ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 
Conclusions 
1. That the difference in the behavior relative to gravity of roots 
in air and in earth is not due to differences in the amount of water 
in the media. 
2. That the difference in behavior is not the result of change in 
the geotonus of the roots, due to their stay in air, whether weakening 
or loss of geotropism, as Sachs suggested, or assumption of plagio- 
geotropism as Nemec reported. 
3. That, as was shown by experiments with media the resistance 
of which to the root's advance could be widely varied, the failure 
of the roots in air to reach the vertical is due to the absence of me- 
chanical resistance to the advance of the root tip through that medium. 
4. That the secondary curvature of roots in earth, sand, sawdust, 
sphagnum or other such media is complete because the resistance of 
these media to the advance of the curved root tip causes passive 
depression of the root and prevents the complete flattening of the 
tip curvature. 
5. That thigmotropism is not a factor in the difference in the 
behavior of roots in air and in earth or other non-fluid media. 
6. That the resistance offered by the medium to movements of 
the root tip influences not only the course of the secondary curvature 
but also the course of the primary curvature, that is, the curvature 
directly following the placing of the root in a position of stimulation. 
The preceding conclusions apply in their entirety to the three 
principal species employed, Viciafaba L. (var. major and var. equina), 
Lupinus alhus L., and Pisum sativum L., although a number of forms, 
mostly Leguminosae, were employed in some of the experiments. 
University of Michigan, 
Ann Arbor 
LITERATURE CITED 
Baranetzky. Ueber die Ursachen welche die Richtung der Aeste der Baum und 
Straucharten bedingen. Flora, Erganzungsband: 138. 1901. 
Berg. Studien iiber Rheotropismus bei den Keimwurzeln der Pflanzen. Lunds 
Universit. Ars-skr. 352; No. 6. 1899. 
Czapek. Ueber die Richtungsursachen der Seitenwurzeln und einiger anderer 
plagiotropen Pflanzentheile. Sitzungsber. Akad. Wien 104^: 1197. 1895. 
Darwin, C. The Power of Movement in Plants. London. 1880. 
Detlefsen. Ueber die von Ch. Darwin behauptete Gehirnfunction der Wurzelspitze. 
Arb. Bot. Inst. Wiirzburg 2: 627. 1882. 
Elfving. Beitrage zur Kenntnis der physiologischen Einwirkung der Schwerkraft 
auf die Pflanzen, Acta See. Fenn. 12: 32. 1880. 
