236 



CAUSES OF VARIATION 



SECTION V EFFECT OF GRAVITY UPON LIVING 

 MATTER 1 ; GEOTROPISM 2 



A germinating seed sends out two sprouts. From whatever 

 position they emerge, one grows downward in response to grav- 

 ity, the other upward in opposition. In other words, the root is 

 positively and the stem is negatively geotropic ; that is to say, 

 each contains within itself some quality that 



j 1 1 puts it into definite relation with the center of 



the earth, but in opposite directions. 



That this tendency is something consider- 

 able is shown by the fact that it is capable of 

 exertion against pronounced resistance, as in 

 burrowing through the soil or persisting against 

 mechanical obstructions. 



This definite relation to gravity seems to ex- 

 ert itself in the manner of inward forces respond- 

 ing to outside conditions ; for if a piece of 

 stem be altered in its position,- future growth 

 readjusts itself as promptly as possible in 



1 C. B. Davenport, Experimental Morphology, Part I, 

 pp. 1 12-124 ! a l so P art H PP- 39 1-402, from which most of 

 the facts here cited are taken. 



2 Two series of terms are in use, of substantially the 

 same meaning: one (see "geotropism" and "geotropic"), 

 with endings derived from the Greek meaning to turn ; the 

 other (see "geotaxis" and "geotactic"), with Greek endings 

 signifying to arrange. We thus have also "chemotropism," 

 with its adjective " chemotropic," over against " chemo- 

 taxis," " chemotactic," " heliotropism," and " heliotactic," 

 etc. The latter endings were supplied when the effect of 

 chemicals, gravity, and light upon free-moving organisms, 

 as bacteria, swarm spores, etc., was first noted, which effects 

 led to definite " arrangements " ; but when similar effects 

 were noted upon larger organisms not free to move, like 

 the fixed plants, but which manifested the effect by turning, 

 the term " tropism " came into use, signifying a turning. 

 This term and its derivatives seem to express more accu- 

 rately what really happens in most cases, and they are 

 coming to be preferred in all generalized considerations. 

 Hence in the text the term " geotropism " is preferred to 

 " geotaxis " and similarly in most places for all correspond- 

 ing terms. 



FIG. 25. Influence of 

 geotropism: be- 

 havior of a grow r - 

 ing plumule in 

 righting itself after 

 being placed in a 

 horizontal posi- 

 tion, as at i. 

 After C.B. Daven- 

 port, from Stras- 

 burger 



