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| PLANTS | Glossary L-Q |
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Pieces |
| Palæos: | |
Plants |
| PLANTS | Glossary L-Q |
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Unit Cladogram |
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Pieces |
Lateral meristem: same as vascular cambium. See cambium.
Leaf: a flattened, photosynthetic structure of a plant arranged on a stem.
Leptoid: Cells involved in solute storage and transport in Bryophyta. Analogous, possibly homologous, to the phloem of tracheophytes. See image at hydroid.
Leptosporangium:
sporangium developed from a single superficial cell. Leptosporangiate:
having leptosporangia. This is a derived character state in the fern
clade; compare to eusporangium.
Image from the Botany
125 site of Prof. Curtis Clark, California State Polytechnic Univ. at
Pomona.
Lignin:
wood. Lignin is formed by the removal of hydroxyl groups from sugars,
creating phenolic compounds and short-chain alcohol ligands. Lignin polymers are
heavily cross-linked. There is great variation in lignin, even within the
same plant. Undoubtedly, it has evolved to have a somewhat random
structure to foil enzymatic attack. Lignin is, in fact, extremely
difficult to dissociate -- so much so that even as late as 1980 almost nothing
was known of the detailed chemistry of lignin, the second most common biopolymer
on the planet (next to cellulose). Fortunately, considerable progress has
been made since then. The basic monomer of lignin can be thought of as
4-alkylcatechol. The alkyl ligand is a 1 to 3 carbon chain which may be
substituted with hydroxy or keto ligands at any or all positions. Both phenol
groups and the alkyl substitutions may cross-link to other monomers or their
side chains, yielding a fairly light, but strongly cross-linked and randomly
ordered mass, i.e., wood. What is more, the chemical
structure virtually guarantees that any intermediate degradation product will be
a highly toxic phenolic compound. Lignin was a rather early product of
plant evolution. The necessary implication is that, contrary to most
current thought on the subject, land plants were subject to biological attack
(although not necessarily attack by herbivorous animals) by at least the Middle
Devonian, and probably even earlier.
Ligule: small protuberance from the base of a leaf.
Lobed margin (leaves) margin indented one quarter or more of the distance from the margin to the mid-vein or (where this is lacking) to the long axis of the leaf; compare entire margin and toothed margin leaves
Locule, alternatively loculus (pl. loculi): a chamber, compartment or other limited volume which is used to accumulate or store either gametes or spores. Typically, the locule contains only gametes or spores. Thus a sporangium is a spore-producing organ, but a locule within the sporangium is where the spores are accumulated before release.
Manoxylic: wood type that contains abundant parenchyma; typical of cycads; compare with pycnoxylic.
Megagamete: the "female" gamete, analogous to an egg and sometimes referred to as an ova (Latin for egg).
Megagametophyte: in heterosporous plants and in seed plants, the female gametophyte produced by a megaspore.
Megaphyll a leaf with
branched veins and a flat area for receiving sunlight.
Megasporangium: a sporangium that produces megaspores; see also heterosporous.
Megaspore: a large, haploid (N=1) spore of a heterosporous plant that produces a megagmetophtye (female gametophyte)
Meiosis:
A process common to almost sexual reproduction in eukaryotic cells. The
homologous chromosomes of a diploid nucleus first exchange homologous genes
(alleles) on a roughly random basis, so that the resulting chromosomes carry a
mixture of genes from each parent. The nucleus then divides normally (by
mitosis) to yield two diploid daughter nuclei. Finally, the nuclei divide
again, but now without DNA replication, to yield four haploid cells. Image
from the Online
Biology Book by Dr. Mike Farabee of Estrella Mountain Community College.
Meristem: a plant tissue containing cells that are actively dividing (or have the potential to do so). Meristem found in the tip of roots and stems, the apical meristem, is responsible for the growth in length (primary growth) of these organs. The cambium is a lateral meristem that is responsible for increase in girth (secondary growth) in perennial plants. Some plants also have intercalary meristem, as in the stems of grasses, for example. These are responsible for their continued growth after cutting or grazing has removed the apical meristem of the shoots. Tiscali.
Mesophyll: parenchymal tissue between the upper and lower epidermis of a leaf.
Microgamete: the "male" gamete, analogous to sperm.
Microgametophyte in heterosporous plants and in seed plants, the male gametophyte produced by a microspore.
Microphyll:
a leaf with only one vascular bundle. Image from the website of Prof.
Gerald Carr at the Univ. of Hawaii at Manoa.
Microsporangium: a sporangium that produces microspores. Also see heterosporous.
Microspore: a small, haploid (N=1) spore of a heterosporous plant that produces a microgametophyte (male gametophyte).
Monocarpic: a plant that produces reproductive propagules only once in its lifetime.
