ARRANGEMENT OF ORGANS. 3 



that the whorls alternate one with another, as may be 

 represented by this symbol : 



9 10 11 12 13 14 15 16 

 12345678 



The simplest illustration of this arrangement is seen 

 in the ease of decussate leaves, where those organs 

 are placed in pairs, and the pairs cross one another at 

 right angles. This may be expressed by the following 

 symbol : 



7 8 



5 6 



3 4 



1 2 



Thus, while in both the annular and the spiral modes of 

 development the individual members of each complete 

 series necessarily alternate one with another, in the 

 former case the series themselves alternate, while in 

 the successive arrangement they are placed directly 

 one over the other. There are, of course, exceptions, 

 but the rule is as has been stated, and the effect is to 

 prevent one leaf from interfering with the- development 

 and growth of its neighbours. 



In the case of the whorled or simultaneous arrange- 

 ment the conditions of growth must be uniform on 

 all sides, but in the successive or spiral disposition 

 the conditions influencing growth act with unequal 

 force, on different sides of the stem, at the same 

 time. In the whorl there is an illustration of 

 radiating symmetry, while in the spiral arrangement 

 there is a transition to the bilateral symmetry. There 

 are frequent passages from one to the other even under 

 normal circumstances ; thus, while the one arrange- 



