| Pit connection is a vital channel in plant tissues or cells.Cells in internode of bamboo stem are arranged in a vertical direction,and the pits is the key way to support the penetration and transmission of substances along horizontal direction.Thus,it is of great significance to make thorough studies into the characteristics of pit in bamboo cell wall,which not help promote the profound interpretation of cell wall structure and function,but also enlighten the high valueadded utilization of bamboo resources and the development of bionic structure and material.However,researches about pits in bamboo have been rarely reported so far due to the difficulties in sample preparation and research methods of bamboo.This paper systematically studies the basic characteristics of bamboo pits,which could provide a scientific basis for exploring the relationship between the structure and function of pits.And the results will help fill the gaps in the comprehensive understanding of bamboo pits.In this paper,a kind of common economic bamboo spieces(Phyllostachys edulis(Carr.)J.Houz.)served as major research object.Microscope systems field-emission environmental scanning electron microscope(FEG-ESEM)with large depth field and high resolution features were used,in combination with 3D resin casting and common anatomical techniques to study basic characteristics of bamboo pits.Based on the detailed division of the spatial location and types of bamboo cells in vascular bundles,the three-dimensional structure of pit and pit pair on the main types of cells such as bamboo vessels,parenchyma cells and fiber cells were analyzed.Characteristics including quantity,shape and size,structure,type,distribution,the pit aperture,the pit membrane,and the pit cavity,arrangement of microfibrils,and pit pairs and channels in vascular bundle were studied to complete systematic understanding of bamboo pits.The conclusions are as follows:(1)Small vessel members with diameter of 2~5 to 20~80μm were found arranged in vascular bundle by parallel or scattered form,in the cell wall of which,presenting a large amount of bordered pits and connecting each other by them.Pitting presented original annular,spiral and pitted thickening in vessel,and mainly in opposite or mixed with opposite-alternate between treachery elements.The pit border in metaxylem vessel was obvious,and the inner and outter aperture was about 0.6~0.6μm and 1.1~3.8μm respectively.Besides,the inner aperture was further subdivided into three types namely PⅠ,PⅡ and PⅢ.Microfibrils near pit aperture showed streamline arrangement,and twisted between immediate pits.The pit membrane were either covered by incrustation and amorphous substances or almost clear.Tyloses were found both in protoxylem and metaxylem vessels.Distribution of pits in vessel demonstrated highly preference on location,that is,pits in vessels from the same vascular bundle always lay towards the center of the vascular bundle geometry.(2)Pits in parenchyma cells were often sheltered by cell inclusions like starch,and some short parenchyma cells also found containing small particles.Pits distributed less in ground tissues than in vascular bundle,meanwhile,distribution in longitudinal cell wall were far more than that of transverse cell wall.The size of inner aperture was about 1.4~2.5 microns,and pit aperture demonstrated varieties of forms from simple pit to bordered pit.Primary pit field often been left in thinner cell wall.A unique type of branched pit cavity was found in parenchyma.In this structure,a main pit cavity ramified into several branches(usually two in moso bamboo)at certain point to connect several adjacent cells by pit pairs accordingly.Between parenchyma cells,pits composed simple pit pairs and occasionally a few bordered or half-bordered ones.Opposite pitting also exist in some parenchyma cells.In addition,a special type of parenchyma cell near vessel was found possessing abundant pits including simple and bordered ones.(3)In or between phloem fiber cells and libriform fibers,pits connected each other by simple pit pairs.While between fiber-tracheid cells,and between fiber-tracheid and libriform fiber,it turned out to be bordered pit pairs and half bordered ones,respectively.Pits in fibers were usually slight(smaller than 1μm),sparse,and holding a narrow and long pit cavity.Microfibrils arrangement near outer aperture showed vortex pattern in primary cell wall and streamline pattern in secondary cell wall.Pits from inner cell wall view demonstrated a crack or fissure form probably with warts around.Adjacent to the vessel,a kind of special fiber cells presented almost the largest number of pits than any other fibers.In addition,fiber pits showed a gradually reduce tendency from the fiber sheath periphery to the center position.It was a common phenomenon that multiple consecutive connections between adjacent fiber cells emerging in the same horizontal plane.(4)Half bordered pit pairs connected the vessel and parenchyma cell and a small amount of bordered pit pairs were also found.Pitting in certain parenchyma cells and vessels formed alternate or opposite-alternate type.Vessel pits in some cases connected one branched pit cavity in parenchyma to form a special pit pair.Connection between fiber and parenchyma cell may be simple pit pair in majority.The pit pair between fiber cell and vessel usually occured in the smooth side of vessel wall.Pits on transverse distribution presented a rule of reducing outwards in vascular bundle.(5)Connections between adjacent vessel elements in longitudinal direction were mainly formed by single perforation or occasionally a scalariform one;Longitudinal parenchyma cells were connected by pits in the transverse cell wall;There was usually no pit between longitudinal adjacent fiber cells.Within the vascular bundle,pits connections along the longitudinal direction was much higher than that of horizontal. |
