2 BOTANICAL GAZETTE [JANUARY 
to grow upward. This conclusion holds good to the present 
time, but Knight’s explanation of how gravitation could affect 
roots so differently from stems is of historical interest only. He 
considered the root curvature as a purely passive bending due to 
great flexibility in the curving zone. 
After Knight we need take no special notice of the explana- 
tions given of curvature in growing plant organs until the time 
of Hofmeister and Frank, though several authors (4, 16, 40, 70) 
wrote upon the subject in the interval. The two men named 
engaged in a controversy as to whether the curve which takes 
place in roots that are placed in a horizontal position is purely 
passive as claimed by Hofmeister (27, 28), or is an active curva- 
ture as asserted by Frank (21, 22). The latter proved his point 
by showing experimentally that the curving root can be made to 
lift a weight. Frank also introduced the term geotropism to 
designate the response of plant organs to the action of gravi- 
tation. 
Sachs, in his earlier work (61), explained the curvature of 
plant organs by supposing a difference in the rate of growth on 
the opposite sides of the curving organ. Later, influenced by 
the work of de Vries (71-75), he regarded the difference in 
growth of the two sides as conditioned by greater turgor on the 
convex side. De Vries’s conclusions rested principally on the 
fact that plant organs in the early stages of curvature are 
completely straightened on plasmolysis, and he supposed 
the cells of the convex side must have been more highly 
stretched by osmotic pressure. The osmotic substances that 
were especially effective were said to be organic acids and their 
salts (75). 
Sachs has added much to our knowledge of curvatures pro- 
duced in plant organs by external stimuli. In his investigation 
of roots (62) he found that the cortex of roots grows faster than 
the axial strand, that split roots curve with the cut face concave, 
and that any part of the cortex derives its nutriment from that 
part of the axial strand lying in the same transverse zone. He 
found by external measurement, as well as by microscopical 
