36 Mil. SOLLY ON THE POTATO MUllRAIN. 



the smallest ravages ; tlie tuliers wliich were imperfectly ripened, 

 and those which were still quite young at that period, for the 

 most part sustained the greatest injury. 



Before proceeding to examine the nature of the effects pro- 

 duced in the tubers by this disease, it will be necessary to describe 

 the chemical composition of ordinary potatoes. Potatoes consist 

 of starch, fibre, gum, resin, azotised matter, inorganic salts, and 

 water ; the relative proportion of these various substances varies 

 considerably, depending on the variety of tlie potato, the soil in 

 which it grew, and the peculiarities of the year in which it was 

 formed. Generally speaking, tiie proportion of water constitutes 

 three-quarters of tlie whole bulk, 75 being about the average 

 per centage, but it varies from 66 to 82. The following analysis 

 of a bread-fruit potato, of 1842, will serve to give a tolerable 

 approximation to the ordinary composition of the tubers : it is 

 selected out of many similar analyses. 



Starch 13020 



Gum, with a portion of sugar .... 3050 



Fibre 4000 



Soluble azotised matter —Albumen . . . 953 



Insoluble azotised matter— Gluten . . . 1150 



llesin and wax ...... 750 



Water /•')H31 



Potash and Soda, with (organic acids.) . . 982 



Sulphuric acid . . 48 



Chlorine ... C7 



Earthy phosphates ...... 1G3 



Lime ........ 17 



Magnesia ....... 24 



Oxide of iron ...... 9 



Silica ........ 



looboo 



Tiie composition of potatoes varies also considerably in the dif- 

 ferent stages of their growth ; when young, the proportion of 

 earthy matters, azotised matter, and water, is frequently much 

 higher tlian wlien ripe ; the proportion of water is almost always 

 from 5 to 10 per cent, higher than in the mature tubers, being 

 often higher than 80 ])er cent. On examining plants attacked 

 with the disease, it was found that in some the disease first made 

 its appearance in the leaves, in others the stems were first aftected, 

 whilst others again had sound and healthy to})s whilst the tubers 

 were affected ; in every case however \\hich I examined, the 

 under-ground stem, or that part of the main stem of tlie plant, 

 just below the surface of the soil, fiom which the roots proceed, 

 was found to be more or less decayed ; in a great many cases the 

 roots were entirely rotted, and fresh roots had evidently been 

 formed by the action of the still growing haulm. 



The composition of the sound tubers of diseased plants did not 



