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: 3 
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 grotv 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, 0 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 
s Op. cit., p. 523. 
6 Ueber den Einfluss von Licht und Temperatur auf den Turgor. Halle 
(1895). 
