DISCUSSION OF DEPENDENT PLANTS 389 



responding in any way to some application of energy which 

 serves as a sthnulus. Professor W . Pfcffer tlius ilhistrates the 

 chain of events that occurs : An alarm clock whicli is wound 

 up, but not going, receives a shake (stimulus) which starts its 

 wheels, so that after a time (latent period) the clock sounds 

 its alarm (result). The sensitiveness of the clock to the jar 

 which starts it, corresponds to the irritability of the proto- 

 plasm. Among the stimuli wliich call forth responses from 

 protoplasm are heat, light, electricity, gravity, pressure of 

 external objects, and contact with substances which produce 

 chemical effects on the protoplasm. 



360. Examples of responses to stimulation. The earlier 

 chapters of this book are full of instances of irritability called 

 into action by appropriate stimuli. Indeed, the whole life his- 

 tory of a plant is a series of responses to stimuli ; every newly 

 sown wheat held in which the grain lies comparatively in- 

 active during a succession of cold days, and then, when 

 warmer weather comes, suddenly begins to sprout, is an ex- 

 cellent illustration of the response of protoplasm in the seed 

 to heat. 



Eespoiises to light are very common, one of the most evi- 

 dent being the jDrompt excretion of (jxygen noted when a 

 green aquatic plant arranged as shown in Fig. 12 is brought 

 from darkness into sunlight. 



The response to gravity is shown by the downward growth 

 of the roots and upward growth of the stem in very young 

 seedlings. 



The response to pressure is most evident in the coiling of 

 a tendril about any slender support, which begins almost as 

 soon as the tendril touches the foreign object. 



Responses to chemical stimuli are extremely common and 

 of great importance in the life of the plant. One familiar 

 instance is the manner in which roots grow toward masses of 

 fertilizer or rich soil (Fig. 348). Another case is the huddling 

 of swarms of bacteria about a filament of any green alga 

 which is giving off oxygen during photosynthesis. 



