ORDER GNETALES
Gnetales is most highly evolved group of gymnosperms .
Gnetum
Most of the species are lianes and climbers except few shrub and trees . G. gnemon is a small tree and resembles in its appearance to that of angiosperms . G. trinerve is a parasite . G. ula is most common species found in India . Gnetum species are found in sea shore areas and north east himalayas .
Root is tap root . Stem bears two types of branches - dwarf shoot of limited growth and long shoot of unlimited growth . All branches are of same types in G. gnemom . Foliage leaves are present only on dwarf shoot . Long shoot bears pairs of reduced scale leaves . Foliage leaves are arranged in opposite decussate manner .
Both the roots and shoots of Gnetum show secondary growth . Young stem and leaves show similarities with that of dicot angiosperms . Old stem in some species shows eccentric rings of xylem and phloem due to excessive growth on one side . This is characteristic of angiospermic lianes . Vessels are present in the xylem .Variations in the size and various types of perforation in the end walls occurs in the vessels of Gnetum . Origin of vessels is different from that of angiosperms . Gnetum shows uniseriate , biseriate and multiseriate medullary rays . Shoot apex of Gnetum can be interpreted in the terms of tunica corpus concept .
Gnetum is usually dioecious . Cones are organised into infloresceces . Sometimes cones are terminal in position . Cone axis has two bracts at the base and is divided into nodes and internodes . Whorl of circular bracts are arranged one above on nodes , these rings are called collars . Flowers are present in these collars . Each male flower has two unilocular anthers attached on a stalk enclosed within perianth . At maturity anthers come out by elongation of stalk . Above the male flowers whorl of abortive female flowers is present .Abortive ovule has single envelop . Female cone has a single ring of female flowers or ovules just above each collar . Normal ovules have 3 envelops . One of the ovule in a whorl may be enlarged to mature in seed , others become abortive . Micropylar tube of ovule is derived from inner envelop . Pavement tissue develops from nucellus in Gnetum .
Male and female gametophytes show more affinity with the angiosperms than gymnosperms . Female gametophyte follows tetrasporic type development . Gnetum approaches the angiospermic condition in complete elimination of prothallial cell . Microspore of Gnetum sheds at 3 nucleate stage . Fertilization is siphonogamous . Archegonia are absent in female gametophyte . Before fertilization endosperm is cellular at chalazal end but after fertilization whole endosperm becomes cellular in most species . Peculiar cell is responsible for embryo formation in G. ula . Secondary suspensor cell is responsible for formation of embryo in most species . Gnetum has 2 cotylendons ( like dicot angiosperms ) . Seed germinatuin is epigeal . Polyembryony is common in Gnetum .