Monoecious: used to describe a species in which a single plant produces both male and female gametes.
Monophyletic: a group descended from a single common ancestor within the group being studied. The term is also used in a slightly different sense. A "monophyletic group" usually refers to a clade, i.e. an organism and all of its descendants. Thus Sinraptor and Allosaurus are (very likely) monophyletic within Allosauroidea. However they are not, by themselves, a clade, because they do not include all descendants of their last common ancestor.
Monopodial: of branching patterns, Monopodial branching is a pattern of branching in which there is one main shoot with lateral branches emerging from it. Many firs and spruces exhibit monopodial branching patterns, therefore the Christmas tree image is a good example of monopodial branching. Branching in which there is a clear central branch with lateral branches that develop some distance behind the end of the central branch.
Monosaccate: a pollen grain with one saccus or buoyant bladder. Compare bisaccate.
Nucellus: ovule tissue within which an embryo develops (embryo sac); homologous with the megasporangium of a seed plant.
Oogamy: reproduction in which the female gamete is large and non-motile and the male gamete small and motile.
Operculum: L. for lid or cover. In botany, nearly any kind of lid or cover.
Osmotic pressure: Most biological membranes are impermeable to many of the solutes found in the cell. If this were not so, all of the valuable biomolecules in the cell would simply diffuse out and be lost. However, the same membranes are often more or less permeable to water. Since there are many solutes trapped in the cell, the "concentration" of water is lower in the cell than outside, i.e., there are more water molecules per unit volume outside the cell than inside. Diffusion of water through the membrane works both ways and is completely random. However, water "concentration" is higher outside the cell. That is, there are more water molecules in contact with the membrane on the outside than on the inside. So, there will be a net flux of water into the cytoplasm until the concentrations equalize. The osmotic potential is the measure of the net tendency of water to enter the cell. Real cells can't usually behave in this fashion, since the cell will expand and ultimately burst. The problem is handled in numerous different ways, depending on the cell type. In plant cells (and various others), the cell membrane is confined within a semi-rigid cell wall. Water enters the cell only until the elastic reaction force of the cell wall equals the outward force caused by water molecules crowding into the cell. At steady state, the plant cell then maintains a rather high internal pressure, referred to as turgor pressure. Turgor pressure serves as a sort of internal hydrostatic skeleton which helps to support -- and even move -- the plant structures. [I have lost the original source for most of this useful paragraph -- my apologies to the author.]
Ovary: the enlarged basal portion of a carpel,
where the ovules are borne; the ovary differentiates into the
fruit.
Ovule: unfertilized seed; the ovule contains the megasporangium with the megagametophyte, surrounded by one or two integuments. See sclerotesta for detailed anatomy of a (fossil) seed.
Palynomorph: a fossil spore or pollen type.
Paraphyletic: a taxon is said to be paraphyletic if it does not contain all descendants of its last common ancestor. So, for example, "seedless plants" is paraphyletic because its last common ancestor would also be the ancestor of plants with seeds. A clade must contain all descendants of its last common ancestor. It is usually fairly important to stick to a consideration of clades. However, that importance can often be outweighed by the utility of being able to discuss the earliest forms of some group. We normally signal that we are talking about such a paraphyletic group by saying "basal seedless plants" or "paraphyletic seedless plants" or by the use of some other, similar linguistic artifice.
Parenchyma: the most common type of plant cell; thin-walled cells varying in size, shape, and function. Parenchyma cells are usually loosely packed cubed-shaped or elongated cells that contain a large central vacuole and have thin, flexible cell walls. Parenchymal cell walls are enriched in pectins and therefore stain pink with Toluidine Blue. They also have simple pits. Parenchymal cells are found in large numbers in the pith and cortex. They are the least specialized cells in the plant and normally act as a sort of reserve of stem cells. They may also serve a number of functions, including food storage (either water or nutrients), photosynthesis, as well as tissue repair.
Parichnos:
an interconnected system of parenchymous strands with many air spaces that
extend throughout the vegetative organs of some arborescent
lycopsids. Essentially, parichnos tissue is parenchyma
with lots of air space between cells. This tissue probably aided photosynthetic
gas exchange in much the same way as do the lenticels of modern trees.
Image: oblique section through a leaf base of "Lepidodendron"
from Zhou et
al. (2004). Scale bar = 500µ.
Pectin:
yet another sugar polymer. In plants, pectin occurs on cell walls of parenchyma
and related cell types where it apparently strengthens the cell wall and assists
in cell-to-cell adhesion. It is frequently extracted and used as a thickener for
preserves.
Perennial: a plant without a definite lifespan.
