In this article we will discuss about the anatomy of gnetum with the help of a suitable diagram.
The root of Gnetum has diarch xylem, the endodermis bounds the multi-layered pericycle. The cortex of parenchymatous cells is filled with starch; the thick-walled fibres are also common in cortex.
Tracheids have uni-seriate, bordered pits with conspicuous ‘bars of Sanio’ whereas pits on vessels are small and multi-seriate, with or without inconspicuous bars of Sanio. Xylem rays are thin-walled, of starch-containing parenchymatous cells. The xylem elements produced after secondary growth are larger than in stems.
The shoot apex in Gnetum is typical angiospermous- it shows tunica-corpus organization.
The young stems of G. ula and G. gnemon have single-layered epidermis of rectangular cells, with a thick coating of cuticle and sunken stomata. The cortex is broad, of 12-16 layers of parenchymatous cells in which the inner cells become fibrous with narrow lumen. In older stems, a ring of parenchymatous cells becomes sclerenchymatous and is referred as the ring of spicular cells. The endodermis and pericycle are indistinct in the stem.
Vascular bundles about 20-24, arranged in a ring (Fig. 18.1B) are collateral and endarch. The primary xylem is of circular bordered pits.
An anomalous secondary growth (Fig. 18.1D) occurs in some species due to additional cambia which arise either from secondary phloem or from cortical parenchyma and results in several concentric rings (Fig. 18. 1E) of secondary xylem.
In old stems, there is periderm. Branched sclereids occur in pith, cortex, petiole and lamina of many species.
The wood of Gnetum has vessels (Fig. 18. 1E, F) with a single pore on end walls with long tracheids of uniseriate bordered pits on both radial and tangential walls. Vascular rays are massive, of considerable width and height.
Leaves are supplied with 3-7 traces. The petiole bundles are first arranged in a U-shape (Fig. 18.1C) and are finally in a single line within the mid-vein region.
Vessels (Fig. 18.1F) of Gnetum are similar to the angiosperm vessel. However, this resemblance is no more than an example of convergent evolution because vessels in angiosperms have evolved from tracheids with scalariform thickenings, whereas the vessels in Gnetum have evolved from pitted tracheids.
Thus, in this respect, there is no affinity between Gnetales and angiosperms. In some species, the secondary phloem (unlike in most gymnosperms) contains companion cells as well as sieve tubes, but these two types of cells arise from different cambium initials and not from the subdivision of sieve-tube mother-cells, as in angiosperms.