Sunday, July 5, 2020

The folded leaves of Iris

In this Bearded Iris  the leaves are folded and flattened,
forming a fan perpendicular to the tip of the rhizome.
Many members of the Iris Family exhibit a peculiar, fan-shaped arrangement of their leaves. Leaves that are lined up on two sides of the stem in a single plane are called 2-ranked, or equitant.  Such an arrangement of leaves is not uncommon, occurring in the Traveler's Palm, Ravenala madagascariensis, for example.

In the Traveler's Palm, leaves are equitant, but have
conventional, spreading blades, with exposed upper and 
lower surfaces.
The leaves of the Iris connect to the rhizome in a circle, as
in most monocots, but above that,the the two
sides fold together tightly forming a narrow channel
through which newer leaves emerge. Still higher, the two
sides  of  the leaf become completely joined together, forming
what  appears to be a simple, sword-shaped leaf blade.

The bud of a new inflorescence pushes up through the
center of the fan.
But what's most interesting in the Iris is that the leaves are folded, with the two sides fused together into a seemingly simple structure.  It's as if someone has taken a hot iron and pressed the whole clump of leaves into a flat sheet in preparation for mounting in a herbarium. You can see such leaves in many members of Iris, Gladiolus, and related genera. It has evolved independently in unrelated monocots such as Acorus (Acoraceae) and Lachnanthes (Haemodoraceae).

Such folded leaves are called unifacial (one-faced), because both sides are actually the same side - technically the abaxial side. The upper, or adaxial side of the leaf is totally internalized.

You can see the folding most obviously at the bases of the leaves where the two sides remain separate to form a leaf sheath. New leaves emerge from the center of the fan through the folded bases.
The inflorescence results from the elongation
of the rhizome tip, with long internodes
between leaves that are reduced in size. Each
leaf is open at the base, but fused into a
solid upper portion.
Like the leaves in the main fan, those on the inflorescence
stalk are open at the base, but fused together in the upper part.
Ultimately, the spectacular flowers of the Bearded Iris open, beginning at the top. Other flowers
will emerge from the bracts lower down. Incidentally, this is a rare sight in central Florida, where these pictures were taken. Only recently have "reblooming" varieties of the Bearded Iris been grown successfully here.

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