EXTERNAL INFLUENCES AS CAUSES OF VARIATION 275 



the diffusing molecules on certain elements of the skin influence 

 the tension of the muscles," causing motion. 



The female fly is attracted by meat, the same as are larvae, and 

 " as soon as the fly is seated on the meat chemical stimuli seem 

 to throw into activity the muscles of the sexual organs, and 

 eggs are deposited on the meat." This chemical stimulus is 

 about all there is of the wonderful " instinct" by which insects 

 are led " always to deposit their eggs in exactly the right places." 



These and similar examples show the effect of certain chem- 

 icals upon free-moving organisms. It remains to illustrate their 

 effect upon the direction of growth among plants, which are not 

 free to move. 



First of all, we are to note the effect of certain chemicals upon 

 the tentacles of insectivorous plants. Darwin noticed that " when 

 drops of water or solutions of non-nitrogenous compounds are 

 placed upon the leaves of the sundew, Drosera, the tentacles 

 remain uninflected ; but when a drop of a nitrogenous fluid, such 

 as milk, wine, albumen, infusion of raw meat, saliva, or isinglass 

 is placed on the leaf, the tentacles quickly bend inwards over 

 the drop." 1 Darwin found that of " nine salts of ammonia tried, 

 all caused inflection, and of these the phosphate was the most 

 powerful," and that " sodium salts in general caused inflection 

 with extreme quickness." This action of nitrogenous, phos- 

 phatic, and other chemicals common to animal life, was the 

 same upon the tentacles of insectivorous plants as upon the 

 tentacles of lower animals subsisting upon the same kind of 

 food. 2 Thus plant and animal tissues appear to be subject to 

 the same general laws in this regard. 



From this point of departure, common to both plant and ani- 

 mal, we note that the animal, free to move, does so in response 

 to this class of stimuli. What does the plant do that cannot 

 move, even as tentacles move ? In other words, how does chemot- 

 ropism affect the direction of growth among higher plants ? 



Roots in general are supposed to grow toward oxygen, and 

 pollen tubes will certainly turn toward the stigma of the flower, 



1 C. B. Davenport, Experimental Morphology, pp. 335-336. 



2 See the experiment on actinians previously quoted from Loeb, Physiology of 

 the Brain, pp. 4950. 



