PLANT DISEASE AND THE " VICIOUS CIRCLE." 
is due to a disturbed equilibrium in the distribution of turgor." In 
other words, the process of premature cleavage is due to an arrest of 
metabolic activity causing pathological turgor at the basal zone 
followed by a progressive loss of activity. 
A similar process accounts for the premature shedding of twigs 
or shoots which may occur as early as July. The oak and the poplar 
are frequently affected, the ground being in some cases thickly strewn 
by the detached twigs. 
The destruction of organs may also be due to the attacks of 
parasites, some of which display a preference for certain organs, such 
as leaves, twigs, tubers or buds. A lowered state of vitality predisposes 
to such parasitic invasion, which then further lowers nutrition, 
culminating in the total destruction of the organs. 
1. Lophodermium Pinastri. — As an example of a parasitic leaf- 
shedding disease may be mentioned the Pine Needle-cast caused by 
the fungus Lophodermium Pinastri, also called the Pine Leaf-scurf 
or leaf-shedding fungus. 
The disease chiefly attacks the young and tender leaves of the 
Scots and Austrian Pines and other Conifers, and may cause many of 
their leaves suddenly to wilt and drop. The needles when first 
attacked are merely speckled with brown spots containing the 
mycelium of the fungus. But in the following year they wither, 
turn red or brown and die off in hundreds. This loss of leaves seriously 
weakens the plants and thus hastens the progress of the disease. 
The greater the loss of foliage the less the power of resisting infection. 
It is for this reason that the parasite is most destructive in shut-in 
valleys or low-lying situations, where the trees possess least vitality 
and consequently succumb most readily. 
2. Ustilago. — The destruction of : plant organs as a result of the 
circulus vitiosus is also illustrated by the action of the cereal smuts 
(Ustilago), which attack such grains as are rendered liable to infection 
by lowered vitality. The smuts reduce the ovules to a black powdery 
mass of spores, which are carried away on the wind or otherwise 
dispersed, leaving nothing but the bare axis on which the flowers 
were originally situated. 
III. — The Termination of Life. 
Many examples might be given of the death of a plant as a result 
of the morbid process under discussion. One fatal disease due to a 
fungus and one due to a bacillus may be referred to. 
Dasyscypha Willkommii. — The well-known larch canker, associated 
with the Dasyscypha Willkommii, is an example of a fungus disease 
which is responsible for the loss of an enormous number of trees in 
our woodlands. The larch is indigenous in the Alps, where there is 
a long winter season, followed by a short or no spring, and by a short 
hot summer. Owing to the rapid transition from winter to summer 
the larch buds open very rapidly when once they start. Hence the 
period during which the foliage is young and tender and susceptible 
