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A central strand of xylem occupied up to a third of the diameter of the stem. In the lower parts of main stems it was circular, enlarging before it divided prior to dichotomous branching. Higher up it became elliptical in cross-section, corresponding to the two rows of fertile branching units.
Psilotum is a genus of fern-like vascular plants.It is one of two genera in the family Psilotaceae commonly known as whisk ferns, the other being Tmesipteris.Plants in these two genera were once thought to be descended from the earliest surviving vascular plants, but more recent phylogenies place them as basal ferns, as a sister group to Ophioglossales.
Psilotum nudum, the whisk fern, [3] is a fernlike plant. Like the other species in the order Psilotales, it lacks roots. [4]Its name, Psilotum nudum, means "bare naked" in Latin, because it lacks (or seems to lack) most of the organs of typical vascular plants, as a result of evolutionary reduction.
A typical absorption cross-section has units of cm 2 ⋅molecule −1. In honor of the fundamental contribution of Maria Goeppert Mayer to this area, the unit for the two-photon absorption cross section is named the "GM". One GM is 10 −50 cm 4 ⋅s⋅photon −1. [1] [2]
The stem anatomy of ferns is more complicated than that of dicots because fern stems often have one or more leaf gaps in cross section. A leaf gap is where the vascular tissue branches off to a frond. In cross section, the vascular tissue does not form a complete cylinder where a leaf gap occurs.
The gram (10 −3 kg) is an SI derived unit of mass. However, the names of all SI mass units are based on gram, rather than on kilogram; thus 10 3 kg is a megagram (10 6 g), not a *kilokilogram. The tonne (t) is an SI-compatible unit of mass equal to a megagram (Mg), or 10 3 kg.
The stem is either underground or aerial. The leaves may be microphylls or megaphylls . Their other common characteristics include vascular plant apomorphies (e.g., vascular tissue ) and land plant plesiomorphies (e.g., spore dispersal and the absence of seeds ).
The stems are usually without branches and smooth at the base, except for a single groove on the upper surface of the lower section of stem. [ 4 ] Leaves are oblong in shape, sometimes broad as well, and are roughly 20-30mm in length, 4-8mm wide. [ 3 ]