312 BOTANICAL GAZETTE [MAY 
tensions do not exist, and that downward curvature is purely 
mechanical, in confirmation of the theory originally proposed by 
Knight in 1806 (9). Frank, a few years later, demonstrated 
conclusively that the apogeotropic curvatures of roots are not 
mechanical, or due to the plasticity of the root tip and its own 
weight, but are due to active physiological processes (7). 
Despite the facts presented by Frank, Hofmeister maintained 
in a later paper: 
Alles ist so concludent wie méglich und lasst nur den Schlus zu, dass bei 
der Abwartssenkung der dussersten Ende wachsender Wurzeln eine nahe hin- 
ter der Spitze gelegener Querabschnitt der Wurzel in ahnlicher Weise passiv 
dem Zuge der Schwere folge, wie ein ziber Brei oder ein Tropfen steifen 
Lacks (10). 
Frank, however, firmly established the active part taken by 
the root in producing curvatures, and all later researches upon 
the subject are based upon this fact, and attention has been 
directed to the localization of the sensory and motor tissues, and 
the determination of the individual factors active in curvature. 
Cieselski made an examination of the mechanism of the 
curvature of roots in 1871, in connection with his remarkable 
researches upon the general nature of irritability of these organs 
1). He noted for the first time the greater density of the pro- 
toplasm of the concave side of the organs, a fact confirmed by 
Kohl in unicellular or coenocytic plants, and later in stems and 
tendrils by Sachs, and further examined by myself. The greater 
density of the protoplasm in the concave side of a tendril is not 
conditioned upon curvature, however, but is a distinct morpho- 
logical character apparent in the earlier stages of development, 
before the special forms of irritability characteristic of these 
organs are exercised or even manifested. This aggregation of 
the protoplasm upon the concave side of isodiametric unicellular 
and multicellular organs has been described by Wortmann as the 
primary factor in the cause of curvature, an explanation which 
has been found inadequate for reasons which need not be dis- 
cussed here. During a period from 1871 to 1873, Sachs devoted 
a large share of his attention to the curvatures of roots, and, SO 
