644 Mutations 



the leaves, which has been replaced in the teasel- 

 family by a grouping in pairs. In order to 

 prove the validity of this assertion, it will be 

 necessary to discuss two points separately, viz. : 

 relative positions of the leaves, and the manner 

 in which the alternate position causes the stems 

 to become twisted. 



Leaves are affixed to their stems and branches 

 in various ways. Among them one is of 

 wide occurrence throughout the whole realm 

 of the higher plants, while all the others are 

 more rare. ' Moreover these subordinate ar- 

 rangements are, as a rule, confined to definite 

 systematic groups. Such groups may be large, 

 as for instance, the monocotyledons, that have 

 their leaves arranged in two opposite rows in 

 many families, or small, as genera or subdivi- 

 sions of genera. Apart from these special 

 cases the main stem and the greater part of the 

 branches of the pedigree of the higher plants 

 exhibit a spiral condition or a screw arrange- 

 ment, all leaves being inserted at different 

 points and on different sides of the stem. This 

 condition is assumed to be the original one, 

 from which the more specialized types have 

 been derived. As is usual with characters in 

 general, it is seen to vary around an average, 

 the spiral becoming narrower and looser. A 

 narrow spiral condenses the leaves, while a 



