Friday, May 4, 2012

The Invention and Reinvention of Trees


Most trees - plants with permanent, elevated,  leafy shoot systems - depend on wood for physical support and nutrient transport.  Wood consists of annual layers of secondary xylem, deposited by a cylindrical meristem called the vascular cambium.  The vascular cambium in most trees also lays down rings of secondary phloem, the necessary sugar transport tissue that carries food from the leaves back to the roots and other developing organs.  This is the standard model of trees and shrubs found throughout the gymnosperms and dicots (eudicots, magnolids and other ancient flowering plant lineages). 

Getting tall has its advantages in competing for light, dispersing seeds, etc., and on numerous occasions,  plants without a vascular cambium have found ways to do so.  Though perhaps not strictly-speaking trees, they are all interesting experiments that lasted for millions of years, or are still with us (e.g. palms, bamboos). The monocots, in particular, have a number of different forms of gigantism arising from rhizomatous ancestors all without any wood at all.

These tree ferns, growing in the temperate
rain forest of Australia, achieve considerable
height with their root-clad, upright rhizomes.
One very ancient form of non-woody gigantism, the tree fern, is still with us.  All forms of tree-like growth begin with low-growing herbaceous plants, usually with an underground rhizome system.  In the case of the tree fern, the rhizome has essentially "gone vertical."  This slender upright stem is strengthend by masses of fibers, but no wood.  It has no secondary growth and its "trunk" does not get thicker over time.  It is just about as thick at the top where the stem tissues are being laid down, as it is lower down. The thickness of the tree fern stem is enhanced by a thick mantle of fibrous  adventitious roots  (the tree fern fiber of horticultural commerce) that collectively serve as a water-absorbing sponge.  A massive terminal bud makes a single rosette of large compound leaves atop the thick stem apex and rarely branches.  Plants of this general form are sometimes referred to as pachycauls (“thick stems”), or rosette trees.  Palm trees and cycads are other common examples. 

Lepidodendron and Sigillaria were ancient relatives
of modern clubmosses.  Like the giant horsetails
featured earlier in The First "Bamboos," they had
meager layers of wood, but no secondary phloem.
From Smith, Cryptogamic Botany. 1955, Fig. 128

The first upright plants with a vascular cambium that produced layers of wood developed in parallel among club mosses and horsetails.  The problem was that they could not also produce layers of secondary phloem (food-conducting tissue) toward the outside, and their longevity was limited by that of the original phloem.   When a vascular cambium came along that could alternately produce xylem to the inside and phloem to the outside, truly large and long-lived trees became possible, and this led to the early explosion of seed plants (the first such trees were actually the seedless progymnosperms, which are believed to the the ancestors of the first seed plants).

As discussed earlier, the first monocots were seed plants that returned to the ancient underground lifestyle.  In the process, they lost all ability to make a vascular cambium.  So when various monocots found themselves in situations where getting taller would be advantageous, they had to reinvent the wheel, so-to-speak.  Bamboos spread underground via rhizomes like ordinary grasses, but their hollow, upright, leafy shoots have gotten taller and taller over time, adding thick bundles of fibers to their culm walls to support that upright growth.  In parts of Asia, they are aggressive enough to displace ordinary trees for many square miles.


The trunk of this Pigafetta palm growing
in Papua New Guinea, develops its full
thickness at the top, as the massive leaf
bases expand.

Palms like this Ptychosperma develop many
upright stems from a branching
underground rhizome system.

Palms appear also to have originated from underground plants.  Many still spread by rhizomes like the bamboos.  Their upright leafy shoots are not hollow, but filled with hard fibrous bundles, or sometimes with a softer, food-storing center (e.g. the true sago palms, genus Metroxylon).  Some, like the tropical mangrove palm, Nypa fruticans, retain a basically horizontal position, with only leaves and flowerstalks rising vertically.  The saw palmetto of Florida (Serenoa repens) has a similar habit with its stems mainly lying on the ground and occasionally turning upward.  Those palms that become solitary rosette trees, like the coconut, are actually exceptional in having given up their rhizomatous underground system.  They, like all monocots, lack secondary growth, but have enormous buds atop an expanded shoot apex, which is as thick as most of the rest of the trunk. 

Philodendron selloum achieves some modest
height by supporting its stem with prop roots.
Some would-be monocot trees don’t have quite such a thick trunk, but produce a series of adventitious prop roots, both for support and for additional water-absorption.  A simple example is Philodendron selloum, whose relatives usually climb up trees.  Though it can’t claim the tree-like dimensions of palms or bamboos, it is a giant within its family (Araceae).

The screw pine (Pandanus) is a monocot with
long strap-shaped leaves and a fibrous
trunk similar to that of palms.  It supports itself
with prop roots
This giant Pandanus in a New Guinea forest
has prop roots six inches thick.
The screw pines (genus Pandanus) also rely on prop roots to support their upward growth, but are able to achieve true tree size and compete with forest trees.  Unlike palms, screw pines sometimes produce a number of branches, but without secondary growth, the branches are progressively and permanently thinner as they spread their crown. 

Another approach to tree-ness is seen in bananas and some gingers.  What appears to be a trunk is actually mostly the concentric cylindrical bases of the leaves (the leaf sheathes).  Each new leaf that pushes up through the center of this false stem (pseudostem) has a longer cylindrical base than the previous, and so can achieve the proportions of a modest tree.   The true vertical stem rises through the center of the pseudostem only when it is time to flower and fruit.



The herbaceous pseudostem of a banana shoot
builds up as each tubular leaf sheath that
pokes up through the center is longer than
the previous one.
Banana "trees" are really giant herbs.  The soft shoots
bud off of an underground rhizome system and die
after fruiting


























 
A cross-section of a banana trunk
reveals the nested series of leaf sheaths
from which it was built.  The solid circle
in the center is the stalk of the
inflorescence, which pushes the cluster
of flowers to the top of the plant. From Brown,
The Plant Kingdom, 1935. Fig. 92


A most interesting case of monocot gigantism is seen in the Egyptian papyrus (Cyperus papyrus in the Sedge Family, Cyperaceae).   This source of ancient paper and floating bassinets for infant prophets, is mostly a long smooth stem arising from an underground rhizome, with a crowded tuft of grass-like leaves and flowerstalks at its tip.  The smooth stem, from which the valuable fiber is obtained, can be 3 meters tall, and consists of a single elongate internode.  Other sedges have a similar stalk for elevating flowers above a grass-like clump, as do familiar plants like onions and amaryllis.  So papyrus “trees” are basically overgrown flowerstalks.

Papyrus shoots arise from underground rhizomes through the
elongation of a single internode at the base of the globe-shaped
cluster of leaves and flowers. From Kerner and Oliver, The
Natural History of Plants, 1904.

The globe-shaped cluster of leaves and
flowers of the Egyptian papyrus plant are
lifted to tree-like proportions by the
elongating flower stalk.















The most tree-like of all monocots are found in Dragon trees and their relatives (in the genera Dracaena and Cordyline) and in giant aloes.  Though they do not have a conventional vascular cambium, they have evolved a new way of expanding the older stems with a cambium-like layer that continuously produces whole new vascular bundles containing xylem and phloem.

The dragon tree (Dracaena) adds layers of whole
vascular bundles to continually thicken the stems.
From Kerner and Oliver, The Natural History of
Plants, 1904.
So the monocots have been successful in becoming tree-like in a variety of ways without a conventional vascular cambium, adding to their reputation as a varied and highly successful group of plants.




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