Classification
One of the objectives of systematists is to discover a phylogenetic system of classification. The evolutionary history of gymnosperms extends far back in geological time (Figs. 1.1, 1.2); its fossil record begins in the Upper Devonian and spreads over nea
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1 Phyllosperms. Leaves large and much divided, ovules borne on leaves or organs regarded as such: (a) Pteridospermales, (b) Cycadales, and (c) Bennettitales. 2 Stachysperms. Leaves simple, ovules borne on stems: (a) Cordaitales, (b) Ginkgoales, (c) Coniferales, and (d) Taxales including Taxus, Torreya and Cephalotaxus; separated from Coniferales. Florin (1948) upheld the separation of Taxales as an Order, coordinate in rank with the Cordaitales, Ginkgoales and Coniferales (restricted). However, he included only Taxus, Torreya, Nothotaxus; Amentotaxus, Austrotaxus and their fossil relatives, and retained Cephalotaxus within Coniferales. Chamberlain ( 1920, 1935)-almost simultaneously with Sahni-recognized two main groups amongst gymnosperms: C. Biswas et al., The Gymnosperms © Springer-Verlag Berlin Heidelberg 1997
Classification
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1 Cycadophytes. (a) Cycadofilicales, (b) Bennettitales, and (c) Cycadales. 2 Coniferophytes. (a) Cordaitales, (b) Ginkgoales, (c) Coniferales, and (d) Gnetales. The main difference between Sahni's and Chamberlain's classification is the emphasis on the primary characters. Sahni's main basis is the morphological nature of the ovule-bearing organs. This, according to Scott (1923), is mainly a theoretical distinction for groups like Cordaites, Bennettitales and Coniferales. Chamberlain emphasizes mainly the differences in habit, stem anatomy and leaves. Arnold (1948) gave a palaeobotanical foundation to Chamberlain's (1935) system. He interpreted the class Gymnospermae as an artificial polyphyletic group where the seed habit arose independently in cycadophytes and coniferophytes. Arnold, therefore, recommended that the class Gymnospermae should be dropped from the systems of classification. He divided Gymnosperms into three phyla. 1 Cycadophyta. (a) Pteridospermales, (b) Cycadeoidales, and (c) Cycadales. 2 Coniferophyta. (a) Cordaitales, (b) Ginkgoales, (c) Taxales, and (d) Coniferales. 3 Chlamydospermophyta. (a) Ephedrales, and (b) Gnetales. According to Arnold, the Cycadophytes and Coniferophytes represented natural groups and had separate origin. Pant (1957) modified Arnold's classification: Division 1. Cycadophyta: Class Pteridospermopsida: (a) Lyginopteridales, (b) Medullosales, (c) Glossopteridales, (d) Peltaspermales, (e) Corystospermales, and (f) Caytoniales. Class Cycadopsida: (g) Cycadales. Class Pentoxylopsida: (h) Pentoxylales. Class Bennettitopsida (Cycadeoideopsida): (i) Bennettitales (Cycadeoidales). Division 2. Chlamydospermophyta: Class Gnetopsida: (a) Gnetales, (b) Welwitschiales. Division 3. Coniferophyta: Class Coniferopsida: (a) Cordaitales, (b) Coniferales, (c) Ginkgoales. Class Ephedropsida: (d) Ephedrales. Class Czekanowskiopsida: (e) Czekanowskiales. Class Taxopsida: (f) Taxales. Bierhorst (1971) presented a slightly different classification: 1 Cycadopsida: (a) Pteridospermales, (b) Cycadales, (c) Cycadeoidales, and (d) Caytoniales. 2 Coniferopsida: (a) Cordaitales, (b) Coniferales, (c) Taxales, and (d) Ginkgoales. 3 Gnetopsida: (a) Ephedrales, (b) Gnetales, and
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