A subdivision or class of * vascular plants containing a single living genus, Equisetum (horsetails) with about 23 species, but having a rich fossil record. The most conspicuous feature of sphenopsids is the aerial jointed stem, which bears whorls of leaves inserted at each joint. The stems arise at intervals from a perennial creeping rhizome and may either be all photosynthetic or differentiated into two types, green sterile stems and colourless fertile stems. The stems are grooved and consist of a central cavity surrounded by a ring of smaller cavities in the cortex (see vallecular canals ). These alternate with still smaller cavities associated with the protoxylem (see carinal canals ). The sporangia are borne on sporangiophores in terminal strobili. The spores are all approximately the same size but give rise to separate male and female gametophytes. The antherozoids are multiflagellate. The Sphenopsida is divided into four orders, the Equisetales and the extinct Calamitales, Sphenophyllales, and Pseudoborniales. The arborescent Calamitales, which flourished in the Carboniferous, most closely resemble the Equisetales. |
