MOVEMENTS OF IRRITATION. 4.V.) 



The objects (e.g., four seedlings of Phaseolus, Pisum, or Vicia 

 Faba) are fixed on the cork, when the main root has just emerged. 

 We fix them with their main roots making different angles with 

 the axis, and find after some days (it is best to experiment at a 

 comparatively high temperature, say 20 C.) that all the roots 

 have lengthened in the original direction of growth, assuming 

 uniform movement of the apparatus. Occasional curvatures 

 may appear in the roots ; they are not, however, geotropic nuta- 

 tions, all similarly directed, but spontaneous nutations. 



As regards the lateral roots, these, after a time, grow out from 

 the main roots of the seedlings which have been rotating from 

 the commencement of germination. The angle which they make 

 with the main root, the so-called " proper angle " of the lateral 

 roots of the first order, is determined in our experiment by in- 

 ternal causes alone. This proper angle is in different objects, 

 and in the individual lateral roots of a particular main root, by 

 no means always the same. In general it happens, as can easily 

 be made out by measuring the angles, that the lateral roots at the 

 base of the root are directed almost or exactly at right angles to 

 the main root, while the later ones have an acute " proper 

 angle"; they are inclined towards the apex of the main root. 

 Frequently also the lateral rootlets bend in the form of an arc. 

 Seedlings of Phaseolus are particularly suitable for investiga- 

 tions of this kind. In this plant lateral roots are also usually 

 given off by the short hypocotyl, which, as must be specially men- 

 tioned, exhibit an obtuse "proper angle." 



In order to subject plants to one-sided illumination, but elimin- 

 ate the one-sided influence of gravity,' the axis of the clinostat 

 must be directed parallel to the incident rays of light. The plane 

 of rotation of the objects must make a right angle with the rays 

 of light. The pots, in which the plants grow, are fixed in one of 

 the rings depicted in Fig. 151, in the manner already described on 

 p. 456. For more detailed information see 178. 



A cube of bread, with sides 4-5 cm. in length, is moderately 

 (not too much) moistened with water. It is best to sterilise the 

 cube while moist by exposing it for some hours in a crystallising 

 glass covered with a sheet of glass to a temperature of rather 

 more than 100 C. in a drying chamber. The cube is now pushed 

 over the brass shaft (80 cm. in length) of the clinostat, one end 

 of which communicates with the clockwork, while the other rests 

 on the friction rollers. The shaft is perfectly horizontal and 



