The shape of parenchyma cells is almost equal in diameterpolyhedron(See the overview figure), but it can also be differentiated into star awn, branch and arm shape.
Parenchymatousmorphological structure andphysiological functionLess specialization, but with greater developmentPlasticityTherefore, in the process of plant development, parenchyma can further develop into more specialized tissue.
The parenchyma tissue usually occupies the most part in the plant body. It is contained in roots, stems, leaves, flowers and fruits, and is responsible for absorption, assimilation, storage, ventilation, transmission and other nutritional functions, so it is also called the nutritional tissue or basic tissue.[2]
The parenchyma cells have potential cell division ability, and there are many developed cell gaps between the cells. Under the stimulation of certain external factors, the cells can reverse differentiate and recoverMeristematic ability, change tomeristem(Meristem) to promote theWound healing, regeneration, formationAdventitious rootorAdventitious bud。Under the condition of in vitro culture, isolated parenchyma cell clusters, even single cells, can grow into a completeplant。
The parenchyma is mainly related to the vegetative activities of plants and has important functions such as assimilation, storage, ventilation and absorptionphysiological function, is carried out by plantsphotosynthesis（Photosynthesis）、Respiration(Respiration), storage nutrients and variousmetaboliteThe base of synthesis and transformation of“Trophic tissue”It is called.
distribution
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Thin wall tissue is widely distributed in plants, accounting for the majority of plant objects, and mainly exists in the ground or fundamental, such as the cortex and pith of stem and root（pith），mesophyllAll or most of the parts of the flower, the pulp of many fruits and the endosperm of seeds are composed of parenchyma.
characteristic
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1. The cells are living cells with large volume;
2. Usually sphericalpolyhedronAnd other shapes, arranged loosely;
Low degree of differentiation, with potentialMeristematic ability。In some cases, it can be transformed into biological tissue or further developed into other tissues, so the recovery of traumaAdventitious root, adventitious bud generation, cutting propagation and grafting survivaltissue cultureAnd so on.
type
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According to the different physiological functions of parenchyma, it can be divided into the following seven types:
basic parenchyma
The basic parenchyma is mostly distributed in the root, stem and other organs, such as cortex and pith, which play the role of filling parenchyma.
The cells of basic parenchyma are colorless,Cross sectionIt is spherical or polygonal, with no obvious difference in length and width, almost equal in diameter, and has living cellsProtoplastIt is a vegetative living cell.
In plants, the basic parenchyma plays the role of filling, so it is also called filling parenchyma.
Assimilating parenchyma
Assimilating parenchyma, named for its close relationship with photosynthesis, mostly exists in light sensitive parts of plant surface.It is characterized by the presence ofchloroplast, which can carry out photosynthesis, so it is also called green parenchyma（chlorenchyma）。For example, between the upper and lower epidermis of plant leavesmesophyllContains chloroplasts, especially nearUpper epidermisOfpalisade tissueThere are more chloroplasts in cells.
leafThe green body also exists in the cortex or the inner part of the stem or even the pith.There are also some assimilative parenchyma in young parts of plants, such as green stems, branches and the outside of fruits.In some plants with degenerated leaves (such asEphedraIn the stem, there are several layers of chloroplast containing parenchyma cells under the surface to replace the assimilated parenchyma cells of the leaves, which often appear obvious vacuolization and form a high degree of chamber structure, thus facilitating the process ofgas exchange。
Storage parenchyma
Storage parenchyma is a special kind of accumulated plantsAfterinclusion, such asStarch granule, protein particlesPseudocrystal, fat globules, oil drops and othersOrganic matterAn organization of etc.Mainly distributed in rootsrhizome, seeds and fruits.Sweet potatoOfroot tuber, potatotuber、peas and beansCotyledons andCerealsThe storage parenchyma is particularly developed in the endosperm of grains.The above after contents generally exist in the storage parenchyma cells in a dissolved stateCytosolSome of them are distributed in the cytoplasm in liquid or solid state, and a few of them can also containcell wallThickening exists, such asNux vomica, coffee andPersimmonIsoseededEndosperm cellThe thickened cell wall is formed byhemicelluloseFormed.The storage parts and components of the aftercontents vary with the physiological types of plants,beetOfFleshy rootScallioncormDissolved in squamous cell fluidamide, protein and sugar;Potato tuberAnd many othersRhizomeAmide and protein were dissolved in the cell fluid of parenchyma tissue, while starch was contained in the cytoplasm of parenchyma tissue;Others such askidney beanThere are proteins and starch in the parenchyma cells or cytoplasm of pea cotyledons.
