310 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY, 
and other systems sympathetically, and these secondary disorders 
in their turn injure the organ primarily affected. Thus is the circulus 
vitiosus established. Doubtless to some extent one organ can give 
vicarious assistance to another in difficulty. But this power of relief 
is limited since there is no great provision of reserve. 
In the diseased plant similar injurious interdependences are 
observed. 
A common illustration is afforded by root starvation leading to 
an inadequate supply of nutritive material to the assimilating leaves. 
Their metabolic activity is impaired, and such impairment reacts 
injuriously on all other organs. Growth and development are checked 
and there is less material to transport. Further root starvation takes 
place, and the sequence of events is repeated and intensified. 
A similar concatenation of injurious factors may start from any 
other tissues. If the leaves of a plant are so feebly illuminated that 
assimilation is reduced to a minimum, far-reaching results obtain. 
The stem remains thin ; the development of the cambium layer is 
checked ; the supplies of nutriment passing to the roots are insufficient 
for their growth and for the formation of new root-hairs. Absorption 
of water and salts is interfered with, and this in turn further curtails 
the functional activity of the leaves. Here also is established a 
mutual causal relation between disease of various organs. 
There is, however, a striking difference in the manifestation of the 
morbid process in animals and in plants. 
In animals, owing to the higher differentiation of organs, numerous 
specific circuit vitiosi are met with, and fresh examples are frequently 
discovered. Thus disease of the blood provokes disease in other 
tissues, which in their turn pour products of perverted activity into 
the blood, and it is possible to study this endless chain of disorders 
link by link. 
Future research may yield similar processes in plant pathology. 
But at present specific effects of morbid reactions are almost a terra 
incognita owing to lack of detailed knowledge of functional disorders 
in plants apart from those caused by attacks of parasites. On the 
other hand, the general principle is in universal operation, manifesting 
itself not by specific results but by the production of a lowered 
resistance to morbific agencies, and playing a part of great importance 
in the growth and life-history of the plant. 
It may be of interest to summarize briefly some causes of lowered 
resistance, and then to discuss its effects. Immaturity of tissue is a 
common cause of impaired resistance. The epidermis in early life 
is both tender and thin, and may be penetrated by bacteria or fungi 
that are powerless to injure plants whose epidermis has become 
cuticularized or replaced by cork. On the other hand, advanced 
years also predispose to infection. In young coniferous trees well 
provided with resin canals injuries in the cortex are at once sealed 
by an exudation of turpentine and thus protected from wound fungi, 
while in older trees turpentine and resin are less freely exuded as 
