ARRANGEMENT OF ORGANS. 5 
that the whorls alternate one with another, as may be 
represented by this symbol : 
i) 10 11 12 13 14 $615 16 
Ii 2 3 A, oD 6 7 8 
The simplest illustration of this arrangement is seen 
in the case 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 : 
aN 
i 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 interferig 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- 
