126 

 APPENDIX. 



THE BESTING SPORES OF THE POTATO 

 DISEASE. 



Extracted from the" Gardener's Chronicle'* July 1875. 



I now give in conclusion an illustration of the perfectly 

 mature resting-spore of Pernospora infestans, as seen im- 

 bedded in the substance of the Potato leaf. These resting- 

 spores, which carry n the winter life of the fungus, are 

 not restricted to the leaves, for I find them sparingly in 

 both haulm and tuber, although I have at present se^-n the 

 best specimens in the leaves. The engraving given here- 

 with (iig. 19) shows a transverse section through a black 

 spot of one of the leaves from Chiswick, and the resting- 

 spore is seen at A nestling in amongst the cells of the leaf. 

 An antheridium, B, and two oogouia (c. c.,) from which 

 such resting-spores arise, may be seen in the cut, and the 

 old common form of the fungus will be noticed breaking 

 through a hair on the upper surface of the leaf, which) is a 

 very uncommon occurrence. The situation of the resting 

 spores can generally be ascertained on the leaves by noticing 

 the slightly thickened and very dark spots, for the bodies 

 are commonly in these spots. It is, however, an extremely 

 doubtful matter either to get them out, or, indeed, to see 

 them when imbedded, for, when mature, they are black- 

 brown in colour, and only a little larger in size than the 

 leaf-cells. These leaf-cells are also intense brown-black in 

 colour from contact with the hurtful mycelium, and almost 

 as hard as wood. The best way to see the resting-spores is 

 to macerate the leaves for several days in water, and then 

 setting them free by crushing the spot between two slips 

 of glass, The presence of the fungus in the leaf makes the 

 cells very thick and woody as well as black, so that in 

 crushing the leaf cells the resting-spore is not uncommonly 



