GROWTH AND MOVEMENT 427 



over some 70 C. The humidity of the air shows like hourly, daily, and 

 seasonal fluctuations, and the tree may be thrashed by a parching wind 

 or wrapped in a dripping fog. A gentle shower, torrential rain, or hail 

 may fall upon it within the hour ; and with a change of season it may be 

 weighed down by sleet and snow. The underground parts suffer less 

 extreme variations of temperature than the top. The water content 

 of the soil swings from the drought of summer to the saturation of late 

 winter and spring, and the solutes vary more or less in concentration 

 with the rains and evaporation. Combine all these in as many ways as 

 possible, and some idea is obtained of the variations in external con- 

 ditions which may affect the plant. 



Adjustment. 1 To many of these a plant must be able to adjust itself 

 on pain of death, and suitable response to others is advantageous. The 

 plant is indeed a self-adjusting * mechanism, whose reactions are often- 

 times more delicate than those of our own bodies, with all their special 

 senses and complicated sense organs. Thus, many a tendril is sensitive 

 to a mechanical stimulus which we cannot perceive, even by the tip of 

 the tongue, the portion of the body most sensitive to contact ; and some 

 plants distinguish differences of illumination which are inappreciable to 

 the eye. On the whole, it is perhaps fair to say that plants are more 

 responsive than animals. The plant has mostly to take what comes and 

 make the best of it ; the animal often takes shelter from unfavorable 

 conditions or migrates to a gentler climate. 



Intricate relations. It is extremely difficult to disentangle the com- 

 plex of forces acting on a plant and to assign to each its special influence. 

 Out of them all only a few have yet been isolated. What are known 

 as general or formative stimuli, namely, the totality of physical conditions, 

 external and internal, which determine the general course of develop- 

 ment and consequently the form of the plant as a whole or of any par- 

 ticular organ, furnish especially intricate problems, because it is so dif- 

 ficult to alter only one condition experimentally, or to evaluate the 

 influence of those which cannot be controlled. Experience is showing, 

 too, that so-called special stimuli, i.e. those which act locally, such as 

 gravity, light, -heat, etc., are interrelated, and their effects are unexpect- 

 edly interwoven. No phase of plant life requires more careful experi- 

 mentation and more caution in inference than the study of stimuli and 

 the responses to them. 



1 This term must be understood as if it were applied to a steam engine or a dynamo, 

 both of which adjust themselves automatically to thir " load." 



