202 CROCUS, WARMTH. [CH. VIII 



of 15-5°, closing slightly again on being replaced in a 

 temperature of 13° C. 



(252) Crocus: mechanism of the movement. 



The following instructions are based entirely on 

 Pfeffer's^ account of the experiments, in which he showed 

 that when a crocus or tulip opens it does so because of 

 the accelerated growth on the inner faces of the segments, 

 and vice versa when it closes. A series of 4 or 5 minute 

 dots (about l*5mm. apart) are made with black spirit- 

 varnish on both surfaces along the region of curvature, 

 which in the crocus is the lower J or ^ of the perianth 

 segment. The distance between the marks must be 

 measured with great care by means of an eye-piece 

 micrometer: the amounts of growth observed do not 

 exceed 3 p.c, and it is therefore necessary to use a 

 magnifying power of something like x 80, and a micro- 

 meter with which the distance between the marks on the 

 flower is about 200 divisions of the micrometer. To get 

 accurate measurements it is necessary to sketch each of 

 the varnish marks, noting on the drawing a corner or 

 projecting point from which the reading is taken. The 

 readings are easily taken on the outside of the perianth 

 segment, and by removing the opposite segments the 

 inner marks can also be observed. The readings are 

 assumed to have been taken on a closed flower, which is 

 then placed in a room warmer than the first by 6° — 7° C. 

 and after \ hr., during which the flower opens, the readings 

 are again taken. On the inner side the marked region 



^ Physiologische Untersuchungen, 1873, p. 167. 



