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The Seedling Types of Gymnosperms and Their Evolutionary Relationships

Ye Neng-gan, Gou Guang-qian, Liao Hai-min, Zhang Zhu-lin   

  • Received:1900-01-01 Revised:1900-01-01 Online:1993-11-18 Published:1993-11-18
  • Contact: Ye Neng-gan

Abstract:

 The three types and eight subtypes of seedlings are recognized in the
gymnosperms in the present paper. They are: 1. The Cycas Type: cotyledons 2,
with  absorptive function, absorbing nutrients from the endosperm (gametophyte);
hypogeal;  including three subtypes: (1) The Cycas Subtype: internodes not elon-
gated;  leaves simple, pinnate;  (2) The Ginkgo Subtype: internodes elongated;
leaves fan-shaped;  (3) The Araucaria Subtype: internodes elongated;  leaves linear.
2.  The Pinus Type:  cotyledons 2 to numberous,  with both absorptive and
photosynthetic Functions;  epigeal;  including three subtypes: (1) The Cunninghamia
Subtype: cotyledons 2- 4;  internodes elongated;  leaves linear;  (2) The Pinus
Subtype: cotyledons numerous;  internodes not elongated;  leaves needle-like (3)
The Ephedra Subtype: cotyledons 2;  internodes elongated;  leaves scaly. 3. The
Gnetum Type: with an absorptive function foot at the base of the hypocotyl, ab-
sorbing nutrients from the endosperm;  cotyledons 2, with photosynthetic function,
epigeal, containing two subtypes: (1) The Gnetum Subtype: cotyledons similar to
foliar leaves, like the pinninerved leaves of the Dicotyledons;  internodes elongated;
(2) The Welwitschia Subtype: cotyledons linear, internodes absent, the whole
plant with only a pair of leaves. Detailed descriptions and a key to the three
types and eight subtypes are presented in the paper. It is considered that the ori-
gin of the gymnosperms is not monophyletic but polyphyletic. That is to say, the
seed of gymnosperm is polyphyletic as a result of parallel evolution in different
groups of the progymnosperms. According to the morphological characters of the
seedling types we propose that there have existed four evolutionary lines in the
gymnosperms, namely: the Cycas line, the Ginkgo line, the Conifer line and the
Gnetum line, and the evolutionary process of each line is explained. The evolu-
tionary relationships among the three types and eight subtypes are discussed.
There is a foot in the Gnetum Type, which is uncomparable with any seedling
type of seed plants, since it has its unique developed line and differentiated into
the Gnetum Subtype and the Welwitschia Subtype. The evolutionary tendency is
from hypogeal to epigeal in the other types and subtypes. Cycas and Ginkgo are
relict with seedling types belonging to the Cycas Subtype and the Ginkgo Subtype
respectively, which have maintained an ancient character-hypogeal. Therefore,
the evolution of seedling types is from the Araucaria Subtype to the Pinus Type,
and the latter itself differentiated into three subtypes. Moreover, it is explained
why there are stomata on the hypogeal cotyledons of Cycas and Ginkgo when the
footed embryo of the progymnosperms changed to the embryo of the Cycas type
via neoteny, the foot was lost, and the first two leaves on the stem tip were ar-
rested, changing to the cotyledons of the Cycas Type and replacing the foot of the
progymnosperms as an absorptive organ. The stomata on the cotyledon epidermis
of Cycas and Ginkgo are a residue of the first two leaves of the progymnosperms.

Key words: Gymnosperms, Seedling type, Evolutionary line, evolutionary relationship

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