251 
MICROSCOPIC OBSERVATIONS. 
By THOMAS TAYLOR. 
Under date of April 25, 1873, Martin McKenzie, of Boston, Mas- 
sachusetts, informed this Department that he had planted in his hot- 
house, on January last, a healthy Early Rose potato, and also one of the 
same variety affected with the fungus, Peronospora infestans. While 
growing, both were treated as nearly alike as possible, were supplied 
with a superabundance of water to induce fungus growth, and were 
subjected to about 70° of temperature. Both varieties sprouted and 
sent forth the usual supply of rootlets, stalks, leaves, and tubers. 
The original tubers used as seed in these experiments, with their 
appendages, were forwarded to this Department for microscopic exami- 
nation, the object being to ascertain the relative value of diseased 
potatoes, as compared with healthy ones, when used for seed. The 
so-called healthy seed-potato, with its appendages as received, was 
Separately examined for fungoid forms, and especially for that of Per- 
onospora infestans, but neither its spores, mycelium, nor fruit were found 
on them. 
My attention was next directed to the unhealthy “ seed-potato,” its 
stalks, leaves, rootlets, and tubers. On the surface of the tubers were 
plainly visible, to the naked eye, small brown spots, represented by S, 8S, 
S, I, (Fig. 6,) some of which I carefully removed with a pen-knife. These 
i placed in strong nitric acid for several minutes. Other portions were 
submitted to the action of boiling caustic potash. As the mineral acids 
and alkalies render starch soluble and transparent, or rather transform 
it into new substances which are transparent and soluble, fungoid 
forms, if present, are more easily discovered by the use of these sol- 
vents. Potash has also the power of rendering the albuminates of the 
Skin soluble, and by its use the fungoid masses S, 8, S are more easily 
separated from the cellular matter of the potato. The fangoid cells 
are also rendered more transparent. When the brown spots S, S, 
S are viewed under a power of about 275 diameters, amber-colored 
budding cells, represented by a, are observed, from which protrudes 
amber-colored branched and jointed mycelium, as represented by 4, 
5, and 6. Before examining the rootlets (see arrows) I submitted 
them to boiling caustic potash a few minutes, to render them soft and 
easily compressed, and brought into focus. 7, 8, 9, and 10 represent 
their mycelium and rootlets. Their relative size is represented by the © 
scale A, C. A, A represent the parenchyma cells of the rootlets, B their 
vascular bundles, and C the mycelium on the rootlets. The cells a have 
the same relative size. 2 represents the vascular bundles of the tubers. 
The dark spots on the young tubers, 3, 3, are composed of clusters of 
budding spores, as represented by a, being of the same form and color 
as those of the parent tuber. The dotted lines on 1 represent the cel- 
lular structure of the skin of the tuber highly magnified. 
The stalks and leaves of the unhealthy Early Rose potato, although 
presenting to the naked eye a healthy appearance, were found to be 
interspersed with amber-colored mycelium peculiar to potato-rot. The 
only appearance observable to the naked eye was brown spots on the 
new tubers, the skins of which otherwise were smooth and transparent. 
Their fungoid markings, to an inexperienced eye, would not readily 
attract attention, as they appear like small portions of brown earth. 
