Botany

• Publisher: Collins
• Year: 2017
• ISBN:
  9780008241858
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

Pp

P680 a special form of CHLOROPHYLL a with a light absorption peak of 684 nm. It is one of the constituents of the photosystem II light-gathering centre of photosynthesis. The absorption of light energy by the P680 molecules causes, among other things, the formation of a strong oxidant that can then oxidize water. This results in the evolution of molecular oxygen and initiates electron flow, which continues, via photosystem I, through to the production of reduced NADP. See also P700.

P700 a special form of CHLOROPHYLL a with an absorption peak for light of 700 nm wavelength, found within the photosystem I light-gathering centre of photosynthesis. Other ordinary chlorophyll a molecules are also present and these absorb light energy and pass it to P700, creating a charged form, P700⁺. Electron transfer occurs from photosystem II, electrons being passed on ultimately to produce NADPH, the reduced form of NADP. See also P680.

pachytene see PROPHASE.

Pacific coast forest the most dense coniferous FOREST in the world, extending along the coast and mountain ranges just inland from California to southern British Columbia. It is reknowned for its giant conifers, including the coastal redwood (Sequoia sempervirens), big tree (Sequoiadendron giganteum), and Douglas fir (Pseudotsuga taxifolia), which are among the tallest trees in the world, some reaching heights of 110 m. and girths in excess of 20 m. It includes some of North America’s most important OLD-GROWTH FOREST.

padang a type of KERANGA found on PODSOLS in southeast Asia.

paedomorphosis see NEOTENY.

pairing see SYNAPSIS.

Palaearctic a BIOGEOGRAPHICAL REGION that comprises Europe, Russia and the former Soviet states, northern Arabia and the Mediterranean coastal strip of north Africa.

palae- prefix denoting ancient, archaic, early, or primitive.

paleobotany see PALAEONTOLOGY.

Palaeocene see TERTIARY.

palaeoecology the study of extant and fossil organisms with a view to establishing the nature of the life of past ages, the environmental conditions that prevailed, and the interactions between them. Details of the environment in which given fossil organisms lived can be assumed from a knowledge of the prevailing climatic conditions in which the rocks that contain them were formed. However, difficulties arise in distinguishing between the environment in which the organism lived and the environment in which it was buried. See also ASSEMBLAGE, PALYNOLOGY, REFERENCE SPECIES.

palaeolimnology the study of the history and development of freshwater ecosystems.

palaeontology the study of FOSSILS. The branch dealing with the study of plant fossils is termed palaeobotany, which includes palaeoethnobotany, the study of fruit and seed remains found in archaeological sites. Palaeontology requires a knowledge of the way in which fossilization takes place and of the type of rock in which the fossil is found, a knowledge of the environmental conditions that prevailed when the rock was laid down, and an appreciation of the geological time scale so that the fossil can be dated.

palaeosol a soil that was not formed during the present period of PEDOGENESIS. It may simply have been formed earlier, or it may have been buried, or buried and later exposed.

palaeospecies a group of organisms known only from their fossils, which differs from all other known groups.

palaeotropical floristic region see BIOGEOGRAPHY, FLORISTIC REGION, PLANT GEOGRAPHY.

Palaeozoic the era of geological time between about 570 and 225 million years ago that succeeds the Precambrian and precedes the Mesozoic. It is subdivided into six periods, the CAMBRIAN, ORDOVICIAN, and SILURIAN (lower Palaeozoic), and the DEVONIAN, CARBONIFEROUS, and PERMIAN (upper Palaeozoic). See also GEOLOGICAL TIME SCALE.

pale- see PALAE-.

palea (pale, valvule) the upper of the pair of BRACTS beneath each floret (flower) in the inflorescence of a grass, the other being the LEMMA. The palea is usually thin, narrow, and parallel sided. It usually has two ribs, which may project as prominent keels. Compare GLUME. See illustration at poaceae.

palindrome see REPEATED SEQUENCE.

palisade mesophyll photosynthetic CHLORENCHYMA tissue composed of more or less elongate cells arranged in radial columns. Palisade mesophyll occurs with SPONGY MESOPHYLL as a ground tissue in the leaves of many MESOPHYTES. Here the palisade mesophyll is usually on the adaxial (upper) side of the leaf, whereas in many XEROPHYTES it forms the bulk of the mesophyll and is present on both sides of the leaf. When the stem has taken over much or all of the photosynthetic work of the plant, the cortex may consist largely of palisade parenchyma.

