42 EXPERIMENTS IN BLUEBERRY CULTURE. 



The young rootlets of the blueberry before they branch are ex- 

 ceedino-ly slender, varying from 0.002 to 0.003 of an inch (50 to 75 fi) 

 in diameter. This makes them very susceptible to actual drying and 

 they are easily killed by it. This characteristic has an important 

 bearing on the treatment of these plants when in pots. The matter 

 is discussed on pages 65 to 67. 



(14) The rootlets of healthy plants of the swamp blueberry are inhab- 

 ited BY A FUNGUS, OF THE SORT KNOWN TECHNICALLY AS AN ENDOTROPHIC 

 WYCORRHIZA." 



As already stated, the ultimate rootlets of the blueberry are very 

 fine, their diameter varying from 0.002 to 0.003 of an inch (50 to 75 fi). 

 In rootlets of the smaller size about three rows of epidermal cells are 

 visible in a lateral view, in the larger rootlets about five rows. In a 

 newly grown rootlet not contaminated with soil particles these epi- 

 dermal cells, and, indeed, all the underlying cells as well, are as trans- 

 parent as glass, and were it not for the difficulties due to the refrac- 

 tion of light the examination of the contents of the cells would not be 

 difficult. As a matter of fact the study of the contents of the live 

 cells is difficult, their intelligent examination requiring the use of an 

 oil immersion objective and microscopic enlargements of 1,000 to 

 1,500 diameters. The darkened window installation for a microscope, 

 devised by Dr. N. A. Cobb, of the Bureau of Plant Industry, and 

 used in his laboratory, has been found almost indispensable in this 

 work. 



Clean rootlets may be procured readil}^ from active blueberrj^ 

 plants in the open spaces between half-rotted leaf blades, in clean 

 sand, in live sphagnum, or at the outer surface of the ball of soil in 

 earthen pots. Rootlets taken from live sphagnum are especially clean. 

 They are conveniently studied when simply placed in water on a 

 microscope slide under a thin cover glass held in place by a ring of 

 paraffin. 



Ordinarily the only thing visible in one of the live epidermal cells is 

 the minute cell nucleus lying close to the cell w^all. The protoj^lasmic 

 membrane lining the cell is very thin and is invisible except where it 

 is thickened to envelop the nucleus. The remainder of the cell is 

 filled with the colorless cell sap. An examination with medium en- 

 largements will show some of the cells faintly clouded in appearance. 

 A higher poAver, such as is afforded by a 2-mm. oil immersion objec- 

 tive and a 12-mm. eyepiece, with proper illumination, will resolve the 

 cloudiness into a mass of fungous threads, or hyphcT?. These may be 

 few, making only two or three irregular turns about the interior of 

 the cell, as occasionally found, or they may be more numerous, even 

 occupying the whole sap space, as shown in figure 17, in a dense knot 



"The spelling rnyrorhlzn is also in jrood standing and is used in many (lermau, 

 English, and American botanical works. 

 193 



