MODIFICATIONS OF FORM 



169 



leaves of each succeeding whorl alternate as a rule with those of the 

 preceding, so that they occupy the spaces between them, an arrange- 

 ment that is very convenient in the packing of the crowded parts into 

 small compass in the bud. A transition to higher numbers in the cycle 

 may be seen in the individual plant. Thus in Fuchsia, which has 

 usually decussate leaves, a very strong shoot may bear alternating 

 whorls of three. In Lysimachia vulgaris, and in the Privet, a like 

 variability is common. It is styled meristic variation, and one factor 

 in producing it is probably the size of the apical cone, which, when 



Fig. 127- 

 Young leafy shoot of sycamore seen from above : sfiowing how with very little 

 overlapping the leaf-blades form a mosaic. The spaces unoccupied centrally will 

 be filled as the younger leaves expand. 



large proportionally to the leaf-primordia, can accommodate a larger 

 number of young leaves at the same level. Such variations are 

 common in the floral region, where cyclic arrangements prevail. 

 (Compare Floral Diagrams in Appendix A.) 



But in most Dicotyledons, and very generally in Monocoty- 

 ledons, the arrangement of the leaves is alternate ; that is, they are 

 seated singly, each at a different level upon the axis. The arrange- 

 ment is often such that an ascending spiral line may be drawn round 

 the stem so as to thread together the bases of them all. Such arrange- 

 ments are therefore described as spiral. That the cyclic and spiral 

 modes of arrangement are not essentially distinct from one another 

 is shown by the fact that both may appear successively in the same 

 plant. For instance, in the Sunflower, the seedling starts with paired 