The material accumulation in the storage parenchyma changes with the changes of plant physiological activities. The starch deposition in the stems and roots of woody plants changes with the seasons. However, in the storage organs such as tubers, bulbs and rhizomes, when the storage materials are transferred to the growing organs, theirProtoplastJust die.
StoredAfterinclusionStarch is widely distributed in cortex, medullaConducting tissue, fleshy leaves（bulbScale)Rhizome, fruitcotyledonAnd the parenchyma cells of the seed endosperm.In addition to organic matter, parenchyma cells also store inorganic minerals, such ascalcium oxalate、calcium carbonate、silicon dioxideAnd various kinds of knotscrystal。SomeParenchyma cellThe protoplasts are still preserved after the production of crystals, and some of them die after the development of crystals.
StoredNutrientsIt is mainly used as the energy source for the further development or reproduction of plants themselves, which can be used in seedroot tuberAnd the development of tuberous plants.Often, after the seedlings are produced, the substances stored in these organs are also transformed and decomposed.
Water storing parenchyma
Water storage parenchyma is a kind of parenchyma with abundant water in cells.The cells are large and have a large size rich in water or viscous juicevacuoleThus forcing the cytoplasmnucleusOnly a thin layer clings to the cell wall.Sticky juice expands when encountering water and has the ability to increase cell absorption and water retention.Water storage parenchyma cells can also exist inProtoplastNeutralize the cell wall.manyXeric OfSucculent plant, such ascactus、aloe、agave, Sedum Sedum, Echinacea, etcchlorophyllThe water storage parenchyma cells filled with water are also likePalisadeCells lengthen like that.The fleshy plants with this kind of water storage parenchyma can adapt to the arid environment of desert, rocky beach and other water scarce areas.
In general, there is no separate water storage parenchyma in the underground storage organs of plants, but in parenchyma cells containing starch and other nutrients,water contentIt can also be very high.For example, the tubers of potatoes are stored with water for germination in the air and for the tubers to start growing.High water content is not only the characteristic of underground storage organs, but alsoAerial stemAnd most of the characteristics of air born buds.
Aerenchyma
Aerial parenchyma is a kind of parenchyma with well-developed intercellular spaces to ensure air circulation.mesophyllInSponge tissueAndaquatic plant(e.gcalamus、rushThe aerenchyma in the cortex of roots and stems is the most typical.The volume of air in the intercellular space of mesophyll aerenchyma is 7.7~71.3% of the mesophyll volume.aquaticangiospermThe parenchyma is especially developed, forming a interconnected ventilation system in the body.The oxygen produced by photosynthesis of leaf camp can enter the root through the ventilation system.Because the cell gap is full of air, it can also enhance the buoyancy andSupportability。This type of aerenchymaStructure and functionIt is the result of long-term adaptation and evolution of plants.
In ventilated parenchyma, there are two ways to form intercellular space:
① Schizoblastic gap - the direct junction of adjacent cell walls is formed by splitting or separating to different degrees from each other.cell divisionFace and stem orpetioleThe longitudinal axis of is parallel to and perpendicular to the surface of the initial gap, so these gaps are often surrounded by many cells.Cortex of root and stem andmesophyllSponge tissueThe parenchymatous cell spaces in all belong to this category.In aquatic plants andMonocotyledon, such ascalamus、rush、ElodeaThe intercellular space is particularly developed in the stems and leaves.
② Lysogenic intercellular space - mainly due to the destruction and autolysis of cells forming intercellular space in succession during the growth process, resulting in large cavities.It is commonly seen in the saclike orTubular tissue, such as corn, horsetail andCyperaceaeIn the roots of plants.
