Pethybridge — The Verticillium Disease of the Potato. 91 



XI. Explanation of Plates. 



Plate II. 



Fig. 



1. A potato plant derived from a tuber naturally infected with Verticillium 



albo-atrum, and showing in the upper leaves of the central stalk the 

 first external symptoms of the disease. Photographed August 12th, 

 1913. 



2. The same plant completely killed as a result of the attack. Photographed 



September 17th, 1913. 



3. The plant on the left (a) was inoculated in a sprout on May 11th, 1914, 



before planting, with a pure culture of V. albo-atrum. The photograph 

 was taken on July 14th, 1914. The gradual and premature death of 

 the foliage proceeding from below upwards will be observed. The 

 control plant (b) was not inoculated, and was perfectly healthy. 



4. The same two plants photographed six days later when the inoculated 



plant was quite dead, but the control still healthy. 



Plate III. 



1. The mycelium of V. albo-atrum developing at the cut surfaces of wood 



vessels in the vascular ring of an old tuber which had produced a 

 diseased plant, x 50. 



2. The black mycelium of V. albo-atrum developed in pure culture. No 



chlamydospores are present, x 220. 



3. Portion of a tuber which had produced a diseased plant, with the cortical 



tissues removed and showing the strongly browned network of woody 

 tissue which still contained the mycelium of the fungus, x 1£. 



4. Longitudinal section through the xylem of an affected potato-stalk 



showing the blackened resting mycelium in the vessels and adjacent 

 elements of the wood, x 224. 



SCIENT. PEOC. R.D.S., VOL, XV., NO. VII. P 



