In every cactus species, the cortical bundles produce secondary phloems.
2.
These xylem strands have a cambium, and a secondary phloem forms.
3.
Lacking secondary phloem, unifacial cambium plants developed alternative strategies to long range nutrient transport.
4.
The vascular cambium cells divide to produce secondary xylem to the inside and secondary phloem to the outside.
5.
It is the source of both secondary xylem growth inwards towards the pith, and secondary phloem growth outwards to the bark.
6.
As the stem increases in diameter due to production of secondary xylem and secondary phloem, the cortex and epidermis are eventually destroyed.
7.
In very old cortical bundles located at the base of very old shoots, the secondary phloem may attain a thickness of several millimeters.
8.
The vascular cambium of " Calamites " was " unifacial ", producing secondary xylem towards the stem center, but not secondary phloem.
9.
As a result, tissues beyond the secondary phloem including the epidermis and cortex, in many cases tend to be pushed outward and are eventually " sloughed off " ( shed ).
10.
The vascular cambium forms new cells on both the inside and outside of the cambium cylinder, with those on the inside forming secondary xylem cells, and those on the outside forming secondary phloem cells.
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