. The points of many of the leaves turn brown, curl
up, and die.

The ascending sap, not being fully elaborated by the diseased leaves,
oozes out through the skin of the stalk in a thick, viscous state, and
the plant to all appearance is in a state of consumption.

At this stage the ever-present minute spores of the _Botrytis infestans_
eagerly pounce on the sickly plant, fastening themselves on its most
diseased parts. The _Botrytis infestans_ is a cryptogamous plant, and is
included in the Mucidineous family, (moulds.) It is a vegetable parasite
preying upon the living potato plant, like lice or other animal
parasites upon the animal species.

At first this mould forms webby, creeping filaments, known in botanical
language as mycelium. These root-like fibres then branch out, sending
out straight or decumbent articulated stems. These bead-like joints fill
up successively with seeds or spores, which are discharged at the proper
time to multiply the species.

Under favorable conditions of warmth and moisture, the mycelium spreads
very rapidly. Spores are soon formed and matured, to be carried to
plants not yet infected. Rains also wash the seminal dust down the
plant, causing it to fasten and grow on the vine near the ground. The
roots of the parasite penetrate and split up the stalk even to the
medullary canal.

These roots exude a poisonous substance, which is carried by the
elaborated descending sap down to the tubers, and as the largest tubers
require the largest amount of elaborated sap for their development, they
will, consequently, receive the greatest quantity of the vitiating
principle, and will, on digging, be found a mass of rottenness, when the
smaller ones are often but slightly affected. The _Botrytis infestans_
can not gain a lodgment on vines that are truly healthy and vigorous,
high authority to the contrary notwithstanding.

Healthy varieties, growing in a sheltered situation on dry, new soil, to
which no nitrogenous manures have been applied, can not be infected,
though brushed with other vines covered with the fungus. Different
varieties, and sometimes different members of the same variety, are not
always alike affected by the disease, though growing in the same hill.

As will be noticed, the potato disease is rather an effect than a cause,
and appears to have been designed to prevent members enfeebled by
accident or otherwise from propagating their species by putting such
members out of exist