A New Morphological Interpretation of the Female Reproductive Organs in Ginkgo biloba L., with a Phylogenetic Consideration on Gymnosperms

Abstract

Abstract:

The ovuliferous structure of Ginkgo biloba L. has been variously inter-

preted morphologically. As a result the systematic position and the relationship

with other gymnosperms of this ancestral gymnosperm have long been under dis-

pute. In the present paper, a brief survey of the main views as to the nature of

the ovuliferous structure is givcn. Based on morphological and teratological data

previously reported, a new interpretation is proposed. The essential points are

summarized as follows:

1. In morphological essence, a fertile dwarf shoot with some ovuliferous struc-

tures in Ginkgo biloba L. might as a whole be nothing but a megasporophyll

strobilus (female cone), which is shared actually by all the conifers in the

gymnosperms. The fertile dwarf shoot has appearance extremely similar to that of

the vegetative dwarf shoot, suggesting that in Ginkgo biloba L. the vegetative or-

gans and the reproductive organs have not been yet well differentiated, and thus

its megasporophyll strobilus might represent one of the most primitive compound

strobilus types.

2. In Ginkgo biloba, the ovuliferous structure borne in the axil of a scale

leaf (sometimes a normal leaf) on the dwarf shoot, together with the scale leaf it-

self, might be the homogenous organ corresponding to the bract-scale and

seed-scale complex of the compound female strobilus of the typical conifers. The

complex is a relatively isolated reproductive unit on the strobilus. The normal

leaves and the scale leaves on the dwarf shoot might be equivalent to the

bract-scales in the typical cones, though the normal leaves still retain the vegetative

nature as the foliage leaves on the vegetative shoot. The stalk hearing ovules at its

top might be equivalent to a seed-scale of the typical cones.

3. The megasporophyll strobilus in Ginkgo biloba, namely a whole fertile dwarf

shoot as mentioned above, seems to show much more primitive characteristics

than those of typical conifers. In this plant it is very difficult to distinguish the fer-

tile dwarf shoot from the common vegetative dwarf shoot before reproduction

time. Moreover, its megasporophyll strobilus often exhibits more atavistic abnor-

malities than those of other conifers. All the evidence indicates that the primitive

ancestor of conifers might have had the fertile organs which might be of basically

identical morphology as vegetative shoots, except that in the fertile organs there

might exist numerous fertile leaves bearing one or many ovules.

4. The longer stalk of the ovuliferous structure in Ginkgo biloba might have

come from mainly a secondary elongation growth of the seed scale, and only a lit-

tle part of it might be the remains of the original shoot. The fork structure bear-

ing ovules at the top of the stalk might be the rudementary part of the petioles of

the only two extremely reduced megasporophylls. The collar around the base of

the ovule might be a secondary protective structure.

5. A correct morphological interpretation of the female strobilus in Ginkgo biloba

is doubtless of important significance for our better understanding of the evolution

of the female reproductive organs in conifers. According to our interpretation men-

tioned above, together with the concept of the bractscale and seed-scale complex

proposed in the present paper, which is mainly based on the concept of the

seed-scale complex propose by Florin, here we put forward an evolutionary theory

of the bract-scale and seed-scale complex. According to this theory, the female reprodu-

ctive organs of the ancestral conifers should be very similar, as mentioned above, to

the sterile foliage shoot except that the former might have some fertile leaves which

could produce ovules at reproduction time. This ancestral female reproductive or-

gan type might have had evolved towards two directions and thus formed two

main evolutionary lines. One is represented by the genus Cycas and we may call

it the Cycas Evolutionary Line (C-line), in which the megasporophyll strobilus is

monopodial, with the fertile leaves and sterile bracts occurring directly on the

main axis. The Cycadaceae is the only living gymnosperm member along this evo-

lutionary line. The second line is represented by all the conifers including Ginkgo,

which all have the structure of the bract-scale and seed-scale complex, and thus we

called it the "Bract-scale and Seed-scale Complex Evolutionary Line" (BS-line).

The members along this line have multipodial female strobilus, i.e. compound

strobilus. On the main axis occur some sterile vegetative bracts. In the axils of

some or most of the bracts occur the seed-scales. The seed-scales are actually the re-

mains of the extremely, or smetimes completely reduced fertile shoots. Each part

of the bractt-scale and seed-scale complex and the main axis of the strobilus could

have undergone independent or correlated changes, and thus have had formed vari-

ous types of strobilus which are found in the living conifers.

6. Our theory on the evolution of the bract-scale and seed-scale complex seems

to support the division of all the gymnosperms into two major groups as pro-

posed by Chamberlain, and is also in favour of the placement of Ginkgo biloba

into the conifers as the most primitive member along BS-line.

7. Based on their similar morphological characters, it can be considered that

Ginkgo biloba might have close relationships with the Nageiaceae, Ephedraceae,

Welwitschiaceae and Araucariaceae. All these groups have multinerved leaves with-

out costa. These living gymnosperms might have a more direct relationship with

the ancestral cordaites.

Key words: Ginkgo biloba L, Female reproductive organ, Bract-scale and seed-scale complex, Systematics of gymnosperms

Fu De-zhi, Yang Qin-er. A New Morphological Interpretation of the Female Reproductive Organs in Ginkgo biloba L., with a Phylogenetic Consideration on Gymnosperms[J]. .

A New Morphological Interpretation of the Female Reproductive Organs in Ginkgo biloba L., with a Phylogenetic Consideration on Gymnosperms

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