Thursday, January 26, 2012

How the grass leaf got its stripes

In monocot leaves, the veins are fundamentally parallel to one
another from the time they enter the leaf base from the stem. 
Some monocot leaves are distorted into other shapes, causing the
veins to bow out from one another.
[For a more complete story of the adaptations and evolution of grasses and other monocots, see Chapter 9 of my book, Plant life - a brief history]

A blade of grass, if you look closely, has long stripes running from its base to its tip.  These are technically the veins containing vascular tissues: xylem and phloem.  These veins conduct water and minerals up into the blade and sugar from photosynthesis down into the stem and roots.  The arrangement is what textbooks call parallel venation.  Each vein enters the broad leaf base independently, and remains parallel and separate from all the others up to the tip of the leaf. 

This anatomical feature of grass leaves speaks to the difference between the traditional division of flowering plants into dicots and monocots (of which grasses are perhaps the epitomy).  Though dicots are no longer recognized as a single distinct clade, they do exhibit a suite of characters by which they differ sharply from the monocots, and therefore can be referred to informally.  

Typical dicot leaves, for example, have a net venation, in which a few veins enter the leafblade via a narrow petiole and then branch repeatedly into a netlike pattern. 
The leaves of dicotyledonous trees, shrubs and herbs have a
 netted pattern of venation developing from the branching
of one or a few primary veins. 
From Kerner & Oliver.  The natural history of plants.  Gresham. London. 1904. 

These contrasting leaf types are the result of a different pattern of growth.  Dicot leaves form as miniatures, with just a few veins, then expand in all directions, adding finer veins between the main veins as they go.  Monocot leaves, on the other hand, produce new leaf tissue at the base, which pushes the older part of the leaf upward.  The many parallel veins appear when the leaf is young and very short, then new tissues are added to the bottom of each as the leaf gets longer.
The leaves of this Liquidambar tree, a dicot,
form in miniature in the terminal bud, then
expand rapidly.  The net-like pattern of
venation developes as ever finer veins are
added between the earlier veins.

The distinctive pattern of growth of the grass leaf is found throughout the monocots, at least in their seedlings, and it appers to have begun in the ancestral monocot as an adaptation to growth from an underground stem.  The tips of the monocot leaves become mature and stiff when they are very short, so as to be able to push through the soil, while the softer, growing bases are nestled below ground among the bases of older leaves.

Many monocots retain this subterranean existence with their primary stems taking the form of rhizomes, bulbs, and corms.  Those that don't, such as single-stemmed palm trees, still have seedling leaves that push up from below the ground.  

The leaves of most dicots, on the other hand, are adapted to form at the tips of exposed twigs, with the miniature versions forming within buds, then expanding rapidly to full size.

The Amaryllis plant consists of an underground bulb, with new leaves arising from the central bud.  One can demonstrate the basal growth of the leaves by making a mark near the base of a new leaf.  The mark rises as new tissues are produced below it.

Seedlings of dicots ( a and b) develop leafy shoots above ground, with the terminal bud and new leaves exposed to the elements, while the seedlings of monocots (c and upper right inset) have highly compressed shoots that keep the terminal bud below ground.  Leaves arising from the buds then push up through the soil, adding new tissues only at the base.

Keeping the main stems, buds and leaf bases below ground is advantageous in variety of conditions.  Plants can survive winters, dry seasons, fires, and grazing by remaining dormant below ground.  This is where grasses excel, giving them mastery over plains and savannas.  The tops of the grass blades can be chewed to the ground, and quickly restored by the basal growth.  But it also serves semi-aquatic plants well, by allowing leaves and shoots to rise above the water level through basal growth.
A number of dicots, such as
this aquatic pennywort, have
petioles that can extend through
basal growth to lift their blades
above water.  Something like
this may have preceded the first

It is therefore not certain whether monocots were initially adapting to aquatic conditions or to harsh seasonal conditions.  They may have been preceded by aquatic dicots that also have the ability to extend their leaves upwards through basal growth of their petioles (leaf stalks).  One theory is that in the ancestral monocots, basally growing petioles eventually lost their conventional leaf blades, and the petiole became wider to become the new kind of blade.

From basic monocots with underground rhizomes, bulbs, and corms, many specialized forms have evolved, including tree-like members of the palm, pandan, and Dracena families. Epiphytic orchids, aroids and bromeliads have rhizomes that creep along tree limbs.  Banana plants use basal growth to lengthen their cylindrical leaf sheaths into a false stem, while Egyptian papyrus lifts a globular crown above water through basal growth of its flowerstalk.  And finally, the giant grasses known as bamboos rapidly reach tree height by regions of basal growth in each of their internodes.