482 PHYSIOLOGY 
deleterious substances arising in the dead region, and so cause their 
injury or death; and third, because they have no nervous system, put- 
ting into quick communication sound distant organs with hurtful stimuli 
from the dead ones. Yet these differences, on the surface so marked, 
are in reality not fundamental, for what is general death in the animal 
is in reality only an extension of local death to the several tissues and 
organs more rapidly than in plants. But each part dies at its own rate 
and only because the interruption of the activity of one organ has created 
conditions unfavorable to the other. 
Irreversible reaction. The phenomena of death are not easily de- 
scribed. Certain changes in the appearance of the cytoplasm are visible 
under the microscope (such as are familiar in fixed cells and are too com- 
monly thought of as the normal appearance of cytoplasm), chiefly ag- 
gregation and vacuolation; but the significance of these is not known. 
Alteration in the chemical processes and different behavior, especially 
permanent insensitiveness to external stimuli, are the most important 
marks of death. During life the protoplasm is constantly adjusting itself 
to new conditions, each response suited to the stimulus, whether in a 
favorable or unfavorable direction. These responses of normal life 
are assumed to be reversible, as are many chemical reactions. But 
when the responses to severe stimuli become irreversible in too great 
measure, the possibility of readjustment to new stimuli is past ; this con- 
dition -is death. 
Diseases, Plants are often killed by diseases which may arise from 
the disturbance of function wrought by external agents, such as the ele- 
ments of climate, the solutes of the soil, gases in the air, etc. Or disease 
may be due to the invasion of the body by parasites, which rob the host 
of food, interfere with its water supply, or upset some necessary function. 
A study of diseases forms a great field in itself, plant pathology, under 
which name therapeutics, the study and application of remedial measures, 
is also usually comprehended. It is one of the divisions of botany which 
is of great economic importance, and one whose study has reached its 
highest level in this country, where the remedial and preventive meas- 
ures devised save annually many millions of dollars. The knowledge 
of infectious diseases has been most extensively developed, but therein 
a great field for investigation still lies open, and a still greater one in the 
more difficult study of functional disorders. 
Mechanical injury. Mechanical injuries often lead to death, es- 
pecially because they expose the plant to infection by bacteria and fungi. 
