XI, C, 5 
Copeland: Growth Phenomena of Dioscorea 
241 
SUMMARY 
1. Previous observations, that a nutation of shoots of Dioscorea 
ceases in darkness, are in general correct. 
2. Especially active stems may nutate and twine around a 
support in darkness. 
3. Professor Newcombe’s observation that the failure to twine 
in darkness is due to changes a number of centimeters from the 
apex is correct. 
4. The rate of growth of vigorous young shoots is but slightly, 
if at all, influenced by the illumination. 
5. The elongating region is much shorter in darkness than 
in light. The part of the stem which executes the movements, in 
active nutation in light, almost, or quite, ceases to elongate 
in darkness, and it is for this reason, that twining ceases in 
darkness. 
6. The short elongating region in etiolated shoots may be ex- 
plained biologically as a selected adaptation to the condition 
under which young shoots in nature are most likely to find 
themselves in darkness — this is, in the soil, where a long grow- 
ing region would be just as dangerous as the production of ample 
leaves. 
7. The growing shoots of Dioscorea are excellent material for 
the analysis of the influence of temperature or other external 
conditions upon growth, into: 
A, effect on the growing region; 
B, effect on the metabolic processes, which make food available; and 
C, translocation of food to the growing region. 
Low temperatures, applied either to the food store, or to the stem 
through which the food must pass to the growing region, result in 
prompt checking of growth. 
8. It is suggested that the blasting of the growing point and 
its replacement by a branch, which at first grows at a right 
angle to the axis from which it springs, is a selected phenomenon, 
by which the plant, the shoot of which is under unfavorable con- 
ditions, tests a wholly different line, instead of using itself up 
in one attempt to reach a place where conditions are good. 
