J Syst Evol ›› 1993, Vol. 31 ›› Issue (6): 505-516.

• Research Articles • Previous Articles     Next Articles

The Seedling Types of Gymnosperms and Their Evolutionary Relationships

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

  • Published:1993-11-18

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 elongated; 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, absorbing 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 origin 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 evolutionary 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 arrested, changing to the cotyledons of the Cycas Type and replacing the foot of the progymnosperms as an absorptive organ. The stomata on the cotyledon epidermisof Cycas and Ginkgo are a residue of the first two leaves of the progymnosperms.

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