A lenticel is a porous tissue consisting of cells with large intercellular spaces in the periderm of the secondarily thickened organs and the

bark Bark may refer to: * Bark (botany), an outer layer of a woody plant such as a tree or stick * Bark (sound), a vocalization of some animals (which is commonly the dog) Places * Bark, Germany * Bark, Warmian-Masurian Voivodeship, Poland Arts, e ...

of woody stems and roots of dicotyledonous flowering plants. It functions as a pore, providing a pathway for the direct exchange of gases between the internal tissues and atmosphere through the bark, which is otherwise impermeable to gases. The name lenticel, pronounced with an , derives from its lenticular ( lens-like) shape. The shape of lenticels is one of the characteristics used for tree identification.

Evolution

Before there was much evidence for the existence and functionality of lenticels, the

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

record has shown the first primary mechanism of aeration in early vascular plants to be the

stomata In botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is b ...

. However, in woody plants, with vascular and cork cambial activity and secondary growth, the entire epidermis may be replaced by a suberized periderm or

in which the functions of the stomata are replaced by lenticels. The extinct arboreal plants of the genera '' Lepidodendron'' and '' Sigillaria'' were the first to have distinct aeration structures that rendered these modifications. "Parichnoi" (singular: parichnos) are canal-like structures that, in association with

foliar A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...

traces of the stem, connected the stem's outer and middle cortex to the

mesophyll A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ...

of the

leaf A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ...

. Parichnoi were thought to eventually give rise to lenticels as they helped solve the issue of long-range

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

transport in these woody plants during the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

period. They also acquired secondary connections as they evolved to become transversely elongated to efficiently aerate the maximum number of vertical rays as well as the central core tissue of the stem. The evolutionary significance of parichnoi was their functionality in the absence of cauline stomata, where they can also be affected and destroyed by pressure similar to what can damage to stomatal tissue. Evidently, in both conifers and Lepidodendroids, the parichnoi, as the primary lenticular structure, appear as paired structures on either side of leaf scars. The development and increase in the number of these primitive lenticels were key to providing a system that was open for aeration and

gas exchange Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a ...

in these plants.

Structure and development

In plant bodies that produce secondary growth, lenticels promote gas exchange of oxygen,

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

, and

water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous p ...

. Lenticel formation usually begins beneath stomatal complexes during primary growth preceding the development of the first periderm. The formation of lenticels seem to be directly related to the growth and strength of the shoot and on the hydrose of the tissue, which refers to the internal moisture. As stems and roots mature lenticel development continues in the new periderm (for example, periderm that forms at the bottom of cracks in the bark). Lenticels are found as raised circular, oval, or elongated areas on stems and roots. In woody plants, lenticels commonly appear as rough, cork-like structures on young branches. Underneath them, porous tissue creates a number of large intercellular spaces between

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

. This tissue fills the lenticel and arises from

cell division Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ...

in the phellogen or substomatal ground tissue. Discoloration of lenticels may also occur, such as in mangoes, that may be due to the amount of lignin in

cell walls A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...

. In oxygen deprived conditions, making respiration a daily challenge, different species may possess specialized structures where lenticels can be found. For example, in a common mangrove

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

, lenticels appear on pneumatophores (specialized

root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...

s), where the parenchyma cells that connect to the aerenchyma structure increase in size and go through cell division. In contrast, lenticels in grapes are located on the

pedicels In botany, a pedicel is a stem that attaches a single flower to the inflorescence. Such inflorescences are described as ''pedicellate''. Description Pedicel refers to a structure connecting a single flower to its inflorescence. In the absenc ...

and act as a function of

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

. If they are blocked, hypoxia and successive

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

accumulation may result and lead to cell death.