Peristome: a combination of Greek roots roughly meaning "stuff around the mouth." In mosses, this means the ring of peristome teeth around the orifice of the spore capsule that control spore dispersal. See discussion of the moss sporophyte.
Petal: one of the whorls of a flower; petals may be brightly colored.
Phloem: conducting tissue of vascular plants. Phloem differs from xylem both structurally and functionally. Functionally, phloem transports solutes both up and down the plant, while xylem functions as a one-way transport system for moving water from the roots to the leaves (or equivalent). Phloem carries sugars and other direct or indirect products of photosynthesis to the stem and roots, generally during the day. Phloem also carries ions and soil nutrients up from the roots to the stem and leaves, generally at night. Xylem makes considerable use of the cell walls of dead or enucleated cells. Phloem uses living cells. Xylem uses capillary action and suction, like a straw, to transport water. Phloem uses osmotic pressure to move solutes. A high concentration of solute is loaded inside cells of the phloem at a source, such as a leaf where sugars are produced. This creates a diffusion gradient that draws water into the cells. The resulting pressure causes a flow to occur. If the dissolved sugars or other chemicals carried along with the sugars are removed from the phloem at another place in the tree for use (a sink such as a root or fruit), the decline in concentration of sugar causes water to move out of the phloem cells. Because water is moving in at a source and out at a sink, there is a mass flow of water and substances in the phloem. In trees, phloem is shed annually with the bark. Water and Chemical Movement Beneath the Bark
Phyllid: the leaf of a bryophyte. By many more exacting definitions, this does not constitute a leaf, hence the purported need for this apparently pointless addition to your vocabulary.
Phyllotaxy: the pattern in which leaves are arranged around a stem.
Pinna: first order of subdivision of a compound
leaf or fern frond. Davallia tasmanii image adapted from vonKonrat
et al. (1999).
Pinnule: the second order of segments of a compound leaf or fern frond.
Pinnulet: as you have undoubtedly guessed, the third order of segments of a compound leaf or fern frond.
Pistil: the "female" organs (megagasporangium) of a flower, including the stigma, style, ovary, and ovule.
Pit:
the thin, remnant primary wall of tracheids, i.e., primary wall
which has not been covered by lignified secondary wall. Pits are
involved in translocation of fluids from the xylem. See
also, pit membrane.
Pit membrane: in many sources, the term pit refers to the absence of secondary wall, i.e. the "hole" in the secondary wall, while the primary wall is the pit membrane. The distinction becomes important in detailed xylem anatomy because vessels may have pits but usually lack pit membranes. In that case the pit is simply a hole in the vessel wall.
Pith: a central unspecialized parenchymous ground tissue in monocot roots and dicot stems.
Plesiomorphic: a plesiomorphic character of a taxon is one which it shares with an ancestral group. It is not a unique, derived character (synapomorphy or apomorphy), but a primitive character which tells us nothing about its evolutionary position.
Pollen: the microspore of seed plants that contains the microgametophyte (male gametophyte)
Pollination: the transfer of pollen from the pollen organ to the ovule; for example in flowering plants from stamens to the stigmatic surface of the carpel.
Polystelic: having more than one stele.
Primary growth: growth in length, controlled by the apical meristem
Primary pinna: leaflet of first subdivision of a fern frond or compound leaf.
Primary xylem: xylem is found in leaves
and young stems or in young roots it is referred to as primary xylem.
Primary xylem is xylem derived from apical meristem,
rather than cambium. It contains fiber
cells and tracheids
(pipe-like) elements which progressively develop secondary, lignified
walls until little of the primary wall is left except at pits.
These pits operate to permit fluids to move between tracheids. The precise
centrifugal series of secondary wall structures, and the shape and pattern of
mature pits are useful in taxonomic and forensic work.
Propagule: in plants, a propagule is whatever structure functions to reproduce the species: a seed, spore, stem or root cutting, etc.
Protonema: first growth from a germinating spore, especially, the thread-like process from a germinating moss (Bryophyta) spore.
Protostele: a type of stele with a solid core of primary xylem.
Pseudomonopodial: branching a type of branching characterized by the presence of a main branch which has outgrown the other of the dichotomous pair, resulting in a main axis with smaller lateral branches. A type of branching where the apical meristem appears to divide to form two branches, one of which is dominant resulting in an upright main axis with distinct side branches.
Pteridophyte: a paraphyletic group including (roughly) all Rhyniophyta except Spermatophytata, i.e. stem rhyniophytes, lycophytes, ferns and horsetails.
Pychnoxylic: dense wood that contains little parenchyma; typical of Archeopteris, and conifers; compare to manoxylic.
Pyrenoid: a starch storage structure found in the chloroplasts of green algae.
Quadriseriate: in four ranks or rows.
checked ATW051012