[Illustration:  No. 4]

No. 4 represents a longitudinal section of Potato-stalk with germinating zoospore, the germ-tube of which has pierced the cell-wall, and is growing inside the cell, as shown at +.

[Illustration:  No. 5]

[Illustration:  No. 6]

No. 5 affords a view of another piece of tissue of the stem of a Potato plant, and shows the hyphae of =Phytophthora infestans= running in the cell-walls; (=a=) nucleus of a cell; the other contents shown are crystals and chlorophyll corpuscles.

No. 6 is a section of a Potato tuber:  A, the cell-walls; B, the starch grains; C, the mycelial hyphae.

Spraying Potato plants twice or thrice with Bordeaux mixture has proved effective in warding off the attack of =Phytophthora infestans=, and the practice is now freely adopted, especially in humid districts.  The first application should be given towards the end of June or early in July, immediately the haulm is sufficiently developed.  The Bordeaux mixture is made in the proportion of four pounds of pure copper sulphate and two pounds of quicklime to forty gallons of water.  The foregoing quantities will give what is known as the =one per cent.= mixture.  For the =two per cent.= mixture the quantities of copper sulphate and quicklime must be doubled, but the amount of water should remain at forty gallons.  In its effect on the fungus, however, little difference is to be found between the two solutions.  The copper sulphate is stirred into a few gallons of hot water placed in a wooden tub or earthenware vessel.  When quite dissolved, add twenty or thirty gallons of cold water.  The lime, which must be freshly burnt quicklime, is then slaked in another vessel and thoroughly stirred with two or three gallons of water until it is of the consistency of thin cream.  As soon as the liquid is quite cold, filter it through coarse sacking into the copper sulphate solution and add water to make a total of forty gallons.  To be effective, Bordeaux mixture must be applied in the form of a fine spray, and not with a coarse-holed syringe.

The Burgundy mixture, the use of which is preferred by some, acts in a very similar manner to the Bordeaux mixture, and is made in the same way as the latter, except that washing soda (five pounds) is substituted for quicklime.

Those who leave Potatoes to rot in the ground because the crop is not worth digging, or who bury diseased haulm and tubers in a shallow trench, under the impression that it is a safe way of getting rid of worthless vegetation, are simply storing =Phytophthora= for another attack in the event of Potatoes being planted in the same land again.  If buried at all, it must be at a considerable depth, but the effectual method is to destroy all Potato refuse by fire.