GROWTH. 353 



north and south, that in the course of a year the north and 

 south diameter of the stem exceeded the east and west 

 diameter in the proportion of thirteen to eleven. The greater 

 growth in the thickness along the north and south diameter 

 is the consequence of the stretching and loosening of the 

 cortex on the north and south sides of the stem by the 

 action of the wind. Again, Detlefsen has observed that the 

 development of lateral branches on stems and roots produces 

 excentricity which is likewise the expression of local diminu- 

 tion of the transverse tension. The diminished tension is the 

 result of the growth of the lateral branches which tend to 

 loosen the cortex and hence to diminish the tension. There 

 is thus a region extending for some distance downwards from 

 any of the larger lateral branches of the stem and upwards 

 from any of those of the root in which the tension is dimin- 

 ished ; accordingly growth in thickness is more active here, 

 and the annual rings of the stem or of the root come to be 

 excentric. 



We may perhaps include among the phenomena which we 

 are now considering some of the cases of torsion, those, 

 namely, which appear to be due to internal causes. The 

 surface of many organs, especially internodes and leaves, 

 present striae which twist round the long axis of the organ. 

 They are very apparent, for instance, in the internodes of 

 Chara. These striae of cortical tissue are, as Sachs points out, 

 necessarily longer than the organ on which they are present, 

 and this suggests that they are due to a longer duration of 

 the growth of the cortical as compared with the internal 

 tissues, a conclusion which is supported by the fact that the 

 striae make their appearance towards the end of the period of 

 growth in length of the organ. The increase in length of 

 the external tissues is resisted by the internal tissues. If the 

 line of action of this resistance were parallel to the long axis 

 of the organ, the external tissues would not become twisted, 

 but would remain in a state of positive tension : this can be, 

 however, but rarely the case ; hence the result of the tension 

 between the external and the internal tissues is that the 

 former become twisted. This explanation is supported by 

 V. 23 



