32 PHARMACEUTICAL BOTANY 
to stimulation. When matter fails to be irritable or responsive 
to stimuli, we declare it to be dead. The stimuli that excite 
reactions in living matter are of two kinds, viz., intrinsic and 
extrinsic. 
Intrinsic stimuli are inherited stimuli, being carried by the 
chromosomes from parent to offspring. They determine that 
the plant shall conform to a particular type, carry on certain 
activities, pass through a definite life cycle, and detach a portion 
of its own substance for the formation of new individuals of its 
kind. 
Extrinsic stimuli initiate, inhibit, accelerate or modify the 
effects of intrinsic stimuli. They comprise agents of the external 
world such as cold, heat, chemicals, food, water, light, oxygen, 
electricity, gravity, etc. 
Tropisms, TAxres AND Nasties.—The irritable reactions 
manifested by protoplasm and living things to the effects of these 
external agents are termed tropisms, taxies or nasties and will now 
be considered briefly. The term tropism is now largely limited 
to the response of non-motile plants and is usually applied to 
growth movements of these in response to external stimull. 
The response made to a stimulus is usually designated by a 
prefix name which implies the kind of stimulus, as for example, 
the response to light is called phototropism, to gravity, geotropism, 
etc. A response in the direction of the source of a stimulus is 
called positive while one which is away from the source of the 
stimulus is designated as negative. ‘The term taxy (pl. taxies) is 
applied to responses of motile plants and plant cells, as gametes 
and zodspores, which involve locomotion. The term: nasty 
(pl. nasties) is applied to responses of non-motile plants induced 
by stimuli in which the movements have no particular direction, 
as for example movements brought about in some flowers as a 
result of the succession of night and day (nyctinasty). 
TuermorropisM is the response of living substance to the 
stimulus of temperature. All living substance is influenced by 
variations in temperature. Freezing disintegrates it while 
excessive heat causes its coagulation. Active vital phenomena 
are therefore only evident within these extremes, the limits 
differing depending upon the endurance of the organism under 
