Unlike the Potato disease, which spreads from plant to plant through the atmosphere, the fungus of Finger-and-toe infects the ground, and from the first spot attacked the disease spreads rapidly in all directions and in various ways.  It may be carried by the soil adhering to implements or the boots of labourers.  And each patch becomes a new centre of infection which is spread by digging or raking.  Every scrap of infected soil, or of diseased fibre which may be added to the manure-heap, distributes the virus over a wider area, so that Finger-and-toe may suddenly appear in parts of the garden which have hitherto been free from this troublesome pest.  A very simple experiment will prove the certainty and ease with which the spores may be introduced to fresh land.  Macerate the tissue of old Finger-and-toe in water; use this on young isolated plants of Cabbage or Turnip and in a short time the plants will be infected.

The fungus which produces Finger-and-toe is known as =Plasmodiophora brassicae=, and it belongs to the =Myxomycetes=, or {~~}slime-fungi,’ which, as a rule, live upon decaying vegetable material.  The protoplasm of the fungus ramifies among and within the tissues of the roots of attacked plants, and eventually produces an amazing number of spores so small that more than thirty millions would be required to cover a superficial inch.  A microscope of great power is necessary to reveal them to human vision.

[Illustration:  Fungus of finger-and-toe disease =Plasmodiophora brassicae=]

The spores are capable of resting in a state of vitality for a long time, and can easily withstand the frosts of winter.  The illustration shows at A the fungus in its protoplasmic condition, and at B its ultimate sporiferous or ’seed’-producing stage, after the protoplasm has changed to a mass of minute spores (enlarged five hundred and twenty diameters).  When a spore in due course germinates, its protoplasmic contents escape through a small aperture in its wall and begin moving about of their own accord in a slow writhing manner.  The movement is so much like that of the microscopic animal organism found in ponds, and called =Amoeba=, that this tiny mass of moving protoplasm is called =Myxamoeba=, to denote that it is an amoeba-like form produced by one of the =Myxomycetes=.  Each myxamoeba is drawn out at one spot into a fine delicate tail or cilium, as at C, D, E, and is capable of a creeping motion in moisture.  When quite free from the spores, transparent expansions or limbs extend from the bodies of the myxamoebae, as at F, G, and when these organisms, after existing in the soil for a longer or shorter time, reach the roots of cruciferous plants, which they apparently enter through the root-hairs, they again assume the protoplasmic condition shown at A, and live within the cells, at the expense of the nurse-plant.  Other cruciferous plants are less seriously damaged by the pest than