42 BIOLOGY: GENERAL AND MEDICAL 



greener, the flower buds have opened, the buds upon 

 the trees have doubled their size or perhaps burst into 

 tufts of tender leaves, the great bumble-bees of spring 

 fly to and fro in a business-like manner, the toad is 

 piping in the nearby pond, and the snake glides noise- 

 lessly but quickly out of the way as the vibrating earth 

 tells of your approach. 



In these examples the thermic irritability is mani- 

 fested by changes so gradual that it is only through 

 observation of their aggregate results that they can be 

 detected, but examples are not wanting to show that 

 upon those plants and animals so constructed as to en- 

 able us to observe them temperature exerts an immedi- 

 ate response. This is the .case, however, only when the 

 variations are considerable and the changes sudden. For 

 example, if a Mimosa be cautiously approached by either 

 a hot or cold object, the greatest care being exercised 

 to avoid mechanical contact with the plant, the sudden 

 change of temperature is sufficient to excite the 

 irritable cells and provoke closure of the leaves. It is 

 not improbable that the irritability of living matter 

 is susceptible to the stimulating effect of any sudden 

 change. 



The subject must not be dismissed without question- 

 ing whether cold, as well as warmth^ may not have a 

 stimulating effect. Cold has a marked inhibitive action 

 upon the vital processes and when sufficiently intense 

 may terminate them; but that it acts as a stimulus as 

 well is not impossible, for certain bulbs are found to 

 grow more rapidly, and their plants to bloom more 

 quickly if they are exposed for a short time, before 

 planting, to an unusually low temperature. 



The diminishing temperature of approaching winter 

 may be responsible for the plentiful growth of hair and 

 feathers in many animals and birds. 



The application of cold to the human skin is followed 

 by vasomotor stimulation resulting in contraction of 

 the peripheral blood vessels, and the effect of a cold 



