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Cortex   [ Botany - Biology ]
Adjective: Cortical

Dictionary of botanic terminology - index of names

     
  The cortex is the outer layer of an organ or structure in plant or animal
(For example the bark of a tree, or the rind of a fruit)
 
     

The cortex in a plant is the primary tissue between the outer layer (epidermis) and the central vascular bundles core (called phloem  in the stem and pericycle in the root]
It is composed mostly of undifferentiated cells, usually large thin-walled parenchyma cells and also often  the outer cortical cells acquire irregularly thickened cell walls, and are called collenchyma cells.
Some of the outer cortical cells may contain chloroplasts (chlorenchyma)

Its main function is the storage of starch and/or water; often, it also provides aeration of cells to allow respiration.  Compare with: Pith,
 


Cross section of a stem of Astrophytum myriostigma
The cortex is the pale green tissue between the outer epidermis and the stele ( vascular bundles) in the centre.

 



Stele of Echinocactus grusonii whit cortical bundles branching off from cauline bundles and from leaf/bud traces.

The particularity of the Cactus Cortex

Cacti have a particular increased cortex width in which the photosynthetic outer cortex is farther away from the vascular bundles of the stem (Stele) Most cacti still have a relatively narrow pith, so in broad, globose barrel cacti the epidermis and chlorenchyma are often several centimetres (and as much as 40 cm in Ferocactus and Echinocactus)  from the xylem and phloem of the shoot’s vascular bundles. Over such distances, diffusion of water is extremely slow, diffusion of sugars is even slower. Even with an extremely thick covering of cuticle and wax, any epidermis will lose some water to dry desert air, and that water must be replaced rapidly if the epidermis and outer cortex tissues are to remain alive. Rapid translocation throughout the voluminous cortex of many cacti is made possible by the presence of a specific system of collateral cortical bundles.
Cortical bundles branch off from stele bundles and from leaf/bud traces, then run outward, tangentially and upward through the cortex, branching extensively and vascularizing all regions of the cortex. Cactus cortex is long-lived, cortical bundles must also function for many years. In all species, cortical bundles produce secondary phloem, and in very old bundles at the base of very old shoots, the amount of collapsed secondary phloem can be many millimeters thick.

In contrast the ability of other plants not belonging to the Cactaceae to produce extra cortical tissues is extremely limited by its inability to vascularize them. Consequently other stem-photosynthetic plants and succulents such as euphorbias, pachypodiums, stapeliads never become as broad as cacti do. It is certainly popular to point out the convergent evolution between euphorbias and cacti, but no euphorbia ever becomes as broad as a barrel cactus, or even as broad as many smaller cacti. The reason is simple: none of the non-cactus stem-photosynthetic succulents have cortical bundles in their stems. They are free to evolve to have a wider, more voluminous cortex only up to the point at which diffusion from the stele becomes too slow to keep the epidermis healthy. One of the broadest euphorbias, E. obesa, achieves its width by widening its stele: it has a broad pith that spreads its set of vascular bundles into a ring with a wide diameter, which is in turn surrounded by a cortex that is only moderately thick. But storing water in pith is more difficult than storing it in the cortex because a pith can absorb a large amount of water and swell only if the wood is flexible, otherwise either the wood would be torn apart or the pith would be prevented from expanding enough to absorb much water


 

 


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