Systematic Atavism 653 



the tissues. At other times the straightening 

 may have taken place directly internal to a leaf, 

 and it is torn and may be seen to be attached to 

 the stem by two distinct bases. 



Summing up this description of the heredi- 

 tary qualities of our twisted teasels and of their 

 mechanical consequences, we may say that the 

 loss of the normal decussation is the cause of all 

 the observed changes. This special adaptation, 

 which places the leaves in alternating pairs, re- 

 placed and concealed the old and universal ar- 

 rangement on a screw line. In disappearing, it 

 leaves the latter free, and according to the rule 

 of systematic atavism, this now becomes active 

 and takes its place. If the fibrovascular con- 

 nection of the leaf -bases were lost at the same 

 time the stems would grow and become straight 

 and tall. This change however, does not occur, 

 and the bases of the leaves now constitute a con- 

 tinuous rope instead of separate rings, and 

 thereby impede the stretching of the internodes. 

 These in their turn avoid the difficulty by twist- 

 ing themselves in a direction opposite to that 

 of the spiral of the leaves. 



As a last example of systematic atavism I will 

 refer to the reversionary changes, afforded by 

 the tomatoes. Though the culture of this plant 

 is a recent one, it seems to be at present in a 

 state of mutability, producing new strains, or 



