In sphenopsida (Horsetails), the sporophyte is differentiated into root, stem and leaves. The leaves may be expanded or scale-like and are always arranged in whorls. Plants belonging to this group are also called arthrophytes because the whole plant body is composed of large number of joints.
Main stem is not smooth, it has large number of ridges and furrows. Each node has whorl of branches. The sporangia are born on structures called sporangiophores, aggregated to from strobili.
Each sporangiophore has a slender stalk and an expanded disc at its free end. The sporangia appear on the underside of the disc. The thalloid gametophytes grow upon clayey soil and on mud, e.g., equisetum (Fig. 9.16).
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b |
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Fig. 9.16: Representative of three of the subdivisions of vascular plants (a) club moss lycopodium (b) A horsetail. Equisetum (c) A tree fern.