706 IMBRICATIVE ASTIVATION. 
spiral in all the flowers of plants of the same kind is uniformly 
from left to right or with the hands of a watch, the parts of the 
whorl may be termed imbricate+-; when the direction of the 
Fig. 110, spiral is uniformly from right to left or con- 
2 trary to the hands of a watch, the parts are 
imbricate—. If, however, the spiral does 
not maintain a uniform direction, but winds 
in some flowers of the plant to the right and 
in others to the left, the parts of the whorl 
may be termed imbricate +. 
4. Imbricative æstivation is the most 
common arrangement of both sepals and 
petals. It admits of much variety dependent upon the number 
and the relative position of the external and internal parts. 
Also, this variety of arrangement is displayed much more in the 
corolla than in the calyx. 
5. In a dimerous imbricate whorl, the two edges of one part 
overlap both edges of the other part (Fig. 110), as in the calyx 
of the Spring Beauty (Claytonia Caroliniana). This is an alter- 
nate two-ranked arrangement. 
6. In a trimerous imbricate whorl, one part is wholly external, 
one is wholly internal and the third is intermediate, that is, has 
Fig. 111. Fig. 112. 
+ — 
one edge in and the other edge out, as in the calyx and corolla of 
Trillium. Here there is an obvious spiral arrangement and the 
successive members of the cycle may be numbered 1, 2 and 3. 
The spiral makes a single turn to the right or to the feft at- 
cording as 3 lies to the right or left of 1, and the whorl is imbri- 
cate-+ (Fig. 111), or imbricate— (Fig. 112), accordingly- TP 
is the 3-ranked or 4 arrangement of leaves. ne 
7. A tetramerous imbricate whorl presents two cases- ; 
‘ 
