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Home >>Botany Dictionary>>Secondary Thickening Sex Chromosomes
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Seed Ferns - Include members of Pteridospermales, an extinct order of gymnosperms known from fossil records of Devonian to Triassic period. They possessed fernlike fronds similar to those of fern order Marattiales and their stems showed secondary thickening and plants showed evidence of seed formation. Ovules were present along margins or on surface of megasporophylls that were similar to foliage leaves. Seeds were characteristically radially symmetrical.
Segmental Interchange - Refers to the exchange of pieces, taking place between two chromosomes which are not homologous.
Seedling Blight - Refers to a diseae which is affecting germinated seedlings thereby resulting in slowing of growth and yellowing or wilting of leaves, rotting of roots and lesions at soil level. Causative fungal organisms include Pythium, Phytophthora, Rhizoctonia, Fusarium and Helminthosporium etc.
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Seed Plants - Refers to the member of division Spernatophyta in classifications that are considering seed habit and presence of vascular tissue atleast of equal importance or may be former being more important. Examples include ail seedbearing vascular plants that are divided into classes Gymnospermae and Angiospermae.
Secondary Thickening. - Refers to the formation of additional vascular tissue by the activity of a cambium.
Secondary Tissue - Tissue that is formed by secondary thickening.
Secondary Xylem - Refers to the xylem derived from cambium in plants. It shows secondary growth and has two systems: axial (vertical) system that is derived. from bursiform initials of cambium and composed of mainly treachery elements and fibers and ray (horizontal) system derived from ray initial of cambium and composed mainly of ray parenchyma forming rays. Secondary xylem is having shorter treachery elements than the primary xylem and these are arranged in regular rows. Commercial wood is secondary xylem.
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Seedless Fruit - Refers to a fruit that is developing without fertilization so that seed does not form in the fruit. See parthenocarpy.
Seedling - The young plant which is developing from a germinat ing seed.
Segment HalfCell - Refers to one of the two smaller cells cut off from the pialac cells of most Acrogynae. They ultimately give rise to a 'leaf'
Segregate - (1) Keep separate.
(2) In taxonomy a group which is based on part of an earlier group.
Segregation - Refers to the separation at meiosis of the members of a pair of allelomorphs.
Segregation Ratio - Refers to the proportion of one type of off spring to another type of offspring taking place as a result of separation of alleles . during meiosis.
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Section - (1) A rank in taxonomic hierarchy which is above the rank of series but subordinate to genus. Section name is printed it italics having a. capital first letter and is either having same form as generic name or, if a plural adjective, agrees in gender with generic name. Section name gets preceded by abbreviation 'sect' to show its rank e.g., chenopodium sect Pseudoblitum. (2) A thin piece of material which is cut so as to be capable of being examined by a microscope.
Sectorial Chimaera - A chimarea in which a plant consists of two of more types of tissue, which are arranged in crosssection as sector of a circle.
Seed - A product of a fertilized ovule. It consists of an embryo, enclosed in the test a which is derived from the integuments(s). Food reserves mayor may not be present in the endosperm.
Seed Dressing - Refers to a chemical which is applied to seeds for protection against fungal or insect attack Chemicals are applied in the form of slurries or concentrated solutions.
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Seismonasty - Refers to a type of nastic movement which occurs in response to shock as in collapse of leaflets of Mimosa pudica after being subjected to shaking or singeing.
Selection Pressure - Refers to the measure of the action of selection in tending to alter the frequency of a gene in a given population.
Selective - The term applicable to herbicides that kill some plants but not others selectively. Broadleaved plants are usually killed while narrowleaved plants are unaffected. Such herbicides find used in eliminating unwanted plants from a cereal crop and in lawns.
Selective Absorption - Refers to the ability of a cell, or the cells of an organ, to accumulate a particular ion in a higher concentration in its vacuole, than it is in the surrounding medium or alternatively not to accumulate a substance which is present in the surrounding medium, and would normally pass into it by diffusion.
Selective Fertilization - Refers to the fusion of gametes of different sexes in a nonrandom manner.
Selective Transport - Refers to the preferential movement of a particular ion from one part of a plant to another, so that the ion appears in higher concentration in some parts than others.
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SelfFertilization - The fusion of male and female gametes produced by the same individual.
SelfPollination - Involves the transfer of pollen from the another to the stigma of the same flower. More generally it may imply a similar transfer to stigmas on the same plant, or within the same clone.
