52 TEXTBOOK OF BOTANY 



recognize and understand the processes and conditions which bring 

 about the phenomenon he wishes to explain. From this point of view 

 natural events are assumed to follow each other in a cause and effect 

 sequence, and explanations are obtained only through the discovery of 

 the sequence of causes and effects. 



All of us, therefore, are faced with deciding whether we shall accept, 

 as explanations of natural phenomena, those that are based on needs 

 and purposes or those that are based on preceding conditions and 

 processes. 



Interpretations exemplified. Everyone is familiar with the evaporation 

 of water, with the formation of clouds, and with rain. Does water evapo- 

 rate from the earth's surface in order to form clouds, or is this evapora- 

 tion of water merely the consequence of a series of events that may be 

 traced to molecular motion? Do clouds form to produce rain, or are 

 clouds a consequence of a series of events that we may trace back to 

 the evaporation of water? Does the rain fall in order to moisten the soil 

 and keep the streams flowing, or because the force of gravity on larger 

 drops forming in the atmosphere exceeds the forces of air movement 

 and resistance? The origin of the large drops may be traced back through 

 smaller and smaller particles to water vapor and its relations to the 

 temperature of the atmosphere. This brief outline suggests merely a few 

 of the many facts and relations that must be apprehended before these 

 phenomena can be seen as causal sequences. Most persons, however, 

 prefer to seek such data rather than accept as an explanation for rainfall 

 the statement "that it is necessary to wet the soil and to make plants 

 grow." 



Physical and chemical phenomena have for the most part passed out 

 of the stage of personification. It would be hard to find persons who 

 would consider the statement, "Sodium unites with chlorine in order to 

 form sodium chloride or common salt," a satisfactory explanation of the 

 union. No one would explain the flow of water downstream as a striving 

 of the water to get to the sea. Feature writers would never dream of 

 accounting for the movement of electrical energy from dynamos through 

 wires to motors by saying "the energy is needed to turn the motors." 

 Most readers would not be satisfied with the substitution of purpose, 

 necessity, or use, for cause in such examples. 



But when biological phenomena are being described and explained 

 many writers feel no need for securing the results of experimental study. 

 On the contrary, they proceed with their discussion of natural processes 



