Hemiptera plant. 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
Woody plant certain species in family Polygonaceae. These herbs are not truly woody but have hard densely packed stem tissue. Other herbaceous plants have woody stems called a caudex, which is a thickened stem base often found in plants that grow in alpine or dry environments. Under specific conditions, woody plants may decay or may in time become petrified wood. The symbol for a woody plant, based on "Species Plantarum" by Linnaeus is , which is also the astronomical symbol for the planet Saturn. Woody plant A woody plant is a plant that produces wood as its structural tissue. Woody plants are