22 BULLETIN 1417, U. S. DEPARTMENT OF AGRICULTURE 



to those found in the central hypha of the strands. The outer band 

 of hyphse composing the strand forms a sort of protective layer over 

 the "weft of mycelium which apparently develops very rapidlv. 

 (PL 7, C.) 



After the weft has reached a sufficient size, penetration is accom- 

 plished by the formation of a wedge of hyphse which advances 

 between the cells of the lenticels or cork. (PL 8, B and C, and pi. 9, 

 B.) The wedge pushes its way down between these cells in a 

 mechanical fashion. The cells themselves are not attacked, but 

 remain intact and are finally engulfed in the fungous mass. Later 

 they collapse, and the old cell walls stained green by the Pianese 

 differential stain can be observed scattered about in the red-stained 

 mycelial mass. 



That the host plants react to this fungous invasion can be seen 

 in Plate 8, B, where the cork cambium (cell walls stained green) is 

 attempting to wall off the lenticel. Thus, until the hyphse reach the 

 cork cambium, the process of penetration is mechanical and in some 

 respects is similar to the action of ectotrophic mycorrhizas. 



DEVELOPMENT OF THE HYPHiE IN THE TISSUES 



As soon as the fungous wedge reaches the cork cambium, the hyphse 

 become differentiated from that composing the wedge. (PL 8. D. and 

 pi. 11, A.) The hyphse become smaller and more irregular in shape, 

 and the cell contents are less dense and more granular. The nuclei 

 are less prominent, and more cross walls occur. Branching of the 

 hyphse also occurs more frequently. From this point of entrance to 

 the cork cambium the hyphse invade it in all directions and likewise 

 advance into the parenchyma of the cortex. (PL 8, D, and pi. 11, A.) 

 The advance into the cork cambium is very rapid, much more so 

 than into the parenchyma. The l^phse are found at first in between 

 the cells, but later are found in the cells also. The 'cells of the cork 

 cambium and cortex are rapidly broken down, and some collapse. 

 The bast fibers and sieve tubes remain intact and can be observed 

 among the broken-down cells. (PL 9, C.) 



When enough of the parenchyma cells are broken down, a sunken 

 area or depression is formed. (PL 9, C.) While the hyphse have 

 been advancing and breaking down the cells to form a depression 

 the mycelium has been piling up to form a rather thick compact 

 mass of hyphse, similar to the hyphse which were responsible for 

 the penetration between the cork cells. 



By the time the advancing hyphse reach the cambium the depres- 

 sion is rather large. When this stage is reached the depression does 

 not increase in diameter, although the hyphse advance rapidly in 

 all directions from the depression. The cork cells remain intact 

 on each side of the depression even though the cells beneath are 

 badly diseased. (PL 11, A.) 



The hyphse rapidly destroy the cambium in all directions from the 

 points of attack. The stele is also invaded, so that by the time the 

 plant is dead fungous mycelium can be found throughout the stele 

 in the invaded area. This, in short, is a description of the penetra- 

 tion and development of the fungus at only one point. When it is 

 realized that the strand hyphse envelop the taproots of an alfalfa 

 or cotton plant and that many depressions are formed, it is not to 



