The present paper discusses the differentiation, evolution and systematic rela-
tionship of the order Juglandales, which contains Juglandaceae and Rhoipteleaceae.
1. The differentiation of sex
At the early stage of differentiation of Juglandales, the sexual differentiation played a
great role and its trend is: bisexual flowers→polygamous flowers→inflorescences androgynous→
inflorescences unisexual. As a result, flowers in the more advanced taxa are more reduced, i.e.
their perianthes gradually reduced, or even disappeared, while their stigmas become more speci-
alized. This fact indicates that Juglandales is one of the most advanced wind-pollinated taxa.
2. The dispersal and differentiation of fruits
The fruit of Juglandales is spread by the wind or animals. The fruits for animal dispersal
are of the edible parts for animals. They evolved along the two pathways: (1) the wings develo-
ped from trilobed bracts (fused from 1 bract and 2 bracteoles), such as those of Engelhardia
and Oreomunnea, evolved towards reduction, and as a result, their fruits have enlarged into
wingless drupe-nuts (as in Alfaroa). (2) the fruits with two wings (as in Pterocarya)→the fruits
with ring wings (as in Cyclocarya)→typical drupe-nuts (as in Juglans, Carya, Annamocarya).
Therefore, we suggest that the fruits of Juglandales have evolved from wind-dispersed to anim-
al-dispersed.
3. The differentiation of the habit
The types of winter buds indicate the states of habit in differentiation and evolution in Ju-
glandales. In the author's opinion, Juglandaceae is of forest origin in tropical mountains with
seasonal drought. Their primitive groups usually have naked buds (as in Rhoipteleaceae, Engel-
hardia, Oreomunnea, Alfaroa), while their more advanced groups have buds enclosed by scales,
adapted to temperate and relatively dry circumstances and expanding their distributional areas.
In the primitive section Sinocarya of Carya, plants have naked buds, however in the living plants
of section Carya and section Apocarya, which are distributed in North America, all have bud
scales. This evidence shows that the differentiation of habit in Juglandales is from the one ada-
pted to rather moist tropical and subtropical circumstances to the one adapted to rather dry te-
mperate ones.
4. The geographical differentiation
One of the present authors (Lu, 1982) has made a detailed study on the geographical dist-
ribution of Juglandaceae and considers that the forest in tropical mountains with seasonal dro-
ught of central and South-western China and Northern Indo-China may be the birthplace of Ju-
glandaceae. The family Rhoipteleaceae is distributed in Western Guizhou and Guangxi, East-
southern Yunnan and Northern Viet-nam, where the primitive section Psilocarpeae of Engelha-
rdia are also distributed. Therefore it is considered that the above-mentioned speculation is also
applicable to Juglandales. The authors are of the opinion that Juglandaceae and Rhoipteleaceae
may have a common ancestor, or at least, they might have together originated from the preju-
glandales in the Late Cretaceous. Now this opinion still has been debated. Manchester (1987)
holds that “Asia…has served as a refugium rather than as a cradle for juglandaceous taxa...,
An Asian origin also seems unlikely because Asia lies outside of the Normapolles province from
which the family probably evolved. The earliest centre of juglandaceous generic diversity ap-
pears to have been North America.” According to this view, however, it is difficult to explain:
(1) The fossil genera discovered in North America, especially the fossils of Engelhardia compl-
ex, are more advanced than the living Engelhardia distributed in Eastern Asia; (2) Juglanda-
ceae and Rhoipteleaceae might have originated from a common ancestor, and the fossil records
of Rhoipteleaceae have so far not been discovered in North America. These two facts seem to in-
dicate that Juglandaceae have originated from Asia rather than from North America. Unfortu-
nately the fossil records of Juglandales in Asia are inadequate for solving the problem.
5. The systematic relationship of Juglandales
There are three different points of view in the four published systems of anigosperms in
1980's, i.e. (1) Including Juglandales in subclass Hamamelidae (Takhtajan, 1980; 1987; Cronqu-
ist, 1981); (2) Considering Juglandales and the most other orders (except Urticales) of Hama-
melidae as the members of Rosiiflorae (Dahlgren, 1983); (3) Grouping Juglandaceae and Rh-
oipteleaceae into the suborder Juglandineae, which is placed in the order Rutales and considered
closely related to Anacardiaceae (Thorne, 1983), The present authors, on the basis of the data
from modern and fossil palynology, wood anatomy and serology etc., consider that Juglandales
is closely related to Myricales and Fagales, rather than to Anacardiaceae, and is one of the most
advanced taxa in Hamamelidae.
