PLANT DISEASE AND THE "VICIOUS CIRCLE." 311 
styptics. Again, wounds are more slowly occluded by callus in old 
age than in youth. Such predisposing factors enable many organisms 
successfully to effect an entrance and to weaken the host further. 
According to recent researches by Appel, excess of air in the tissues 
associated with insufficiency of water is another common cause of 
lowered resistance. 
Parasitic invaders may abstract their food from the host-plant by 
various methods. Some ramify in the intercellular spaces and middle 
lamellae ; others send haustoria into the actual cells. Many secrete 
enzymes or toxins which destroy cells or cell-walls, the materials of 
which then promote further growth and proliferation of the parasites. 
Hence fresh enzymes or toxins are secreted for the destruction of 
remoter cells, which in their turn fall a prey to the ever-spreading 
invader. Thus the morbid process vires acquirit eundo. 
The effects of circular reactions may be briefly discussed under 
three headings : (I.) The Perpetuation of Disease ; (II.) The Destruc- 
tion of Organs ; (III.) The Termination of Life. These groups, how- 
ever, are by no means sharply defined ; diseases placed in I. and II. 
may under exceptional conditions prove fatal, while diseases placed 
in III. may be so chronic as scarcely to shorten the duration of life, j 
f. — The Perpetuation of Disease. 
The perpetuation of a disease through insufficient chlorophyll 
assimilation has already been referred to. In other cases the morbid 
process may be initiated by living organisms. 
Both the true fungi as well as Schizomycetes (Bacteria) and 
Myxomycetes (Slime Fungi) may be concerned. 
Peridermium Pint. — A striking example may be found in the 
case of pine-blister caused by Coleosporium senecionis (Peridermium 
Pint var. corticola), a fungus which attacks the cortex of the Scotch 
and Weymouth pines amongst others. The hyphae grow in between 
the green cells of the cortex as well as in the bast-tissues, and may even 
penetrate the medullary rays and resin-canals. Other hyphae pierce 
the cells, consume the starch and other food stuffs, and cause a serious 
loss of resin which both soaks into the wood and exudes from the 
bark. This loss of resin involves a serious impairment of vitality. 
Moreover, the effusion of turpentine into the wood interferes with 
conduction in these tissues and lowers the nutrition of the tree, 
especially above the point of attack, since the flow of sap is checked. 
Meanwhile the parasite nourishes itself on the juices which it has 
liberated and on the contents of the cells it has invaded. Thus a 
struggle takes place which may last for many years. If the tree is 
vigorous it may, by the diversion of metabolic material, form sufficient 
cork to shut in and suffocate its enemy. But as a rule the invader 
extends its ravages and converts a robust thriving tree into a dwarfed 
sickly one. The more the parasite can arrest the flow of sap and the 
greater the loss of resin the more is vitality impaired, and the less 
the vitality the more rapid the progress of the invader. Cause and 
