The Differentiation, Evolution and Systematic Relationship of Juglandales

Abstract

Abstract:

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.

Key words: Juglandales, Juglandaceae, Rhoipteleaceae, Differentiation, Evolution, Systematic relationship

Lu An-Ming, Zhang Zhi-Yun. The Differentiation, Evolution and Systematic Relationship of Juglandales[J]. .

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The Differentiation, Evolution and Systematic Relationship of Juglandales

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