Spindle shapedPrimitive cellMeristematic, a vertical string of brick shaped parenchyma cells.It is characterized bySingle pit, short in shape.
The parenchyma in xylem is calledWood parenchyma。Wood parenchyma inconiferous forestThe content is small or lacking.It accounts for about 1.5% of the total volume, but it is compared with the timbers of Podocarpine, Taxodiaceae and Cupressaceae.It is relatively developed and is an important recognition feature of this kind of wood.The content of other coniferous materials is little or no.
The parenchyma cells of coniferous wood are square or rectangular in cross section.The wall is thin, and the cell cavity contains dark resin, so it is also calledResinous cell。The cross section is brown and black, easy to identify.On the longitudinal section, it often forms a string, which can be several centimeters long, and the cells at both ends are relatively sharp.
The difference between parenchyma cells and tracheids: thin wall, single pit, short, and horizontal end wall (because single pit is often bulbous, it is valuable to identify wood).
Because the thin-walled cell cavity contains resinAromatic oil, such as firSabina chinensis, cypress, leachable firCypress oilIt can make the wood have special fragrance.And make woodDurabilityGreatly improved.
In addition to the above seven types, in the 1960s, with the help ofelectron microscopeA kind of specialized parenchyma cell, calledTransfer cell。The cell wall protrudes into the cell cavity, forming many finger like or antler like irregular projectionsplasma membraneOfSurface areaIncrease and be richPlasmodesmata, which is conducive to the transportation and transmission of materials.Most of these cells are distributed in plants, where solutes are concentrated in large quantitiesShort distance transportationFrequent parts, such as the end of leaf veinConducting tissueAround, becomingmesophyllAnd the transport of materials between transport organizations.staygenitalsIn Guanzhong, transfer cells often have many different forms.
Wood rayA tissue formed by the continuous aggregation of many ray cells in the xylem.
When observed under the microscope, the wood rays are mostlyCellsEach individual cell is called ray cell.Most of the wood rays areParenchyma cellform.This kind of single parenchyma cells that constitute wood rays is called ray parenchyma cells.But there are alsoSclerenchyma cellThis thick wall cell is calledRay tracheid。It is the transverse sclerenchyma of wood.It exists in PinusPicea、Larix、Cedar、Cathaya 、pseudotsuga Etc.In addition, in some tree species (with horizontalResin channel）In the middle, there are fat secreting cells in the wood ray.
1. Types of wood rays
According to itsChord sectionThe upper form can be divided into two types.
① Single row wood ray: ray composed of only one row (or occasionally two rows) of wood ray cells.asFir、fir wood、CypressEtc.
②fusiform ray: Many rows of wood rays or in the center of the wood rays, the wood rays are spindly due to the existence of transverse resin channels.It is commonly found in trees with transverse resin, such as pineSpruce、larch、Cathaya argyrophylla、Yellow cedarAnd other genera.
2. Composition of wood rays
Coniferous timberThe wood ray is mainly composed of ray parenchyma cells.But occasionally, the ray tracheids mentioned above are mixed in the rays of some coniferous trees.
① RayTracheid: The transverse tracheid in the wood ray perpendicular to the wood grain isPinaceaeCharacteristics of wood.The cell wall has marginal pits.But few, not many.Aboutaxial1/30 of tracheid.Aspect ratioIt is 5-10.There is no resin in the cavity. It usually appears in the upper and lower edges of ray and parenchyma cells, in 1-2 cases, and occasionally mixed between ray parenchyma cells.The inner wall of the ray tube cell has serrated thickening in some species.asMasson pine，Chinese pineEtc.Some inner walls are smooth, such asKorean pine、Pinus armandii、white bark pineEtc.In the pine genus, the ones with dentate thickening belong to hard pine, and those without dentate thickening belong to soft pine.
