236 The Philippine Journal of Science 1916 
by seedlings, and in this work with Dioscorea I am working with 
stems analogous to the primary stems of seedling plants. The 
primary stem of seedlings most frequently finds itself in darkness 
because buried by the soil; rapid growth is nature’s method of 
bringing the shoot to the light before the exhaustion of its food 
store. If etiolation is an adaptive phenomenon, selected pri- 
marily because it preserved plants that germinated below the 
surface of the ground and enabled the growing point to reach 
and pass the surface, then a short growing region is just as 
natural a feature of this phenomenon as is rapid growth in length. 
The short growing region of the etiolated stem is explained then 
in a biological sense just as is the relatively short growing region 
of roots. A structure elongating where mechanical resistance is 
likely to be encountered has need to be short, as compared with 
the growing region of other structures, which elongate in the 
atmosphere and normally have no outside mechanical resistance 
to overcome. 
I have made no experiments with the change in length of 
growing region and manner of growth, when plants are taken 
from the light to the dark room. When plants are brought from 
the dark room and exposed to the light, the growing region be- 
comes longer. This lengthening of the elongating region (if I 
may use the same word twice together in different senses) 
consists in the retention of the power to elongate on the part of 
the zones that in darkness would cease to grow in length. This 
is easily tested by measuring the same zones for successive days. 
Under constant external and internal conditions, the length of 
the zones that cease to elongate during any day is naturally 
approximately equal to the increase in length on the same day. 
If a plant be brought from the dark room into the open labora- 
tory, it may happen that no zone ceases to grow during the 
next day or even two days; and in any case, the length of the 
region that ceases to grow is much less than the daily increment. 
Thus, in the case of Dioscorea hirsuta, May 23, plant No. 3 on the 
floor of the dark room was brought into the open laboratory. 
During the following day, it grew 6.8 centimeters and the region 
which ceased to grow was only 2.25 centimeters long. The 
increase in length of the elongating region continues until the 
normal length for a plant growing at the same rate in light is 
reached. This seems likely to be accomplished in about three days. 
The remeasurement of zones on successive days is a valuable 
test of the accuracy of one’s measurements and observations by 
these methods. It has just been suggested that if on successive 
days measurements are made of the distance between the same 
