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. 



