rootlets where they are attached to the taproot. 



Entrance is effected by the formation of a hyphal wedge, which 

 pushes its way down between the cork and lenticel cells until the 

 cork cambium is reached. 



The hyphee on reaching the cork cambium become narrower, with 

 longer cells, containing a protoplasm less dense but more granular 

 than the hyphse responsible for the actual penetration. 



These invading hyphse advance in all directions through the cork 

 cambium and into the cortex. The hyphse are present in and between 

 the cells, and in a short time enough of the parenchyma cells are 

 broken down to form a depression in the bark. 