Fruits

Lenticels are also present on many fruits, quite noticeably on many apples and pears. On European pears, they can serve as an indicator of when to pick the fruit, as light lenticels on the immature fruit darken and become brown and shallow from the formation of cork cells Certain bacterial and fungal infections can penetrate fruits through their lenticels, with susceptibility sometimes increasing with its age. As mentioned previously, the term lenticel is usually associated with the breakage of periderm tissue that is associated with gas exchange; however, lenticels also refer to the lightly colored spots found on apples (a type of pome fruit). "Lenticel" seems to be the most appropriate term to describe both structures mentioned in light of their similar function in gas exchange. Pome lenticels can be derived from no longer functioning stomata, epidermal breaks from the removal of trichomes, and other epidermal breaks that usually occur in the early development of young pome fruits. The closing of pome lenticels can arise when the cuticle over the stomata opening or the substomatal layer seals. Closing can also begin if the substomatal cells become suberized, like

cork Cork or CORK may refer to: Materials * Cork (material), an impermeable buoyant plant product ** Cork (plug), a cylindrical or conical object used to seal a container ***Wine cork Places Ireland * Cork (city) ** Metropolitan Cork, also known as G ...

. The number of lenticels usually varies between the species of apples, where the range may be from 450 to 800 or from 1500 to 2500 in Winesap and Spitzenburg apples, respectively. This wide range may be due to the

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

availability during the early stages of development of each apple type. "Lenticel breakdown" is a global skin disorder of apples in which lenticels develop dark 1–8 mm diameter pits shortly after processing and packing. It is most common on the ' Gala' (''Malus'' × ''domestica'') variety, particularly the '

Royal Gala Gala is an apple cultivar with a sweet, mild flavour, a crisp but not hard texture, and a striped or mottled orange or reddish appearance. Originating from New Zealand in the 1930s, similar to most named apples, it is clonally propagated. In ...

', and also occurs in ' Fuji', ' Granny Smith', '

Golden Delicious 'Golden Delicious' is a cultivar of apple. It is one of the 15 most popular apple cultivars in the United States. It is not closely related to ' Red Delicious'. History Golden Delicious arose from a chance seedling, possibly a hybrid of ' Gr ...

', and 'Delicious' varieties. It is more common in arid regions, and is thought to be related to relative humidity and temperature. The effect can be mitigated by spraying the fruit with lipophilic coatings prior to harvest.

Tubers

Lenticels are also present on potato tubers.

Gallery

Prunus serrula bark lenticels, Dawyck Botanic Gardens.JPG, Lenticels on ''

Prunus serrula ''Prunus serrula'', called birch bark cherry, birchbark cherry, paperbark cherry, or Tibetan cherry, is a species of cherry native to China, and is used as an ornamental in many parts of the world for its striking coppery-red bark. Description ...

'' Wild Cherry bark lenticels.JPG, Lenticels on wild cherry or gean Alder bark and callus.JPG, Alder bark (''

Alnus glutinosa ''Alnus glutinosa'', the common alder, black alder, European alder, European black alder, or just alder, is a species of tree in the family Betulaceae, native to most of Europe, southwest Asia and northern Africa. It thrives in wet locations ...

'') with characteristic lenticels and abnormal lenticels on callused areas Pommes de terre Monalisa.png, Lenticels on

potato The potato is a starchy food, a tuber of the plant ''Solanum tuberosum'' and is a root vegetable native to the Americas. The plant is a perennial in the nightshade family Solanaceae. Wild potato species can be found from the southern Uni ...

es of the Monalisa variety Poires williams Vertes et Rouges Savoie.jpg, Lenticels on

Williams pear The Williams' bon chrétien pear, commonly called the Williams pear, or the Bartlett pear in the United States and Canada, is the most commonly grown variety of pear in most countries outside Asia. It is a cultivar (cultivated variety) of the s ...

varieties

Notes

References

*{{cite book , last =Raven , first =Peter H. , author2=Ray F. Evert , author3=Susan E. Eichorn , title =Biology of Plants 7th Ed. , publisher =W. H. Freeman and Company Publishers , year =2005 , pages =586–587 , isbn =0-7167-1007-2 Plant morphology Trees