36 PRESENT DISEASE IN THE POTATOE. 



globules floating about in a fluid among broken down cells 

 and spiral vessels. 



(139.) Such is the course which the disease takes in 

 gangrena humida ; but the appearances in gangrena sicca 

 are somewhat different. In this form of disease, the pota- 

 toe, when entirely affected, may retain its outward form ; 

 and yet, on being cut open, there will be found a large 

 irregular cavity in its centre, which cavity often takes the 

 form of an imperfect X. (Plate vii., fig. 4.) 



(140.) To form this cavity, the mass of the potatoe has 

 shrivelled up, and presents either a white or brown appear- 

 ance. If this matter be examined microscopically, it will 

 be found to consist of starch globules, with perhaps some 

 fine lines which have replaced the structure of the plant, 

 and the entire cellular tissue will have vanished. Under 

 certain circumstances in this case, the tuber becomes as 

 hard and as dry as a bit of wood, and will even in some 

 cases bear polishing. Sometimes a potatoe is found to be 

 partially affected with dry gangrene, in which case it will 

 fall in or contract on one side, showing a pit on its outer 

 surface. 



(141.) A potatoe occasionally shows a cavity in its in- 

 terior, which presents little rounded masses, which, under 

 the microscope, show masses of globules of starch, with 

 fungoid growths. (Plate vn., fig. 10.) 



(142.) When the disease attacks the plant and cuts 

 down the haulm, the crop of tubers is rendered deficient 

 according to the period of growth of the haulm when it 

 dies. This is consonant with facts previously known, for 

 Marshall found that after the haulm was cut he observed 

 the same result. He " took up two rows of equal length, 

 the haulm of one of which had been previously cut, that of 



