xi, c, 5 Copeland: Growth Phenomena of Dioscorea 235 



In all of the pairs of plants tested, it happened only once that 

 the plant in darkness showed a longer growing region than that 

 in light ; in this case, the plant in light proved to have a surpris- 

 ingly short growing region. It happened repeatedly in the course 

 of these observations of paired plants that the plant in darkness 

 showed greater total growth than the plant in light and yet 

 showed a much shorter growing region, in some cases less than 

 half as long. 



We have here, I believe, the whole of the immediate explanation 

 of the conclusion of Professor Newcombe: 5 



Die ummittelbare Ursache des Verlustes des Windens ist der Verlust des 

 einseitigen Wachstums im Stamme eine betrachtliche Entfernung riickwarts 

 von der Spitze — bei den meisen der beobachteten Pflanzen mehrere Zenti- 

 meter riickwarts von der Spitze. 



It is not merely that this zone on the stem loses the faculty of 

 one-sided growth; the region that would execute the circum- 

 nutating movement in light almost ceases to grow at all in dark- 

 ness or does actually cease entirely to elongate. In Pfeffer's 

 Physiology, volume II, page 13, I find the citation of a paper by 

 Strehl said to show that the elongating region is longer in etio- 

 lated than in normal stems. I have been unable to check this by 

 reference to the original publication, which is a Leipzig doctor's 

 thesis of 1874. Without testing at all a variety of stems, I 

 strongly suspect that the condition I have found in Dioscorea 

 will turn out to be quite general. On the one hand, it can be 

 harmonized easily with my old observation, 8 that the turgor of 

 etiolated stems is less than that of normal stems. The lower 

 turgor in the zones which lie beneath that of rapid growth may 

 well be associated with a cessation of growth prompter than 

 would occur if the turgor were higher. 



On the other hand, the short elongating region of stems in 

 darkness invites biological interpretation. It is an old and, I 

 believe, generally accepted idea that the rapid elongation of 

 etiolated or etiolating stems is a response to darkness that has 

 been selected and fixed and is, therefore, inherited, because this 

 rapid growth is likely, in nature, to result in the shoots' reaching 

 light sooner than they would do at the normal rate of growth or 

 in reaching light from positions where the normal rate and 

 manner of growth would result in exhaustion before light could 

 be reached. The typical phenomena of etiolation are best shown 



• Op. cit., p. 523. 



4 Ueber den Einfluss von Licht und Temperatur auf den Turgor. Halle 



(1895). 



