384 EFFECT OF MOLAR AGENTS UPON GROWTH [Ch. XIV 



a movement towards that side or from that side according as 

 the organ is positively or negatively thigmotropic. 



§ 3. Effect op Wounding upon the Direction of 

 Growth — Tkaumatropism * 



Under this head may be considered the effect of a special 

 class of molar agents, namely, those which produce a wound, 

 upon the direction of growth of roots. We may consider first 

 false and then true traumatropism. 



1. False Traumatropism. — ^ When the growing tissue of the 

 radicle of a seedling is destroyed on one side by caustic or heat, 



the radicle turns towards that side (ClE- 

 siBLSKi, '72 ; Darwin, '81 ; Spaulding, 

 '94). This result is satisfactorily ex- 

 plained on the ground that the killing 

 of the growing tissue on one side causes 

 a retardation or cessation of growth on 

 that side ; so that, while the points x, y 

 (Fig. 103), remain nearly stationary, the 

 points a;i, ?/i, opposite to them, grow widely 

 apart. It will be seen at once that this 

 bending belongs to a wholly different 

 category from the thigmotropic curva- 

 tures described in the last section. It 

 is, as Darwin called it, a mechanical 

 bending ; it is a false traumatropism. f 



2. True Traumatropism. — Of widely different cause from 

 the foregoing is the turning due to slight wounding which has 

 been investigated by Ciesielski ('72), Darwin ('81), Det- 

 LEFSEN ('82), WiESNER ('84), and Spaulding ('94). Dar- 

 win irritated the radicle of a bean by touching it near the 

 apex with dry caustic (nitrate of silver). The point of wound- 



* From Greek, rpoC^ua, a wound. 



t A phenomenon allied to this is seen when at an early stage one half of a 

 frog embryo is killed and, consequently, remains of small size ; the larger, 

 growing, side soon comes to bend around the smaller, dead, side. A fuller 

 account of these experiments, made by Eoux and others, upon the frog's egg, 

 must be postponed to the next Part of this work, since it is complicated by the 

 phenomenon of regeneration. 



Fig. 103. — Diagram of a 

 root tip, illustrating 

 false traumatroj)ism : 

 a;?/,region of wounding. 

 The line y-if ran orig- 

 inally parallel to the 

 line x-x', but by growth 

 has been brought to 

 make an angle of 90° 

 with it. 



