secondary xylem and phloem cells arise from the vascular cambium.
secondary xylem and phloem cells arise from the vascular cambium.
where do secondary xylem and phloem cells arise from?
the vascular cambium
in plants with secondary growth, the vascular cambium, originating between the xylem and the phloem, forms a cylinder of tissue along the stem and root. 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. as secondary xylem accumulates, the "girth" (lateral dimensions) of the stem and root increases. 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).
root
phloem sap, which has a higher concentration of sugars and nitrogen, is under positive pressure unlike the more dilute xylem sap. most of the sternorrhyncha and a number of auchenorrhynchan groups feed on phloem. phloem feeding is common in the fulgoromorpha, most cicadellidae and in the heteroptera. the typhlocybine cicadellids specialize in feeding on non-vascular mesophyll tissue of leaves, which is more nutritious than the leaf epidermis. most heteroptera also feed on mesophyll tissue where they are more likely to encounter defensive secondary plant metabolites which often leads to the evolution of host specificity. obligate xylem feeding is a special habit that is found in the auchenorrhyncha among cicadoidea, cercopoidea and in cicadelline cicadellids. some phloem feeders may take to xylem sap facultatively, especially when facing dehydration. xylem feeders tend to be polyphagous; to overcome the negative pressure of xylem requires a special cibarial pump. phloem feeding hemiptera typically have symbiotic micro-organisms in their gut that help to convert amino acids. phloem feeders produce honeydew from their anus. a variety of organisms that feed on honeydew form symbiotic associations with phloem-feeders. phloem sap is a sugary liquid low in amino acids, so insects have to process large quantities to meet their nutritional requirements. xylem sap is even lower in amino acids and contains monosaccharides rather than sucrose, as well as organic acids and minerals. no digestion is required (except for the hydrolysis of sucrose) and 90% of the nutrients in the xylem sap can be utilised. some phloem sap feeders selectively mix phloem and xylem sap to control the osmotic potential of the liquid consumed. a striking adaptation to a very dilute diet is found in many hemipterans: a filter chamber, a part of the gut looped back on itself as a countercurrent exchanger, which permits nutrients to be separated from excess water. the residue, mostly water with sugars and amino acids, is quickly excreted as sticky "honey dew", notably from aphids but also from other auchenorrhycha and sternorrhyncha.
hemiptera