The presence and extent of dentate thickening is one of the characteristics of wood identification.The inner wall of ray tracheid is zigzag thickened toLatewoodThe most developed.Radial sectionUpper mostEasy to observe。In addition, there may also beThread thickening。Such as some spruces and yellow cedars.
② X-ray parenchyma cell: it is the main body of wood ray, and it is the transverse growth parenchyma, which is rectangular;Rectangle or slightly irregular.Striae between ray parenchyma cells and ray tracheidsHole pairbySemi rimmed pityes.
A、Horizontal wall and end wall: The thickness of the horizontal wall of ray parenchyma cells is one of the bases for identifying wood.(If the horizontal wall of the ray parenchyma cell is thinner than the adjacent tracheid wall, it is considered that thecell wallThin).
The horizontal wall thickness isAraucaria lanceolata、PodocarpusCharacteristics of genera and a few genera of Cupressaceae.It is thinner than the horizontal wall;Whether there is one on the horizontal wallPit, is more important (forWood identificationFor).There are marked pits on the horizontal wall of ray parenchyma cells of some woods in Pinaceae, and it is easiest to observe in the late wood part.Such as spruce, larch, yellow cedarhemlock spruce, fircedar、Keteleeria fortuneiandLysimachia roxburghiiEtc.butTaxodiaceae、Araucaceae、Taxaceae、Cephalotaxaceae、PodocarpineThere were no obvious pits on the horizontal wall of the cypress family.There are two families of Podocarpus and Araucaria which have no pit on the horizontal wall.Nodular thickening of the end wall (beaded) is also one of the characteristics of wood identification.The soft pine species andJuniperus。Calocedrus And some other tree species, all with node thickening.
In addition, the junction of the horizontal wall and the end wall of the parenchyma cells was irradiated.haveDent, such aschinese yew。It refers to the indentation on the radial section of the four septa of the ray parenchyma cells. Except for Araucaceae, most of them have.
B、Types of cross field holes:
Cross fieldThe plane where the wood ray parenchyma cells intersect the axial tracheids on the radial section, generally refers toEarly woodpart.The pits in this plane area are called cross field pits.It is the most importantIdentification characteristicsone of.The shape and number of pits in the area are of great significance for wood recognition and classification.Now they are generally divided into five types:
① Panes:Single pit, large and pane shaped.There are 1-3 in each cross field.It is characteristic of many pine species.Such as masson pine, camphor tree, pinePinus yunnanensisEtc.It also exists in the wood of some genera of Podocarpine and Taxodiaceae.But not very typical.
② Picea type: narrow but slightly extended or connotativePit orifice, small in shape.Spruce, larch, yellow cedar andTorreya grandisIt is a typical and obvious characteristic of wood.In the wood of Araucaceae, Podocariaceae, Taxodium and Cedar of Taxodiaceae;Sometimes it appears at the same time as its main type of holes.
③ Cypress type: contained in the pit mouth, slightly wider than the spruce type.Its long axis varies with position.From vertical to horizontal.The number of holes is generally 1-4.byCupressaceaeCharacteristics of.But cedarTsugaandKeteleeriaCan also be found in wood.Like Araucaceae, it usually has a large number and irregular arrangement.
④ Fir type:OvalTo a circular internal pit.The groove opening is wider than the cypress type.Long axis andPit marginagreement.It is the characteristic of most woods of Taxodiaceae.But inAbiesIt is also often found in the woods of Podocarpine and Cedar.However, it is often clustered with its main type of pits.
⑤ Loose type: smaller than window pane, with single pit or narrow edge.No definite shape.When it has a narrow edge, it is similar to the fir type, but the two ends of the pit mouth are sharp, and the size of the pit is different.It is common in pine wood.Such as Pinus bungeana.On the chord section, the ray cells are mostly round, oval or nearly round, and the two ends of the ray are longer than the middle.It is longer in high wood ray than in low wood ray.
Height of wood ray: measured by the number of cells on the chord section or the whole height in mm.Coniferous woodGenerally, the wood ray is not high.The average is 10-15 cells high, and the highest in China ismetasequoia, up to 60, and the lower ones such asCryptomeria fortunei, only a few[3]。
