[^158..3 
IX. On the Origin of Di-trimerous Whorls 
By the Rev. Prof. G. HENSL 
mong the Flowers of Dicotyledons. 
(MA, Lea EGS 
(With two Text-figures. ) 
Read 7th November, 1907. 
IN some papers on Phyllotaxis and A‘stivations *, I have explained the origin of the 
quinary or $ arrangement—or “angular divergence” as it is called—from opposite and 
decussate leaves, originating with the two cotyledons, and giving rise to whorls of fives 
in flowers of Dicotyledons. When this change occurs from opposite to alternate 
arrangements in the leaves—as may readily be seen in stems of the Jerusalem Artichoke 
and Epilobiums,—the 3 necessarily follows, the divergences 3 and 4 not being obtainable 
in this way. 
The origin of the di- and tri-trimerous whorls among Dicotyledons, therefore, must be 
explained in a different manner from that of the simply trimerous whorls of Mono- 
cotyledons. The former are derived from single cycles of the 2 and $ arrangements 
respectively; while the latter consist of successive single cycles of the $. This 
divergence is possible in Dicotyledons, but rare. 
Of the several orders usually placed at the beginning of Floras, some, as Ranunculacee, 
Dilleniace:, Calycanthacez, and Nymphæaceæ, have chiefly quinary flowers; others 
have ternary whorls throughout, or else partly so, especially of the calyx and corolla. 
These orders are often characterized by indefinite stamens and carpels, all being free. 
This is a condition favouring the production of ternary arrangements. The following 
are examples of genera with di- or tri-trimerous floral whorls. The periods indicate the 
four floral whorls—sepals, petals, stamens, and carpels :— 
Asiminu : 3.3+3.0.3 to 15. 
Magnolia: 3.34+8+15.0.0. 
Cocculus: 3; 3+3 .8+3.3+3- 9,3. 
Lardizabala: 343.83843. 3+8 3. 
Berberis: 8; 34+3.34+3.8+3. 1. 
Brasenia : 3+3 . 0 . 12 to 18.6 to 18. 
Cabomba: 8+3.0.343.3. 
Symmeria: 8, 3+3. [3+3] .20 to 50. 2,3. 
Polygonum: 5 . [5] . 5 or 8.3. 
Rumer: 3+3 . [8+3] . 3+3 . 3. 
a Vu 
* “On the Variations of the Angular Divergences of the Leaves of Helianthus tuberosus,” Trans. Linn, Soc. xxv x 
(1868) p. 647; *On the Origin of the prevailing Systems of Phyllotaxis," Trans. Linn. Sot ser. 2, Bot. vol. i. 
(1875) p. 37; « On the Origin of Floral Æstivations, &c.," Trans. Linn. Soc. ser. 2, Bot. vol. i. (1876) p. 177. 
= SECOND SERIES.—BOTANY, VOL. VII. | zy 