Palmae see ARECACEAE.

palmate (digitate, palmately lobed)

1 describing a compound leaf having four or more leaflets arising from a single point. The leaves of the horse chestnut (Aesculus hippocastanum) and of lupins (Lupinus) are examples. See illustration at leaf.

2 describing a form of VENATION in which several equally prominent veins branch out from the base of a leaf blade. See ACTINODROMOUS.

palmelloid describing a type of colonial growth form in which an indefinite number of cells are held together after division in a mucilaginous matrix. Palmelloid forms are characteristic of certain cyanobacteria, e.g. Merismopedia, and various algae and other protoctists.

palmitic acid (hexadecanoic acid) a sixteen-carbon saturated fatty acid. It is the commonest saturated FATTY ACID in plants, forming a high proportion of plant fats. It is also on the synthetic route to other fatty acids, such as STEARIC and OLEIC ACIDS.

palms see ARECACEAE.

Palouse prairie a distinctive type of prairie on fertile loess soils in southeast Washington and neighbouring areas of Idaho USA, which supports a rich growth of native tussock grasses. Most of this prairie is now given over to wheat, and only 1% of the original vegetation remains.

palsa boggy tundra (see BOG), that contains palsas, i.e. mounds or ridges up to 35 m long, 15 m high and 7 m high, found in periglacial areas and composed mainly of peat and containing a permanent ice lens. Palsas occur in damp boggy areas. They are believed to form as a result of frost-heaving of soil, together with the growth of ice inside.

palustrine growing in boggy ground.

palynology the study of living and fossil pollen grains, spores, and similar structures. The diversity of such structures has led to an increasing use of palynological data in systematics, while their resistance to decay has made them an important feature of palaeobotanical and palaeoecological work. See also EXINE. include aearopalynology

pamirs ALPINE vegetation of the high arid plateaux of Tibet. It is composed mostly of sparse tufts of grass and large, cushion plants. Compare PUNAS.

pampas temperate South American GRASSLAND, especially extensive in Argentina. The original grassland was dominated by tussock grsses, with shorter grasses and xerophytic shrubs in the drier areas. Much of the wetter northeastern pampas is now cultivated.

pan a heavily compacted SOIL HORIZON that is either high in clay or cemented with minerals that have been leached out of the upper soil horizons. See HARDPAN, LEACHING, SOIL PROFILE.

panbiogeography an approach to biogeography first developed by Leon Croizat (1894–1982), that is a cartographic method of analysing the spatial structure of the distribution of species and other taxa in an attempt to better understand their origin in time and space.

pandurate fiddle-shaped.

Pangaea see CONTINENTAL DRIFT.

pangenesis a now disregarded theory formulated by Charles Darwin that suggested a mechanism by which acquired characteristics may be inherited. He postulated that there were particles (pangenes) carried in the body fluids from all the organs of the body to the reproductive cells. These particles then influenced the gametes that in turn influenced the characteristics of the succeeding generation.

panicle an inflorescence, associated particularly with grasses, in which the flowers are formed on stalks (peduncles) arising from the main axis. Each stalk is a raceme. The peduncles are long and spreading, as in oats (Avena). The term is also used to describe any type of branching inflorescence. See illustration at inflorescence.

panmixis unrestricted random crossing. It is probably fairly rare but the term may be applied to crossing that appears to be random with respect to a particular character or gene. Compare ASSORTATIVE MATING.

pantocolpate describing a pollen grain having many colpi (see COLPUS).

pantonematic flagellum see UNDULIPODIUM.

pantothenic acid a compound, formula CH₂OHC(CH₃)₂CHOHCONHCH₂CH₂COOH, that is the precursor of COENZYME A. It is synthesized by plants and bacteria but not by vertebrates, which consequently require it as a VITAMIN in the diet.

pantropical distributed throughout the tropics.

papain see PROTEASE.

paper chromatography a widely used chromatographic technique for separating the components of mixtures using absorbent paper as the stationary phase. A sheet or strip of paper, with a concentrated spot of the mixture on a pencil-drawn base line, is vertically suspended in a suitable solvent (the mobile phase), that seeps slowl...

Table of contents

• Title Page
• Copyright
• Preface
• Contributors
• Contents
• Aa
• Bb
• Cc
• Dd
• Ee
• Ff
• Gg
• Hh
• Ii
• Jj
• Kk
• Ll
• Mm
• Nn
• Oo
• Pp
• Qq
• Rr
• Ss
• Tt
• Uu
• Vv
• Ww
• Xx
• Yy
• Zz
• Appendix
• About the Author
• About the Publisher