Wednesday, March 28, 2012

The first "bamboos"

Earlier ("The grasses that would be trees," March 18, 2012), I described the unique pattern of development that results in the tall, lightweight, and very strong stems of bamboos.  The key to the rapid growth of bamboos is a combination of lightweight, hollow construction, plus a process of growth involving intercalary meristems in each internode that all elongate more-or-less at the same time.  Before there were grasses, before in fact there were any seed plants, a group of spore-bearing relatives of ferns discovered virtually the same growth form.  These were the horsetails, formally known as the Sphenophytes. 

Like bamboos, horsetails send up new shoots from buds
on underground rhizomes.  Each bud contains a complete
compressed stem, with many nodes and internodes packed
closely together. Intercalary meristems within each internode
become active at the same time, adding new tissues to each
and raising the stem rapidly.  The true leaves are modified
into toothed, cup-like structures that protect the tender
growing region of each internode.  From Kerner and
Oliver, The Natural History of Plants, 1904, Fig. 190.
Very few of these sphenophytes survive today, but you can see the bamboo-like form in the stems of modern horsetails.  Like bamboos, the horsetail stem is hollow and its wall fortified with fibers.  Also like bamboos, the young horsetail shoot forms as a condensed bud, with nodes and internodes of the entiren stem crowded together.  A basal intercalary meristem in each indernode begins expansion in coordination with all the others in the shoot, resulting in rapid upward growth.  Leaves at each node are reduced to stiff bracts that protect the tender growing region at the base of the internode. 

Giant horsetails, commonly referred to the
genus Calamites, grew like bamboos and
dominated the coal-forming swamps of
the Carboniferous Period. From Smith,
Cryptogamic Botany, 1955,  Fig. 151.

From the late Devonian, Carboniferous and Permian periods, some 350-300 million years ago, giant tree-like horsetails, growing up to 100 feet high, dominated early forests, sprouting from underground rhizomes, just like modern bamboos. They most likely elongated fairly rapidly, but develeped a modest amount of wood to support their large crown of branches.  

A modern horsetail, growing in a ditch beside the
road in Washington State, is just as at-home in the
21st century as its ancestors were 300 million years
ago.  It continues to compete with neighboring
vegetation through its rapid growth from preformed buds
in the spring.  The true leaves are modified into
 bracts that protect the growing  tissues above each node.
Photosynthesis is conducted by tissues in the main
stem as well as by the whorls of slender stems at each node.

Modern horsetails are for the most part fairly modest in size, living in shaded moist areas alongside the descendents of their other ancient companions, ferns and clubmosses.  The largest, up to 8 ft or more in height, are found oddly in moist streamsides in dry areas of Central and South America.  For an image, click on the link below, or if it is no longer active, do a simple web search for Equisetum giganteum:

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