Friday, March 13, 2020

Plant wrappers - leaf sheaths and bracts

While the young leaves of Magnolia
are developing,  they are each wrapped in a white
bract (technically a specialized, bract-like stipule).
Leaves are the most plastic of all plant organs.  That means that they can be modified in endless ways
to result in a mind-boggling variety of shapes. Through evolution via adaptive modification, leaves form an endless array of light-gathering antennas, from the giant fronds of palms to the tiny scales of a juniper twig, but beyond that, have adapted into tendrils, insect-catching traps, and even the parts of the flower.

In the fennel plant, the broad basal portions
of the leaves, the leaf sheaths, overlap to protect
the developing shoot apex.
Today, I'm talking about leaves, or parts of leaves, that form wrappers around tender growing parts of the shoot.  Modified leaves that do so are called bracts, and the modified lower parts of leaves that do so are called leaf sheaths.

A bract is a whole leaf, though it is typically smaller than a regular leaf, simpler in shape, and often colored differently. In some cases, brightly colored bracts serve as part of the apparatus for attracting pollinators, and may even appear to be petals.

A leaf sheath, on the other hand, is the broad basal part of typically large, complex leaves that surrounds the growing tip of the shoot. The rest of the leaf - typically a petiole and a blade - is typically full-sized,

As flowers and leaves emerge from a Crocus corm
in early spring, they are protected by white bracts.

In this bromeliad, Tillandsia cyanea, a fan of colorful
bracts help keep the plant on the radar of pollinators
as the flowers emerge one at a time.

Pachystachys lutea, or yellow shrimp plant, forms
a cone of yellow bracts to attract pollinators to
the white flowers.

As for leaf sheathes, some of the most spectacular are found in palms, but virtually all monocots form a leaf sheath when young.  Leaf sheathes attach to the stem in a complete circle when young, but typically splits apart on one side as the leaf matures and the stem within it expands.  In others, such as the royal palm, the overlapping leaf sheaths of the functioning leaves remain as a smooth, tight, crownshaft.

The leaf sheathes of the royal palms (Roystonea spp.) can be more than  four feet long.  They remain intact as complete
cylinders, forming what is called a crownshaft.  Photo from Palmpedia, photographer not indicated.

The "trunks" of banana plants are made up entirely of leaf sheaths, that may be more than three meters long, wrapped around each other (see "The invention and reinvention of trees")
As each new leaf emerges from the tip of the shoot
of a banana plant, its sheath is longer than the previous
ones.  This builds up a pseudostem of overlapping,
cylindrical  leaf sheaths.

Recall from "The underground plant movement" that the bulb of an onion or amaryllis is also made up of leaf sheaths that fill up with food and water, and are left as storage organs as the leaf blade on top of them dries up and disappears.
In a young onion plant, the leaf sheathes just above
the roots begin to fill with food.

When the onion plant goes dormant for the
season, the food-filled leaf sheathes remain,
forming  the rings of the onion.  The
outermost sheaths dry out to form a
protective tunica.
In many irises, gladioli and other members of the
Iridaceae, the leaf sheath is folded and the entire
shoot looks like it has been pressed with a hot iron.
Note that the newer leaves emerge from the
overlapping, folded leaf sheathes.


  1. Beautifully explained and illustrated, thank you. Where do stipules fit into this discussion?

  2. Excellent question. Stipules are most broadly defined as small, leaf-like appendages, usually in pairs, at the base of a dicotyledonous leaf petiole. They can be leaf-like, scale-like, spine-like or tendril-like. The bracts around the young leaves of Magnolia or Ficus are generally interpreted as highly specialized stipules fused together to form a cap around the next younger leaves. They might be more accurately called "bract-like stipules." Thanks for catching this.


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