3190 BOTANICAL GAZETTE [MAY 
stimulation, and this he believed to explain the curvature. How- 
ever, he did not regard it asa phenomenon of growth in the 
present usage of the term, as is to be seen by the following quo- 
tation and the context: 
Die auf Einwirkung der Schwerkraft eintretende aufwarts Kriimmungen 
horizontaler oder gegen den Horizont geneigter Organe von Pflanzen geschicht 
adurch, dass in der unteren Langshalfte des Organs die Dehnbarkeit die- 
jenigen Zellmembranen zunimmt, welche der Expansion der in Ausde- 
hungsstreben begriffenen Membranen Widerstand leisten. 
As a matter of fact Hofmeister believed that the extension 
of the convex side of a curving root was similar to that shown 
by a pencil of soft wax. 
Sachs, as a result of researches upon shoots and roots, brought 
out in his Handbuch in 1865 (26, pp.g2-96), and again in 1872 
(23) and 1873 (24), agrees in the main with Hofmeister, but 
insists that the exaggerated extension of the convex side of 
curving organs is an actual growth. This idea was applied by 
workers in the Wiirzburg Institute to all curvatures. The devel- 
opment of information concerning turgidity led to an exaggerated 
estimate of the actual part played by its variations in curvatures. 
Since that time, increased turgidity of the cells of the con- 
vex side, decreased extensibility of the membranes of the con- 
cave side, the aggregation of protoplasm on the concave side 
producing a shortening of the longitudinal and a lengthening of 
the radial axes, have each in turn been considered as the motive 
_ forces by investigators engaged with the subject. The thorough 
account of the matter given by F. Darwin in his presidential 
address to the section of biology of the British Association for 
: the Advancement of Science in 1891 (6) renders it unnecessary 
to give the detailed steps here _ 
It seems to be agreed on all hands that the curvatures are 
due to the exaggerated extension of the cells of the convex side, 
_which is accompanied by a diminished extension or contraction 
of the concave side, dependent upon mechanical conditions. = 
The chief contention at present concerns the conditions attend- 
: ant upon the extension of the membranes of the convex side. : a 
