IRRITABILITY. 413 



With regard to traction, we learned, in our consideration 

 of the tensions of the tissues (p. 349) that it promotes the 

 growth of the cell in the direction in which it is applied, as is 

 well shewn in the case of the epidermal and hypodermal cells 

 of etiolated plants. That long-continued traction in one 

 particular direction promotes growth in length of organs in 

 that direction is illustrated by the well-known fact that the 

 stems of trailing plants which grow in streams are longer 

 when the current is swift than when it is sluggish. The effect 

 of traction is apparently only distinctly manifested when the 

 traction is long-continued, consequently it cannot be detected 

 in comparatively brief experiments. Thus in Schwarz's ex- 

 periments upon the influence of centrifugal force upon the 

 rate of growth in length, to which allusion was made above, 

 the traction, though it must have been considerable in view 

 of the rapidity of rotation (in some cases 156 revolutions per 

 minute, when the centrifugal force was twenty times as great 

 as that of gravity), exercised no perceptible influence upon 

 the growth in length of the roots during the experiment. A 

 singular effect of traction was observed by Baranetzky in his 

 researches on the growth in length of stems (see p. 398). 

 He found, namely, that when a stem of a plant on the 

 auxanometer was stretched somewhat by the weight passing 

 over the pulley (see Fig. 44), its rate of growth was retarded. 

 The significance of this fact is not clear, but it would appear 

 that in this case the traction has a stimulating, as opposed to 

 a simply mechanical, effect. 



With this we conclude our account of the influence of 

 external conditions in modifying the rate of growth of organs. 

 In the next lecture we will consider their influence in causing 

 alterations in the direction of growth. 



