526 
ing the body moved the connected parts. 
Plant movements, especially those due to 
changes of turgor, were long looked upon as 
the main evidence of irritability in plants. 
This conception was reflected in the text- 
books of the older day and still survives in 
many of the more elementary works. 
After the bearing of irritability on move- 
ments was firmly established, it came to be 
seen that the regulation of the rate of 
growth and its resumption by certain parts 
which had ceased to grow was accomplished 
through irritability. Growth, therefore, as 
well as movement, had important relations 
to irritability. But during the past decade, 
particularly, a better conception has been 
taking possession of physiological students. 
It is now perceived that all protoplasmic 
functions are initiated or controlled by ex- 
ternal physical or chemical agents. This 
point of view is reflected in that masterly 
treatise of Pfeffer, the second edition of his 
Pflanzen-physiologie. Throughout the first vol- 
ume, discussing the physical and chemical 
phenomena connected with metabolism, the 
ability of the protoplasm to regulate its own 
operations and to control even the physical 
changes in adjacent parts is everywhere 
presented and insisted upon. 
The idea of a stimulus, instead of being 
confined, as it once was, to the action of 
heat, gravity and moisture, has now been 
greatly extended. Any external or internal 
change, slight or profound, gradual or 
sudden, which cails forth a corresponding 
change in the living protoplasm, is to be 
looked upon as a stimulus. The responses 
to stimuli, too, once thought of largely as 
those visible in curvature of motor organs 
or growing parts, are now conceived as of 
great variety. Invisible reactions probably 
outnumber the observable ones. Those pro- 
ducing a change of bodily form must be 
relatively few as compared with those which 
influence the performance of function or the 
course of development. 
SCIENCE. 
[N.S. Von. X. No. 245. 
Diverse and numerous as are the stimul! 
which act upon plants, any conception of 
their operation would be faulty which fails 
to take into account the fact that stimuli 
of many unlike kinds and of unequal in- 
tensity are interacting to bring about the 
peculiar form and behavior of each in- 
dividual plant. Think of the external 
agents which are known to be acting upon 
an ordinary land plant. About the aerial 
part the temperature varies from season to 
season, in our temperate zone changing from 
30° below C. zero to 50° above; it varies 
from month to month and from day to day, 
even from hour to hour. The light differs 
in intensity and direction from day to night 
and from hour to hour. It changes in its 
actinic effect, as the photographer well 
knows, in the course of a few minutes ; a 
variation, by the way, whose effect on 
plants has been entirely unstudied as yet. 
The moisture in the air is hardly the same 
for any two consecutive days; the plant is 
deluged with water for some hours or days 
and dry between rains; it is enveloped in 
fogs and mists; wet with dews at night, 
and all but blistered by the sun during the 
day. Its subterranean part is surrounded by 
solutions whose amounts and composition 
are probably varying hourly; whose con- 
centration and consequent dissociation is 
changing from time totime. The tempera- 
ture of the soil is scarcely the same from hour 
to hour; it varies between day and night, 
from day to day and from season to season. 
Imagine now the numberless combinations 
possible among these varying factors, and 
remember that all these interact as stimuli 
upon the protoplasm. What wonder, then, 
that no two plants are alike ; that Capsella 
may flower at 5 em. height with a few 
minute entire leaves, or may grow ten 
times higher with abundant foliage and 
long racemes of fruitful flowers ! 
This different conception of irritability 
and its relations to the functions of the 
