Systematic Atavism 651 



curve the longitudinal ribs. The straighter 

 and steeper the screw becomes, the more the 

 ribs will become twisted. That this happens in 

 the opposite direction is obvious, without fur- 

 ther discussion. The twisting is the inevitable 

 consequence of the reversal of the screw. 



Two points remain to be dealt with. One is 

 the direct proof of the reversal of the screw, the 

 other the discussion of its cause. The first may 

 be observed by a simple experiment. Of course 

 it proceeds only slowly, but all that is necessary 

 is to mark the position of one of the younger 

 leaves of a growing stem of a twisting indi- 

 vidual and to observe the change in its posi- 

 tion ia a few hours. It will be seen to have 

 turned some way around the stem, and finally 

 may be seen to make a complete revolution in 

 the direction opposite to the screw, and there- 

 by demonstrating the fact of its uncurling. 



The cause of this phenomenon is to be sought 

 in the intimate connection of the basal parts of 

 the leaves, which we have detailed above. The 

 fibrovascular strands constitute a strong rope, 

 which is twisted around the stem along the 

 line on which the leaves are inserted. The 

 strengthening of the internodes may stretch this 

 rope to some extent, but it is too strong to be 

 rent asunder. Hence it opposes the normal 

 growth, and the only manner in which the inter- 



