MOVEMENTS OF IRRITATION. 



437 



FIG. 140. Apparatus .for demonstrating 

 geotropic root curvatures. 



cover the flower-pots or boxes, as the case may be, with large 

 cardboard boxes, or put them in a cupboard, and when the roots 

 have reached a length of 

 about 3 cm. we take out of 

 the sawdust a few seedlings 

 with very straight roots. 

 After careful washing we 

 pass long pins through the 

 seedlings and fix them in 

 the manner indicated in Fig. 

 1-40, the roots being directed 

 horizontally, and the whole 

 being covered by a suffi- 

 ciently large bell-glass. In 



transfixing the seedlings attention must be paid to what was said 

 on p. 390. The rim of the bell-glass dips into water, contained 

 in a glass dish, while the inside of the glass is provided with wet 

 blotting-paper. We place our apparatus in darkness. We may 

 also make the experiment by removing some of the seedlings 

 from the sawdust and simply replacing them with their roots 

 directed horizontally. At a sufficiently high temperature 

 (20 C.-25 C.) we can, even after a few hours (at a low tempera- 

 ture not till later), perceive a downward curvature of the root- 

 tip, more or less sharp according to circumstances. The root 

 executes a positive geotropic curvature, and continues to grow 

 not horizontally but downwards. If seedlings of Phaseolus, etc., 

 are laid in sawdust with their roots directed vertically upwards, 

 the tip of the root soon curves downwards, and then continues 

 to grow in that direction. In my investigations with Phaseolus 

 (at 22 C.) the root-tip had already made a considerable curva- 

 ture at the end of four hours. If seedlings are laid in sawdust 

 with their roots directed obliquely upwards or downwards, 

 geotropic curvatures naturally are made which tend to carry the 

 root ends vertically downwards. 



For further more searching investigations relating to the 

 geotropic downward curvature of roots, we require first of all 

 a special box, such as the one drawn in Fig. 141, and first em- 

 ployed by Sachs. The box consists mainly of strong sheet zinc. 

 The front and back, however, are formed by sheets of glass about 

 20 cm. high and 30 cm. broad. These must not be perpendicular, 

 but inclined at an angle of about 10. The bottom of the box 