SelfSterile - Refers to the inability of gametes of opposite sex, but produced by the same individual, to undergo fusion.
Semantide - Refers to an informationcarrying molecule. DNA as carrier of genetic code is called primary sema tide. RNA as transcription of genetic code is called secondary semantide and proteins as translation of genetic code are called tertiary semantide.
Semiconservative Replication - Refers to a process by which DNA makes exact copies of itself by involving uncoiling of doublestranded DNA molecule. Separated polynucleotide acts as template and new polynucleotide gets formed on it by complementary nucleotides. Hence the new doublestranded DNA molecule possesses once strand from the original DNA molecule and other strand newly synthesized.
Seminal Root - Any of the adventitious roots growing from the base of the stem during early seeding growth and taking over the function of radicle.
Semipermeable Membrane - A membrane which allows the passage of a solvent, but is impermeable to stated dissolved substance.
Senescence - Refers to a period between complete maturity and final death of an organ or organism during which three occurs gradual deterioration in its structure and functioning. It is characterized by accumulation of waste metabolic materials and a decrease in dry weight. Reusable substances are removed from the structure and its respiratory rate increases.
Sensibility - The degree of reaction of plant which is attacked by aparasite.
Sepal - One of the parts (lobes) forming the calyx of a flower. It is generally green, and protects the rest of the flower in the bud.
Separation Disk - A biconcave disk of intercellular material that is found occasionally between the cells of a filament of the blue green algae. They aid in the bradup of the filament into homogonous.
Septicidal - A fruit which opens by the loculi spliting through the middle, and thereby forming several compartments.
Septifragal - Used for describing of the dehiscence of a fruit, when the outer wall breaks away from the walls of the loculi.
Septum - (1) A wall present between two adjoining cells.
(2) A transverse wall present in a fungal hypha, an algal filament, or a spore.
(3) The partition between two loculi of an ovary.
Sequence Hypothesis - A theory postulating how RNA codes the formation of a protein. It is that a particular sequence of nucleotides on the RNA molecule controls the manufacture of a particular aminoacid.
Sequestrol - Refers to a preparation of trace elements which is used to treat deficiency diseases particularly due to lack of iron or magnesium. Elements are cheated for preventing their binding to soil particles and thus promoting their uptake by plant. The preparation having a particular element is named accordingly e.g., sequestering iron.
Sere - Refers to the whole set of communities that are formed during succession in an area and take part in stabilization of the area from extreme conditions thereby bringing about climax. It has one pionee community to start the succession, a number of transitional communities appearing after the pioneers and replacing each other successively and the climax community coming in the last to form the stabilized vegetation in the area.
Serial Endoplasmic Theory (progressiveendosyrnbiotic theory) - Refers to the theory postulating that plastid, mitochondria probably cilia and flagella are having prokaryotic origin and got incorporated into eukaryotic cell through prokaryotic organisms that were living symbiotically within eukaryotic host cell. Plastids are assumed to have originated from organisms like present day bluegreen algae whereas mitochondria from aerobic bacterial organisms.
Series - Refers to a rank in taxonomic hierarchy which is above the rank of species but subordinate to section.
Serine - A water soluble aminoacid in protein.
CH2OH
CHNH2
COOH
Serology - Refers to a method which is used for identifying microorganisms and viruses, their chemical components, and their relations to one another. One acs as an antigen in blood serum, against which the activities of another can be examined.
Serrate - Used for a toothed margin, when the teeth are pointed, and project forwards.
Sessile - Refers to a structure that is unstalked such as a leaf with no petiole or stigma with no style.
Seta - (1) Refers to nervelike extension to a leaf as found in certain species of Selaginella
(2) Refers to the stalk of sporophyte in bryophytes that is supporting the capsule. It elongates rapidly at maturity forbringing capsule at a height which is suitable for spore dispersal. Seta is often having vascular tissue which enables the sporophyte toobtain nourishment from the parent gametophyte.
Set of Chromosomes - A group of chromosomes having one of each kind contained in the nucleus of a gamete.
Sex, Sexual Differentiation - Refers tothe production by an individual orgroup of individuals, of gametes of two types that are differing in size and mobility (male and female gametes) such that the one type can only undergo fusion with the other type.
SexCell - It means gamete.
Sex Chromosomes - Chromosomes of which there occurs a homologous pair in the nucleus of one sex (the homogametic sex) and a dissimilar pair or only one chromosome, in those of the other (the heterogametic sex).
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