236 The Philippine Journal of Science wis 



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 



