272 CAUSES OF VARIATION 



question, providing it is free to do so (as in the case of the 

 lower plants and all animals), or its growth will be directed 

 toward it if (as in the case of higher plants) it is fixed and un- 

 able to move. The former is, strictly speaking, chemotaxis ; the 

 latter is chemotropism. For our purposes it is a distinction 

 without a difference, because in the latter case the plant is 

 unable to indulge in locomotion, and performs the nearest pos- 

 sible act in changing the direction of growth. Both are loco- 

 motion under the circumstances ; both are due to the same 

 cause, a chemical affinity or attraction, and they differ 

 because of differences in the organisms affected in respect to 

 the power of locomotion, not because of differences in the nature 

 of the forces in action. The two terms are, therefore, for our 

 purposes, synonymous as denoting a power of attraction between 

 protoplasm and certain ordinary chemical compounds such as to 

 cause the protoplasm to approach if it be free to move, or, if not, 

 to grow in that direction, which is all that can be done under 

 the circumstances to satisfy the affinity. 



Chemotaxis, or chemotropism as the writer prefers to call 

 it, 1 has but a slender hold upon higher animals. It appears to 

 be localized in the nostrils and to manifest itself only in the 

 sense of smell, agreeable or otherwise ; but in lower organisms, 

 even in many insects, it is apparently not confined to a minute 

 fraction of the surface, but pervades the whole organism with 

 an influence that is all but overpowering. It may of course be 

 aided or opposed by other tropisms, as gravity or light, in which 

 case the total result is the algebraic sum of all the energies 

 operative, but that chemotropism is a force to be reckoned with 

 in development is a fact not to be doubted. 



Examples of chemotropism. 2 Englemann noticed that bac- 

 teria under a cover glass will gather along the margin, or if 

 green algae be introduced they will cluster about them so long 

 as they are producing oxygen, but in the darkness they will not 



1 The ending " taxis " is from the Greek, meaning assortment or arrangement ; 

 the ending " tropism " is from " tropic," to turn. " Chemotropism " is pronounced 

 ke"-mot'r6"-pizm. 



2 Until otherwise noted what follows is a free though not exact transcript from 

 data given in C. B. Davenport's Experimental Morphology, Part I, pp. 32-39. 



