MOVEMENTS OF IRRITATION. 439 



which one angle accurately points to the first mark placed just 

 behind the root tip. In Phaseolus I found, as Sachs mentions for 

 Yicia also, that the root for about an hour continued to grow 

 horizontally ; the first mark becomes somewhat displaced from 

 the point of the paper. After that time, however, a geotropic 

 curvature of the root soon became noticeable, and observations 

 repeated from time to time showed that the curvature can first 

 clearly be proved in the zone lying between the first and second 

 marks ; then later it occurs in the succeeding zones also. We 

 determine that all the zones of the root undergoing growth in 

 length take part in bringing about the geotropic downward cur- 

 vature ; but in order to confirm this important observation we 

 make the following experiment. We use, for example, seedlings 

 of Vicia Faba with roots 2 cm. long. After the growing region 

 of the root has been divided by ink-marks into zones of 2 mm. 

 length, the seedlings are laid in the culture box in the manner 

 above indicated. After about eight hours at 20 C. the horizontally 

 placed root has already made a conspicuous curvature, and we 

 can proceed to measure the growth which has taken place. We 

 use for the purpose a thin sheet of mica on which a number of 

 concentric circles are scratched with a pair of compasses.* It is 

 now best to subdivide the quadrants not into 90 parts, but by easily 

 and accurately performed successive halving into 8, 16, or 32 

 parts. We calculate the length of the arc for each circle. The 

 tabulated values serve for the determination of the lengths of the 

 arcs of the curved roots. The mica sheet is laid against the glass 

 pane behind which the root is situated. We find which circle 

 agrees with the curvature of the convex side of the root, or with 

 a portion of it, fix the mica to the glass by means of strips of 

 gummed paper, and make the further observations. In our case 

 the total growth on the convex side of the root amounts to about 

 4 mm. The entire growing region of the root has participated in 

 bringing about the geotropic downward curvature. The most 

 rapid growth has taken place in the third zone. We further seek 

 pairs of seedlings of Vicia Faba very uniform in development, 

 which have been grown in moist sawdust, and with roots from 

 rr> to 2 cm. in length. From the growing point onwards at 

 intervals of 2 mm. we make fine ink-lines as marks (the first being 

 at the growing point itself), and lay one of the seedlings in the 



* The largest circle may have a radius of 20 mm. 



