ACTION OF FORCES ON ORGANISMS. 55 



animals are exempt from a similar action of similar 

 forces. 



Illustrations of the influence of various external 

 forces upon growth are found in great number among 

 plants. Thus, the sunlight may cause plants to 

 grow toward it, or away from it ; and the amount 

 of light modifies the growth of plants in many other 

 ways. Gravity also has a powerful influence upon 

 the general direction of growth, and moisture deter- 

 mines the direction of growth of roots. 



A number of experiments of very great interest 

 for our present enquiry have been very ably dis- 

 cussed by Professor Detmer ; ^ and his observations 

 seem so important, that I give here extensive quo- 

 tations from his paper — only taking the liberty to 

 translate the quotations into English. 



" In the botanical realm a long series of facts 

 have been ascertained, which teach us that the 

 influence of external conditions upon the plant is 

 often very profound. Their operation modifies not 

 only the outward appearance of the parts of the 

 plant, but in a most striking manner changes even 

 the anatomical structure. 



" If one prepare a cross-section of the bipennate 

 twig of TJmja occidentalis, it will be found, upon 



1 Detmer, " Zum Problem der Vererbung," Pfluger's Archiv f. 

 Physiologie, Bd. 41, 1887. 



