142 THE POTATO DISEASE. 



the brown colour less deeply, often they remain colourless ; the 

 walls of the epidermis alone present a deep colour. ' These, then, 

 are the parts that the parasite does not immediately touch that 

 offer the most perceptible alterations. The cortical and epidermic 

 cellules of the stalk are in great part filled with a watery liquor 

 containing but few granules, and upon the brown spots it is par- 

 ticularly the membrane that present the deep colour. But, in 

 observing the penetration of the germs, and the progress of the 

 mycelium in these parts, one often sees that the coloration of the 

 membrane commences at the point that is first touched by the tube 

 of the parasite. Parting from this point, the brown colour extends 

 itself little by little around the rest of the touched wall, and spreads 

 itself successively upon the most distant cellules, which have no 

 contact with the Peronospora. One thus sees that the parasite 

 alters the point that it first immediately touches, and that the 

 alteration propagates itself upon the perfect elements of the tissue. 

 It is thus that the brown coloration often extends itself to a dis- 

 tance of some centimetres, either in the superficial parenchyma or 

 in the vascular bundles. 



In the tubers the wrinkled and brown parts that characterize the 

 disease are always occupied by the Peronospora. I will not repeat 

 here the numerous descriptions that we possess of the structure, 

 and of the alterations of these parts. I will only add the fact that 

 the mycelium always creeps along the brown cells. It has been 

 already seen, without doubt, by Martins,* who, in describing the 

 diseased tissue, makes mention of intercellulary passages filled with 

 granular matter. In examining attentively the tissue in question, 

 one can easily find these pretended passages, but at the same time 

 one can convince oneself that these are the ordinary tubes of the 

 mycelium, furnished by a proper membrane, often thickened, mak- 

 ing themselves passages amongst the cellules of the parenchyma. 

 It is not always easy to find or follow these tubes, because the 

 brown tissue is too opaque for one to well observe them in the 

 microscope in thin slices, and because in these very thin slices the 

 tubes are frequently cut, and are in consequence little visible. 

 There is, nevertheless, a means of convincing oneself of the pre- 

 sence of the mycelium, and of proving rigorously, at the same 

 time, that the intercellulary tubes belong in reality to the Perono- 

 spora. When a diseased tuber is cut and shielded from dessica- 

 * " Die Kartoffel-epidemie," by Dr. Fr. P. V. Martins, Munich, 1847. 



