282 Journal of Agricultural Research voi. xviii. No. s 



This apparatus (Pi. 35)^ consisted of a modified Joly balance, accurately 

 graduated, and with a vernier for close reading. The lower end of the 

 spring was attached to a metal rod which passed through a short glass 

 tube fixed to the stand of the instrument. Hair lines on both the tube 

 and rod made it possible to determine accurately the point at which the 

 tension on the spring balanced any given weight. Tension was applied 

 to the spring by means of a rack and pinion adjustment. It was possible 

 with this balance to weigh to a milligram, which was well within the 

 limits of experimental error in these determinations. A glass rod, the 

 weight of which was less than the capacity of the balance, was suspended 

 from the pan of the balance, and a small glass needle with a rounded end 

 was attached to the lower end of the glass rod. In operating this appa- 

 ratus a slice of potato was placed on the stage of the instrument, which 

 was so adjusted that the tip of the needle just touched the surface of the 

 potato when the hair line of the indicator on the spring coincided with 

 the hair line of the fixed indicator of the balance. The tension on the 

 spring was then slowly released by means of the rack and pinion adjust- 

 ment until a sudden drop of the needle indicated that the tip of the needle 

 had penetrated the tissue. The position of the column that supported 

 the spring was then noted on the graduated scale. The weight required 

 to balance the pull of the spring at this point was determined and sub- 

 tracted from the weight of the needle. The result was the weight 

 required to push the needle into the potato tissue. 



Inasmuch as the needles used in these experiments were always from one- 

 sixth to one-fourth the average diameter of potato cells, it is evident that 

 in most cases at least the needle was pushed through the cell wall and that 

 the weights obtained were a close approximation of the pressure necessary 

 to break through the cell wall. The needles were drawn from small glass 

 tubing over a micro burner and were drawn out in such a way as to leave a 

 relatively heavy shoulder so that the slender portion which was thrust 

 into the potato was not more than a millimeter in length. It was found 

 that long needles of the small size necessary in this work were very 

 easily broken. They were rounded and slightly larger at the end so 

 that friction against the sides of the puncture would be reduced to a mini- 

 mum. The needles used in these experiments were from 58.3 to 71 

 microns in diameter. In practice 20 determinations were made on each 

 tuber, 10 in the cortex and 10 in the central portion within the ring of 

 bundles. The weights obtained were averaged for each region; and, 

 since the diameter of the needle was known, the weight required to break 

 through tissue per square millimeter of surface was readily calculated. 

 It was shown in this work by using different needles on the same potato 

 that the weights required to puncture potato tissue were about propor- 



'The authors are indebted to Mr. H. K. Sloat, of the Division of Illustrations, for photographing the 

 motion pictures, and to Mr. J. F. Brewer, of the Laboratory of Plant Pathology, for preparation of the 

 plates in this article. 



