BSC 314 Lecture Notes - Lecture 58: Metasequoia, Mesozoic, Aril
28 Jun 2018
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The Gymnosperm Phyla
The four phyla of living gymnosperms are of separate clades or lineages, unlike the
angiosperms, which are a monophyletic, single lineage. “Gymnosperm” means “naked
seed” and the name draws attention to the ovules and resulting seeds that are exposed
openly on the megasporophylls.
Phylum Coniferophyta
The conifers are woody, mostly evergreen trees, with needle shaped or flattened ‐
leaves, which occupy the drier and cooler sites in the world today just as their ancestors
probably did in the Permian. They are the familiar pines, firs, spruces, yews, hemlocks,
and junipers of the Northern Hemisphere forests and the Araucariaspecies of the
Southern. The Sequoias are among the tallest living trees (the Australians credit
the Eucalyptus, an angiosperm, as being the tallest), and the bristlecone pines are
among the oldest living plants. ( The oldest plant, purportedly, is a clone of creosote
bush in the Mojave Desert that is well over 4,000 years of age.)
The pines (pinus species). The leaves of the 90+ species of pines are needles that, in
the seedlings, are borne singly along the stem. As the seedling matures, however, the
needles appear in fascicles (bundles) of several (the number varying by species) on
short shoots covered with scale like leaves. The fascicle is a branch of determinate ‐
growth, a feature of evolutionary significance.
The pine needles are adapted for a xeric environment, one in which water is unavailable
either because it is frozen most of the time or else because it is climatically scarce.
Needles have a thick cuticle, an epidermis, and an underlying hypodermis of thick‐
walled cells, which further protect the mesophyll from drying out. The vascular bundles
are surrounded by transfer tissue of parenchyma cells. Resin canals are present in
regular patterns within the needle. Needles are shed at intervals of two to four or more
years, but not all of the needles on the tree are dropped at the same time. Thus, a tree
remains evergreen.
The wood of pines and conifers (called softwood by lumbermen) in general lacks
vessels and is composed of tracheids with circular bordered pits. Parenchyma is almost
entirely restricted to ribbons of narrow rays. The vascular cambium is bifacial; that is, it
produces secondary xylem (wood) toward the center and secondary phloem toward the
outside of the stem.
The life cycle of a pine is a slow, two year process (Figure‐ ). Pollination occurs in the
spring of one year, and the pollen tube begins its growth towards the megagametophyte
about this time although its goal, the egg, is not as yet differentiated. In fact, the
megaspore mother cell has not yet divided, the megagametophyte does not exist, and
there are no archegonia, let alone eggs within the ovulate cones. This seeming lack of
syncronization is of little concern because it takes the pollen tube over a year to digest
its way through nucellular tissues to the archegonia—which gives ample time for
megagametophyte preparations—and for the immature male gametophyte (the four‐
celled germinated pollen grain) to produce two sperm cells by division of the generative
cell.
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Document Summary
The four phyla of living gymnosperms are of separate clades or lineages, unlike the angiosperms, which are a monophyletic, single lineage. Gymnosperm means naked seed and the name draws attention to the ovules and resulting seeds that are exposed openly on the megasporophylls. The conifers are woody, mostly evergreen trees, with needle shaped or flattened leaves, which occupy the drier and cooler sites in the world today just as their ancestors probably did in the permian. They are the familiar pines, firs, spruces, yews, hemlocks, and junipers of the northern hemisphere forests and the araucariaspecies of the. The leaves of the 90+ species of pines are needles that, in the seedlings, are borne singly along the stem. As the seedling matures, however, the needles appear in fascicles (bundles) of several (the number varying by species) on short shoots covered with scale like leaves. The fascicle is a branch of determinate growth, a feature of evolutionary significance.
