ROOT FUNGUS OF THE BLUEBERRY. 



43 



of interwoven and irregular snakelike coils. These liypha^ are about 

 O.OOOOG to 0.00012 of an inch (1.5 to 3 fi) in diameter. 



On the outer surface of the cells containing these fungous threads 

 others of similar or a little greater thickness may be observed. Some- 

 times they are transparent and their detection requires the same high 

 power of the microscope as do those in the interior of the cells. 

 Sometimes, however, these exterior threads have a pale-brown color 

 and are then readily seen. Their t;urface is smooth, devoid of mark- 

 ings of any kind. Ordinarily the 

 thread wanders loosely along the sur- 

 face of the root giving olf an occa- 

 sional branch and having an occa- 

 sion a 1 septum. Sometimes the 

 threads and their branches may form 

 an open network about the rootlet, 

 but they never form a dense sheath of 

 hyphte such as is characteristic of the 

 mycorrhiza of the oak. 



The connection between the exter- 

 nal and the internal hypha^ is not 

 easy to see at a single observation, 

 for the passage of the hyphse through 

 the cell wall is rarely caught in op- 

 tical section, and even then a clear 

 observation is usually rendered diffi- 

 cult because of refraction. A very 

 clear case, however, was observed in a 

 rootlet of laurel {Kalmia JatifoUa), a 

 shrub which has a mycorrhizal fun- 

 gus similar to that of the blueberry. 

 A drawing of that specimen is shown 

 in figure 18. 



The passage of the fungus through 

 the cell wall may frequently be ob- 

 served in the blueberry by first focus- 

 ing on the external hypha at a point 

 where it appears to have a lateral hump or a very short branch, and 

 then focusing slowly downward. In this way one passes from the 

 external to the internal part of the fungus, having had some portion 

 of the intervening hypha continuously in view. The hypha always 

 appears much constricted at the point where it goes through the 

 cell wall. 



This fungus is of the type named by Frank in 1887 an endotrophic 

 mycorrhiza to distinguish it from an ectotrophic mycorrhiza, such 



193 



Fig. 17. — Mycorrhizal fungus of a blue- 

 berry plant densely crowded in two 

 epidermal cells of the root. (En- 

 larged about 1,200 diameters.) 



